{"id":434,"date":"2021-07-23T09:19:47","date_gmt":"2021-07-23T13:19:47","guid":{"rendered":"https:\/\/pressbooks.bccampus.ca\/aperrott\/chapter\/strengths-of-ionic-and-covalent-bonds\/"},"modified":"2022-06-23T09:01:40","modified_gmt":"2022-06-23T13:01:40","slug":"strengths-of-ionic-and-covalent-bonds","status":"publish","type":"chapter","link":"https:\/\/pressbooks.bccampus.ca\/aperrott\/chapter\/strengths-of-ionic-and-covalent-bonds\/","title":{"raw":"7.5 Strengths of Covalent Bonds","rendered":"7.5 Strengths of Covalent Bonds"},"content":{"raw":"<div class=\"textbox textbox--learning-objectives\">\r\n<h3><strong>Learning Objectives<\/strong><\/h3>\r\nBy the end of this section, you will be able to:\r\n<ul>\r\n \t<li>Describe the energetics of covalent bond formation and breakage<\/li>\r\n \t<li>Use average covalent bond energies to estimate enthalpies of reaction<\/li>\r\n<\/ul>\r\n<\/div>\r\n<p id=\"fs-idp176876736\">A bond\u2019s strength describes how strongly each atom is joined to another atom, and therefore how much energy is required to break the bond between the two atoms. In this section, you will learn about the bond strength of covalent bonds.<\/p>\r\n\r\n<div id=\"fs-idp66065152\" class=\"bc-section section\" data-depth=\"1\">\r\n<h3 data-type=\"title\"><strong>Bond Strength: Covalent Bonds<\/strong><\/h3>\r\n<p id=\"fs-idm12470240\">Stable molecules exist because covalent bonds hold the atoms together. We measure the strength of a covalent bond by the energy required to break it, that is, the energy necessary to separate the bonded atoms. Separating any pair of bonded atoms requires energy. The stronger a bond, the greater the energy required to break it.<\/p>\r\n<p id=\"fs-idp84340368\">The energy required to break a specific covalent bond in one mole of gaseous molecules is called the bond energy or the bond dissociation energy. The bond energy for a diatomic molecule, D<sub>X\u2013Y<\/sub>, is defined as the standard enthalpy change for the endothermic reaction:<\/p>\r\n\r\n<div id=\"fs-idm36464112\" style=\"text-align: center\" data-type=\"equation\">XY(<em>g<\/em>) \u2192 X(<em>g<\/em>) + Y(<em>g<\/em>)\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 D<sub>X-Y<\/sub> = \u0394<em>H<\/em>\u00b0<\/div>\r\n<p id=\"fs-idm12974112\">For example, the bond energy of the pure covalent H\u2013H bond, D<sub>H\u2013H<\/sub>, is 436 kJ per mole of H\u2013H bonds broken:<\/p>\r\n\r\n<div id=\"fs-idp119487888\" style=\"text-align: center\" data-type=\"equation\">H<sub>2<\/sub>(<em>g<\/em>) \u2192 2 H(<em>g<\/em>) \u00a0\u00a0\u00a0\u00a0\u00a0 D<sub>H-H<\/sub> = \u0394<em>H<\/em>\u00b0 = 436 kJ<\/div>\r\n<p id=\"fs-idp244347920\">Molecules with three or more atoms have two or more bonds. The sum of all bond energies in such a molecule is equal to the standard enthalpy change for the endothermic reaction that breaks all the bonds in the molecule. For example, the sum of the four C\u2013H bond energies in CH<sub>4<\/sub>, 1660 kJ, is equal to the standard enthalpy change of the reaction:<\/p>\r\n<span id=\"fs-idp286503296\" class=\"scaled-down\" data-type=\"media\" data-alt=\"A reaction is shown with Lewis structures. The first structure shows a carbon atom single bonded to four hydrogen atoms with the symbol, \u201c( g )\u201d written next to it. A right-facing arrow points to the letter \u201cC\u201d and the symbol \u201c( g ),\u201d which is followed by a plus sign. Next is the number 4, the letter \u201cH\u201d and the symbol, \u201c( g ).\u201d To the right of this equation is another equation: capital delta H superscript degree symbol equals 1660 k J.\"><img src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/CNX_Chem_07_05_CH4bond_img-1.jpg\" alt=\"A reaction is shown with Lewis structures. The first structure shows a carbon atom single bonded to four hydrogen atoms with the symbol, \u201c( g )\u201d written next to it. A right-facing arrow points to the letter \u201cC\u201d and the symbol \u201c( g ),\u201d which is followed by a plus sign. Next is the number 4, the letter \u201cH\u201d and the symbol, \u201c( g ).\u201d To the right of this equation is another equation: capital delta H superscript degree symbol equals 1660 k J.\" data-media-type=\"image\/jpeg\" \/><\/span>\r\n<p id=\"fs-idp85707472\">The average C\u2013H bond energy, D<sub>C\u2013H<\/sub>, is 1660\/4 = 415 kJ\/mol because there are four moles of C\u2013H bonds broken per mole of the reaction. Although the four C\u2013H bonds are equivalent in the original molecule, they do not each require the same energy to break; once the first bond is broken (which requires 439 kJ\/mol), the remaining bonds are easier to break. The 415 kJ\/mol value is the average, not the exact value required to break any one bond.<\/p>\r\n<p id=\"fs-idp26225392\">The strength of a bond between two atoms increases as the number of electron pairs in the bond increases. Generally, as the bond strength increases, the bond length decreases. Thus, we find that triple bonds are stronger and shorter than double bonds between the same two atoms; likewise, double bonds are stronger and shorter than single bonds between the same two atoms. Average bond energies for some common bonds appear in <a class=\"autogenerated-content\" href=\"#fs-idp13638832\">(Figure)<\/a>, and a comparison of bond lengths and bond strengths for some common bonds appears in <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>. When one atom bonds to various atoms in a group, the bond strength typically decreases as we move down the group. For example, C\u2013F is 439 kJ\/mol, C\u2013Cl is 330 kJ\/mol, and C\u2013Br is 275 kJ\/mol.<\/p>\r\n\r\n<table id=\"fs-idp13638832\" style=\"height: 430px\" summary=\"This table has six columns and twenty-four rows. The first row is a header row that labels the columns: \u201cBond,\u201d \u201cBond Energy,\u201d \u201cBond,\u201d \u201cBond Energy,\u201d \u201cBond,\u201d and, \u201cBond Energy.\u201d Under the first \u201cBond\u201d column are the values: H bond to H with a single bond; H bonds to C with a single bond; H bonds to N with a single bond; H bonds to O with a single bond; H bonds to F with a single bond; H bonds to S i with a single bond; H bonds to P with a single bond; H bonds to S with a single bond; H bonds to C l with a single bond; H bonds to B r with a single bond; H bonds to I with a single bond; C bonds to C with a single bond; C bonds to C with a double bond; C bonds to C with a triple bond; C bonds to N with a single bond; C bonds to N with a double bond; C bonds to N with a triple bond; C bonds to O with a single bond; C bonds to O with a double bond; C bonds to O with a triple bond; C bonds to F with a single bond; C bonds to S i with a single bond; and C bonds to P with a single bond. Under the first \u201cBond Energy\u201d column are the values: 436; 415; 390; 464; 569; 395; 320; 340; 432; 370; 295; 345; 611; 837; 290; 615; 891; 350; 741; 1080; 439; 360; and 265. Under the second \u201cBond\u201d column are the values: C bonds to S with a single bond; C bonds to C l with a single bond; C bonds to B r with a single bond; C bonds to I with a single bond; N bonds to N with a single bond; N bonds to N with a double bond; N bonds to N with a triple bond; N bonds to O with a single bond; N bonds to F with a single bond; N bonds to P with a single bond; N bonds to C l with a single bond; N bonds to B r with a single bond; O bonds to O with a single bond; O bonds to O with a double bond; O bonds to F with a single bond; O bonds to S i with a single bond; O bonds to P with a single bond; O bonds to C l with a single bond; O bonds to I with a single bond; F bonds to F with a single bond; F bonds to S i with a single bond; F bonds to P with a single bond; and F bonds to S with a single bond. Under the second \u201cBond Energy\u201d column are the values: 260; 330; 275; 240; 160; 418; 946; 200; 270; 210; 200; 245; 140; 498; 160; 370; 350; 205; 200; 160; 540; 489; and 285. Under the third \u201cBond\u201d column are the values: F bonds to C l with a single bond; F bonds to B r with a single bond; S i bonds to S i with a single bond; S i bonds to P with a single bond; S i bonds to S with a single bond; S i bonds to C l with a single bond; S i bonds to B r with a single bond; S i bonds to I with a single bond; P bonds to P with a single bond; P bonds to S with a single bond; P bonds to C l with a single bond; P bonds to B r with a single bond; P bonds to I with a single bond; S bonds to S with a single bond; S bonds to C l with a single bond; S bonds to B r with a single bond; C l bonds to C l with a single bond; C l bonds to B r with a single bond; C l bonds to I with a single bond; B r bonds to B r with a single bond; B r bonds to I with a single bond; I bonds to I with a single bond; and the last cell in the column is empty. Under the third \u201cBond Energy\u201d column are the values: 255; 235; 230; 215; 225; 359; 290; 215; 215; 230; 330; 270; 215; 215; 250; 215; 243; 220; 210; 190; 180; 150; and the last cell in the column is empty.\">\r\n<thead>\r\n<tr style=\"height: 15px\">\r\n<th style=\"height: 15px;width: 514.533px\" colspan=\"8\" data-align=\"center\">Bond Energies (kJ\/mol)<\/th>\r\n<\/tr>\r\n<tr style=\"height: 31px\" valign=\"top\">\r\n<th style=\"height: 31px;width: 134.567px\" data-align=\"left\">Bond<\/th>\r\n<th style=\"height: 31px;width: 70.4667px\" data-align=\"left\">Bond Energy<\/th>\r\n<th style=\"height: 31px;width: 0.0166667px\" data-align=\"left\"><\/th>\r\n<th style=\"height: 31px;width: 35.4167px\" data-align=\"left\">Bond<\/th>\r\n<th style=\"height: 31px;width: 70.4667px\" data-align=\"left\">Bond Energy<\/th>\r\n<th style=\"height: 31px;width: 0.0166667px\" data-align=\"left\"><\/th>\r\n<th style=\"height: 31px;width: 36.5333px\" data-align=\"left\">Bond<\/th>\r\n<th style=\"height: 31px;width: 70.45px\" data-align=\"left\">Bond Energy<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013H<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">436<\/td>\r\n<td style=\"height: 384px;width: 0.85px\" rowspan=\"23\" data-align=\"left\"><\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">C\u2013S<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">260<\/td>\r\n<td style=\"height: 384px;width: 0.85px\" rowspan=\"23\" data-align=\"left\"><\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">F\u2013Cl<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">255<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013C<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">415<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">C\u2013Cl<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">330<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">F\u2013Br<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">235<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013N<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">390.<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">C\u2013Br<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">275<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013Si<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">230.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013O<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">464<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">C\u2013I<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">240<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013P<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013F<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">569<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013N<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">160<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013S<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">225<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013Si<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">395<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N=N<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">418<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013Cl<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">359<\/td>\r\n<\/tr>\r\n<tr style=\"height: 18px\" valign=\"top\">\r\n<td style=\"height: 18px;width: 135.4px\" data-align=\"left\">H\u2013P<\/td>\r\n<td style=\"height: 18px;width: 71.3px\" data-align=\"left\">320.<\/td>\r\n<td style=\"height: 18px;width: 36.25px\" data-align=\"left\"><img class=\"alignnone wp-image-1494\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5a.png\" alt=\"\" width=\"36\" height=\"16\" \/><\/td>\r\n<td style=\"height: 18px;width: 71.3px\" data-align=\"left\">946<\/td>\r\n<td style=\"height: 18px;width: 37.3667px\" data-align=\"left\">Si\u2013Br<\/td>\r\n<td style=\"height: 18px;width: 70.45px\" data-align=\"left\">290.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013S<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">340.<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013O<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">200<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013I<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013Cl<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">432<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013F<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">270<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013P<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013Br<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">370.<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013P<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">210<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013S<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">230.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013I<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">295<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013Cl<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">200<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013Cl<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">330.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013C<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">345<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013Br<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">245<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013Br<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">270.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C=C<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">611<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013O<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">140<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013I<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\r\n<\/tr>\r\n<tr style=\"height: 19px\" valign=\"top\">\r\n<td style=\"height: 19px;width: 135.4px\"><img class=\"alignnone wp-image-1495\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5b.png\" alt=\"\" width=\"40\" height=\"17\" \/><\/td>\r\n<td style=\"height: 19px;width: 71.3px\" data-align=\"left\">837<\/td>\r\n<td style=\"height: 19px;width: 36.25px\" data-align=\"left\">O=O<\/td>\r\n<td style=\"height: 19px;width: 71.3px\" data-align=\"left\">498<\/td>\r\n<td style=\"height: 19px;width: 37.3667px\" data-align=\"left\">S\u2013S<\/td>\r\n<td style=\"height: 19px;width: 70.45px\" data-align=\"left\">215<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013N<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">290.<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013F<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">160<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">S\u2013Cl<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">250.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C=N<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">615<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013Si<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">370<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">S\u2013Br<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\r\n<\/tr>\r\n<tr style=\"height: 31px\" valign=\"top\">\r\n<td style=\"height: 31px;width: 135.4px\" data-align=\"left\"><img class=\"alignnone wp-image-1497\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5d.png\" alt=\"\" width=\"44\" height=\"16\" \/><\/td>\r\n<td style=\"height: 31px;width: 71.3px\" data-align=\"left\">891<\/td>\r\n<td style=\"height: 31px;width: 36.25px\" data-align=\"left\">O\u2013P<\/td>\r\n<td style=\"height: 31px;width: 71.3px\" data-align=\"left\">350<\/td>\r\n<td style=\"height: 31px;width: 37.3667px\" data-align=\"left\">Cl\u2013Cl<\/td>\r\n<td style=\"height: 31px;width: 70.45px\" data-align=\"left\">243<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013O<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">350.<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013Cl<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">205<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Cl\u2013Br<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">220.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C=O<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">741<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013I<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">200<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Cl\u2013I<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">210.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 31px\" valign=\"top\">\r\n<td style=\"height: 31px;width: 135.4px\" data-align=\"left\"><img class=\"alignnone wp-image-1496\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png\" alt=\"\" width=\"34\" height=\"13\" \/><\/td>\r\n<td style=\"height: 31px;width: 71.3px\" data-align=\"left\">1080.<\/td>\r\n<td style=\"height: 31px;width: 36.25px\" data-align=\"left\">F\u2013F<\/td>\r\n<td style=\"height: 31px;width: 71.3px\" data-align=\"left\">160<\/td>\r\n<td style=\"height: 31px;width: 37.3667px\" data-align=\"left\">Br\u2013Br<\/td>\r\n<td style=\"height: 31px;width: 70.45px\" data-align=\"left\">190.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013F<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">439<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">F\u2013Si<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">540<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Br\u2013I<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">180.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013Si<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">360.<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">F\u2013P<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">489<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">I\u2013I<\/td>\r\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">150.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013P<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">265<\/td>\r\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">F\u2013S<\/td>\r\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">285<\/td>\r\n<td style=\"height: 15px;width: 37.3667px\"><\/td>\r\n<td style=\"height: 15px;width: 70.45px\"><\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<table id=\"fs-idm44464336\" class=\"top-titled\" style=\"height: 165px\" summary=\"This table has three columns and ten rows. The first row is a header row that labels the columns: \u201cBond,\u201d \u201cBond Length in angstroms,\u201d and, \u201cBond Energy in k J \/ mol.\u201d Under the column \u201cBond\u201d are the values: C bonds to C with a single bond; C bonds to C with a double bond; C bonds to C with a triple bond; C bonds to N with a single bond; C bonds to N with a double bond; C bonds to N with a triple bond; C bonds to O with a single bond; C bonds to O with a double bond; and C bonds to O with a triple bond. Under the column \u201cBond Length in angstroms\u201d are the values: 1.54; 1.34; 1.20; 1.43; 1.38; 1.16; 1.43; 1.23; and 1.13. Under the column \u201cBond Energy in k J \/ mol\u201d are the values: 345; 611; 837; 290; 615; 891; 350; 741; and 1080.\">\r\n<thead>\r\n<tr style=\"height: 15px\">\r\n<th style=\"height: 15px;width: 475.35px\" colspan=\"3\" data-align=\"center\">Average Bond Lengths and Bond Energies for Some Common Bonds<\/th>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<th style=\"height: 15px;width: 188.233px\" data-align=\"left\">Bond<\/th>\r\n<th style=\"height: 15px;width: 109.317px\" data-align=\"left\">Bond Length (\u00c5)<\/th>\r\n<th style=\"height: 15px;width: 150.2px\" data-align=\"left\">Bond Energy (kJ\/mol)<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C\u2013C<\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.54<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">345<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C=C<\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.34<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">611<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\"><img class=\"alignnone wp-image-1495\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5b.png\" alt=\"\" width=\"43\" height=\"18\" \/><\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.20<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">837<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C\u2013N<\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.43<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">290.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C=N<\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.38<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">615<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\"><img class=\"alignnone wp-image-1497\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5d.png\" alt=\"\" width=\"44\" height=\"16\" \/><\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.16<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">891<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C\u2013O<\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.43<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">350.<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C=O<\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.23<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">741<\/td>\r\n<\/tr>\r\n<tr style=\"height: 15px\" valign=\"top\">\r\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\"><img class=\"alignnone wp-image-1496\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png\" alt=\"\" width=\"42\" height=\"16\" \/><\/td>\r\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.13<\/td>\r\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">1080.<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<p id=\"fs-idp29959728\">We can use bond energies to calculate approximate enthalpy changes for reactions where enthalpies of formation are not available. Calculations of this type will also tell us whether a reaction is exothermic or endothermic. An exothermic reaction (\u0394<em data-effect=\"italics\">H<\/em> negative, heat produced) results when the bonds in the products are stronger than the bonds in the reactants. An endothermic reaction (\u0394<em data-effect=\"italics\">H<\/em> positive, heat absorbed) results when the bonds in the products are weaker than those in the reactants.<\/p>\r\n<p id=\"fs-idp234028832\">The enthalpy change, \u0394<em data-effect=\"italics\">H<\/em>, for a chemical reaction is approximately equal to the sum of the energy required to break all bonds in the reactants (energy \u201cin\u201d, positive sign) plus the energy released when all bonds are formed in the products (energy \u201cout,\u201d negative sign). This can be expressed mathematically in the following way:<\/p>\r\n\r\n<div id=\"fs-idm40243760\" style=\"text-align: center\" data-type=\"equation\">\u0394<em>H<\/em> = \u03a3D<sub>bonds broken<\/sub> - \u03a3D<sub>bonds formed<\/sub><\/div>\r\n<p id=\"fs-idp118053616\">In this expression, the symbol \u01a9 means \u201cthe sum of\u201d and D represents the bond energy in kilojoules per mole, which is always a positive number. The bond energy is obtained from a table (like <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>) and will depend on whether the particular bond is a single, double, or triple bond. Thus, in calculating enthalpies in this manner, it is important that we consider the bonding in all reactants and products. Because D values are typically averages for one type of bond in many different molecules, this calculation provides a rough estimate, not an exact value, for the enthalpy of reaction.<\/p>\r\n<p id=\"fs-idp17129504\">Consider the following reaction:<\/p>\r\n\r\n<div id=\"fs-idp51383312\" style=\"text-align: center\" data-type=\"equation\">H<sub>2<\/sub>(<em>g<\/em>) + Cl<sub>2<\/sub>(<em>g<\/em>) \u27f6 2HCl(<em>g<\/em>)<\/div>\r\n<p id=\"fs-idp77710320\">or<\/p>\r\n\r\n<div id=\"fs-idp56299152\" style=\"text-align: center\" data-type=\"equation\">H-H(<em>g<\/em>) + Cl-Cl(<em>g<\/em>) \u27f6 2H-Cl(<em>g<\/em>)<\/div>\r\n<p id=\"fs-idp201091440\">To form two moles of HCl, one mole of H\u2013H bonds and one mole of Cl\u2013Cl bonds must be broken. The energy required to break these bonds is the sum of the bond energy of the H\u2013H bond (436 kJ\/mol) and the Cl\u2013Cl bond (243 kJ\/mol). During the reaction, two moles of H\u2013Cl bonds are formed (bond energy = 432 kJ\/mol), releasing 2 \u00d7 432 kJ; or 864 kJ. Because the bonds in the products are stronger than those in the reactants, the reaction releases more energy than it consumes:<\/p>\r\n\r\n<div id=\"fs-idp14860448\" data-type=\"equation\">\u0394<em>H<\/em> = \u03a3D<sub>bonds broken<\/sub> - \u03a3D<sub>bonds formed<\/sub><\/div>\r\n<div data-type=\"equation\">\u0394<em>H<\/em> = (D<sub>H-H<\/sub> + D<sub>Cl-Cl)<\/sub> - 2D<sub>H-Cl<\/sub><\/div>\r\n<div data-type=\"equation\">\u00a0\u00a0\u00a0\u00a0\u00a0 = (436 kJ + 243 kJ) - 2(432 kJ) = -185 kJ<\/div>\r\n<div data-type=\"equation\"><\/div>\r\n<p id=\"fs-idp29639024\">This excess energy is released as heat, so the reaction is exothermic. <a href=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/back-matter\/standard-thermodynamic-properties-for-selected-substances\/\">Appendix G<\/a> gives a value for the standard molar enthalpy of formation of HCl(g), \u0394<em>H<\/em><sub>f<\/sub>\u00b0, of \u201392.307 kJ\/mol. Twice that value is \u2013184.6 kJ, which agrees well with the answer obtained earlier for the formation of two moles of HCl.<\/p>\r\n\r\n<div id=\"fs-idm33428592\" class=\"textbox textbox--examples\" data-type=\"example\">\r\n<p id=\"fs-idp71590928\"><strong>Using Bond Energies to Calculate Approximate Enthalpy Changes:<\/strong><\/p>\r\nMethanol, CH<sub>3<\/sub>OH, may be an excellent alternative fuel. The high-temperature reaction of steam and carbon produces a mixture of the gases carbon monoxide, CO, and hydrogen, H<sub>2<\/sub>, from which methanol can be produced. Using the bond energies in <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>, calculate the approximate enthalpy change, \u0394<em data-effect=\"italics\">H<\/em>, for the reaction here:\r\n<div id=\"fs-idp93467344\" style=\"text-align: center\" data-type=\"equation\">CO(<em>g<\/em>) + 2H<sub>2<\/sub>(<em>g<\/em>) \u27f6CH<sub>3<\/sub>OH(<em>g<\/em>)<\/div>\r\n<div data-type=\"equation\"><\/div>\r\n<p id=\"fs-idp41458304\"><strong>Solution:<\/strong><\/p>\r\nFirst, we need to write the Lewis structures of the reactants and the products:\r\n\r\n<span id=\"fs-idp525280\" class=\"scaled-down\" data-type=\"media\" data-alt=\"A set of Lewis diagrams show a chemical reaction. The first structure shows a carbon atom with a lone pair of electrons triple bonded to an oxygen with a lone pair of electrons. To the right of this structure is a plus sign, then the number 2 followed by a hydrogen atom single bonded to a hydrogen atom. To the right of this structure is a right-facing arrow followed by a hydrogen atom single bonded to a carbon atom that is single bonded to two hydrogen atoms and an oxygen atom with two lone pairs of electrons. The oxygen atom is also single bonded to a hydrogen atom.\"><img src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/CNX_Chem_07_05_CH3OHLew_img-1.jpg\" alt=\"A set of Lewis diagrams show a chemical reaction. The first structure shows a carbon atom with a lone pair of electrons triple bonded to an oxygen with a lone pair of electrons. To the right of this structure is a plus sign, then the number 2 followed by a hydrogen atom single bonded to a hydrogen atom. To the right of this structure is a right-facing arrow followed by a hydrogen atom single bonded to a carbon atom that is single bonded to two hydrogen atoms and an oxygen atom with two lone pairs of electrons. The oxygen atom is also single bonded to a hydrogen atom.\" data-media-type=\"image\/jpeg\" \/><\/span>\r\n<p id=\"fs-idp202067232\">From this, we see that \u0394<em data-effect=\"italics\">H<\/em> for this reaction involves the energy required to break a C\u2013O triple bond and two H\u2013H single bonds, as well as the energy produced by the formation of three C\u2013H single bonds, a C\u2013O single bond, and an O\u2013H single bond. We can express this as follows:<\/p>\r\n\r\n<div id=\"fs-idp76020096\" data-type=\"equation\">\r\n<div id=\"fs-idp14860448\" data-type=\"equation\">\u0394<em>H<\/em> = \u03a3D<sub>bonds broken<\/sub> - \u03a3D<sub>bonds formed<\/sub><\/div>\r\n<div data-type=\"equation\">\u0394<em>H<\/em> = (D<sub><img class=\"alignnone wp-image-1496\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png\" alt=\"\" width=\"23\" height=\"9\" \/><\/sub> + 2D<sub>H-H)<\/sub> - (3D<sub>C-H<\/sub> + D<sub>C-O<\/sub> + D<sub>O-H<\/sub>)<\/div>\r\n<div data-type=\"equation\"><\/div>\r\n<div data-type=\"equation\"><\/div>\r\n<\/div>\r\n<p id=\"fs-idp53734416\">Using the bond energy values in <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>, we obtain:<\/p>\r\n\r\n<div id=\"fs-idm103164880\" data-type=\"equation\">\r\n<div data-type=\"equation\">\u0394<em>H<\/em> = [1080. kJ + 2(436 kJ)] - [3(415 kJ) + 350. kJ + 464 kJ] = -107 kJ<\/div>\r\n<div data-type=\"equation\"><\/div>\r\n<div data-type=\"equation\"><\/div>\r\n<\/div>\r\n<p id=\"fs-idp126256512\">We can compare this value to the value calculated based on \u0394<em>H<\/em><sub>f<\/sub>\u00b0 data from Appendix G:<\/p>\r\n\r\n<div id=\"fs-idp158903840\" data-type=\"equation\">\u0394<em>H<\/em> = \u0394<em>H<\/em><sub>f<\/sub>\u00b0(CH<sub>3<\/sub>OH(<em>g<\/em>)) - [\u0394<em>H<\/em><sub>f<\/sub>\u00b0(CO(<em>g<\/em>)) + 2\u0394<em>H<\/em><sub>f<\/sub>\u00b0(H<sub>2<\/sub>(<em>g<\/em>))]<\/div>\r\n<div data-type=\"equation\">\u00a0\u00a0\u00a0\u00a0 = -201.0 kJ - [-110.52 kJ + 2(0)] = -90.5 kJ<\/div>\r\n<div data-type=\"equation\"><\/div>\r\n<div data-type=\"equation\"><\/div>\r\n<p id=\"fs-idp56832128\">Note that there is a fairly significant gap between the values calculated using the two different methods. This occurs because D values are the <em data-effect=\"italics\">average<\/em> of different bond strengths; therefore, they often give only rough agreement with other data.<\/p>\r\n<p id=\"fs-idp36454672\"><strong>Check Your Learning:<\/strong><\/p>\r\nEthyl alcohol, CH<sub>3<\/sub>CH<sub>2<\/sub>OH, was one of the first organic chemicals deliberately synthesized by humans. It has many uses in industry, and it is the alcohol contained in alcoholic beverages. It can be obtained by the fermentation of sugar or synthesized by the hydration of ethylene in the following reaction:\r\n\r\n<span id=\"fs-idp144672\" class=\"scaled-down\" data-type=\"media\" data-alt=\"A set of Lewis structures show a chemical reaction. The first structure shows two carbon atoms that are double bonded together and are each single bonded to two hydrogen atoms. This structure is followed by a plus sign, then an oxygen atom with two lone pairs of electrons single bonded to two hydrogen atoms. A right-facing arrow leads to a carbon atom single bonded to three hydrogen atoms and a second carbon atom. The second carbon atom is single bonded to two hydrogen atoms and an oxygen atom with two lone pairs of electrons. The oxygen atom is single bonded to a hydrogen atom as well.\"><img src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/CNX_Chem_07_05_Ethanol_img-1.jpg\" alt=\"A set of Lewis structures show a chemical reaction. The first structure shows two carbon atoms that are double bonded together and are each single bonded to two hydrogen atoms. This structure is followed by a plus sign, then an oxygen atom with two lone pairs of electrons single bonded to two hydrogen atoms. A right-facing arrow leads to a carbon atom single bonded to three hydrogen atoms and a second carbon atom. The second carbon atom is single bonded to two hydrogen atoms and an oxygen atom with two lone pairs of electrons. The oxygen atom is single bonded to a hydrogen atom as well.\" data-media-type=\"image\/jpeg\" \/><\/span>\r\n<p id=\"fs-idp27645248\">Using the bond energies in <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>, calculate an approximate enthalpy change, \u0394<em data-effect=\"italics\">H<\/em>, for this reaction.<\/p>\r\n&nbsp;\r\n<div id=\"fs-idp88406224\" data-type=\"note\">\r\n<div data-type=\"title\"><strong>Answer:<\/strong><\/div>\r\n<p id=\"fs-idp24314032\">\u201335 kJ<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div id=\"fs-idm34679504\" class=\"bc-section section\" data-depth=\"1\"><\/div>\r\n<div id=\"fs-idp234492208\" class=\"summary\" data-depth=\"1\"><\/div>\r\n<div class=\"textbox shaded\" data-type=\"glossary\">\r\n<h3 data-type=\"glossary-title\"><strong>Glossary<\/strong><\/h3>\r\n<dl id=\"fs-idp10319376\">\r\n \t<dt>bond energy<\/dt>\r\n \t<dd id=\"fs-idm69489984\">(also, bond dissociation energy) energy required to break a covalent bond in a gaseous substance<\/dd>\r\n<\/dl>\r\n<dl id=\"fs-idp39711472\"><\/dl>\r\n<\/div>","rendered":"<div class=\"textbox textbox--learning-objectives\">\n<h3><strong>Learning Objectives<\/strong><\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<ul>\n<li>Describe the energetics of covalent bond formation and breakage<\/li>\n<li>Use average covalent bond energies to estimate enthalpies of reaction<\/li>\n<\/ul>\n<\/div>\n<p id=\"fs-idp176876736\">A bond\u2019s strength describes how strongly each atom is joined to another atom, and therefore how much energy is required to break the bond between the two atoms. In this section, you will learn about the bond strength of covalent bonds.<\/p>\n<div id=\"fs-idp66065152\" class=\"bc-section section\" data-depth=\"1\">\n<h3 data-type=\"title\"><strong>Bond Strength: Covalent Bonds<\/strong><\/h3>\n<p id=\"fs-idm12470240\">Stable molecules exist because covalent bonds hold the atoms together. We measure the strength of a covalent bond by the energy required to break it, that is, the energy necessary to separate the bonded atoms. Separating any pair of bonded atoms requires energy. The stronger a bond, the greater the energy required to break it.<\/p>\n<p id=\"fs-idp84340368\">The energy required to break a specific covalent bond in one mole of gaseous molecules is called the bond energy or the bond dissociation energy. The bond energy for a diatomic molecule, D<sub>X\u2013Y<\/sub>, is defined as the standard enthalpy change for the endothermic reaction:<\/p>\n<div id=\"fs-idm36464112\" style=\"text-align: center\" data-type=\"equation\">XY(<em>g<\/em>) \u2192 X(<em>g<\/em>) + Y(<em>g<\/em>)\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 D<sub>X-Y<\/sub> = \u0394<em>H<\/em>\u00b0<\/div>\n<p id=\"fs-idm12974112\">For example, the bond energy of the pure covalent H\u2013H bond, D<sub>H\u2013H<\/sub>, is 436 kJ per mole of H\u2013H bonds broken:<\/p>\n<div id=\"fs-idp119487888\" style=\"text-align: center\" data-type=\"equation\">H<sub>2<\/sub>(<em>g<\/em>) \u2192 2 H(<em>g<\/em>) \u00a0\u00a0\u00a0\u00a0\u00a0 D<sub>H-H<\/sub> = \u0394<em>H<\/em>\u00b0 = 436 kJ<\/div>\n<p id=\"fs-idp244347920\">Molecules with three or more atoms have two or more bonds. The sum of all bond energies in such a molecule is equal to the standard enthalpy change for the endothermic reaction that breaks all the bonds in the molecule. For example, the sum of the four C\u2013H bond energies in CH<sub>4<\/sub>, 1660 kJ, is equal to the standard enthalpy change of the reaction:<\/p>\n<p><span id=\"fs-idp286503296\" class=\"scaled-down\" data-type=\"media\" data-alt=\"A reaction is shown with Lewis structures. The first structure shows a carbon atom single bonded to four hydrogen atoms with the symbol, \u201c( g )\u201d written next to it. A right-facing arrow points to the letter \u201cC\u201d and the symbol \u201c( g ),\u201d which is followed by a plus sign. Next is the number 4, the letter \u201cH\u201d and the symbol, \u201c( g ).\u201d To the right of this equation is another equation: capital delta H superscript degree symbol equals 1660 k J.\"><img decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/CNX_Chem_07_05_CH4bond_img-1.jpg\" alt=\"A reaction is shown with Lewis structures. The first structure shows a carbon atom single bonded to four hydrogen atoms with the symbol, \u201c( g )\u201d written next to it. A right-facing arrow points to the letter \u201cC\u201d and the symbol \u201c( g ),\u201d which is followed by a plus sign. Next is the number 4, the letter \u201cH\u201d and the symbol, \u201c( g ).\u201d To the right of this equation is another equation: capital delta H superscript degree symbol equals 1660 k J.\" data-media-type=\"image\/jpeg\" \/><\/span><\/p>\n<p id=\"fs-idp85707472\">The average C\u2013H bond energy, D<sub>C\u2013H<\/sub>, is 1660\/4 = 415 kJ\/mol because there are four moles of C\u2013H bonds broken per mole of the reaction. Although the four C\u2013H bonds are equivalent in the original molecule, they do not each require the same energy to break; once the first bond is broken (which requires 439 kJ\/mol), the remaining bonds are easier to break. The 415 kJ\/mol value is the average, not the exact value required to break any one bond.<\/p>\n<p id=\"fs-idp26225392\">The strength of a bond between two atoms increases as the number of electron pairs in the bond increases. Generally, as the bond strength increases, the bond length decreases. Thus, we find that triple bonds are stronger and shorter than double bonds between the same two atoms; likewise, double bonds are stronger and shorter than single bonds between the same two atoms. Average bond energies for some common bonds appear in <a class=\"autogenerated-content\" href=\"#fs-idp13638832\">(Figure)<\/a>, and a comparison of bond lengths and bond strengths for some common bonds appears in <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>. When one atom bonds to various atoms in a group, the bond strength typically decreases as we move down the group. For example, C\u2013F is 439 kJ\/mol, C\u2013Cl is 330 kJ\/mol, and C\u2013Br is 275 kJ\/mol.<\/p>\n<table id=\"fs-idp13638832\" style=\"height: 430px\" summary=\"This table has six columns and twenty-four rows. The first row is a header row that labels the columns: \u201cBond,\u201d \u201cBond Energy,\u201d \u201cBond,\u201d \u201cBond Energy,\u201d \u201cBond,\u201d and, \u201cBond Energy.\u201d Under the first \u201cBond\u201d column are the values: H bond to H with a single bond; H bonds to C with a single bond; H bonds to N with a single bond; H bonds to O with a single bond; H bonds to F with a single bond; H bonds to S i with a single bond; H bonds to P with a single bond; H bonds to S with a single bond; H bonds to C l with a single bond; H bonds to B r with a single bond; H bonds to I with a single bond; C bonds to C with a single bond; C bonds to C with a double bond; C bonds to C with a triple bond; C bonds to N with a single bond; C bonds to N with a double bond; C bonds to N with a triple bond; C bonds to O with a single bond; C bonds to O with a double bond; C bonds to O with a triple bond; C bonds to F with a single bond; C bonds to S i with a single bond; and C bonds to P with a single bond. Under the first \u201cBond Energy\u201d column are the values: 436; 415; 390; 464; 569; 395; 320; 340; 432; 370; 295; 345; 611; 837; 290; 615; 891; 350; 741; 1080; 439; 360; and 265. Under the second \u201cBond\u201d column are the values: C bonds to S with a single bond; C bonds to C l with a single bond; C bonds to B r with a single bond; C bonds to I with a single bond; N bonds to N with a single bond; N bonds to N with a double bond; N bonds to N with a triple bond; N bonds to O with a single bond; N bonds to F with a single bond; N bonds to P with a single bond; N bonds to C l with a single bond; N bonds to B r with a single bond; O bonds to O with a single bond; O bonds to O with a double bond; O bonds to F with a single bond; O bonds to S i with a single bond; O bonds to P with a single bond; O bonds to C l with a single bond; O bonds to I with a single bond; F bonds to F with a single bond; F bonds to S i with a single bond; F bonds to P with a single bond; and F bonds to S with a single bond. Under the second \u201cBond Energy\u201d column are the values: 260; 330; 275; 240; 160; 418; 946; 200; 270; 210; 200; 245; 140; 498; 160; 370; 350; 205; 200; 160; 540; 489; and 285. Under the third \u201cBond\u201d column are the values: F bonds to C l with a single bond; F bonds to B r with a single bond; S i bonds to S i with a single bond; S i bonds to P with a single bond; S i bonds to S with a single bond; S i bonds to C l with a single bond; S i bonds to B r with a single bond; S i bonds to I with a single bond; P bonds to P with a single bond; P bonds to S with a single bond; P bonds to C l with a single bond; P bonds to B r with a single bond; P bonds to I with a single bond; S bonds to S with a single bond; S bonds to C l with a single bond; S bonds to B r with a single bond; C l bonds to C l with a single bond; C l bonds to B r with a single bond; C l bonds to I with a single bond; B r bonds to B r with a single bond; B r bonds to I with a single bond; I bonds to I with a single bond; and the last cell in the column is empty. Under the third \u201cBond Energy\u201d column are the values: 255; 235; 230; 215; 225; 359; 290; 215; 215; 230; 330; 270; 215; 215; 250; 215; 243; 220; 210; 190; 180; 150; and the last cell in the column is empty.\">\n<thead>\n<tr style=\"height: 15px\">\n<th style=\"height: 15px;width: 514.533px\" colspan=\"8\" data-align=\"center\">Bond Energies (kJ\/mol)<\/th>\n<\/tr>\n<tr style=\"height: 31px\" valign=\"top\">\n<th style=\"height: 31px;width: 134.567px\" data-align=\"left\">Bond<\/th>\n<th style=\"height: 31px;width: 70.4667px\" data-align=\"left\">Bond Energy<\/th>\n<th style=\"height: 31px;width: 0.0166667px\" data-align=\"left\"><\/th>\n<th style=\"height: 31px;width: 35.4167px\" data-align=\"left\">Bond<\/th>\n<th style=\"height: 31px;width: 70.4667px\" data-align=\"left\">Bond Energy<\/th>\n<th style=\"height: 31px;width: 0.0166667px\" data-align=\"left\"><\/th>\n<th style=\"height: 31px;width: 36.5333px\" data-align=\"left\">Bond<\/th>\n<th style=\"height: 31px;width: 70.45px\" data-align=\"left\">Bond Energy<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013H<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">436<\/td>\n<td style=\"height: 384px;width: 0.85px\" rowspan=\"23\" data-align=\"left\"><\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">C\u2013S<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">260<\/td>\n<td style=\"height: 384px;width: 0.85px\" rowspan=\"23\" data-align=\"left\"><\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">F\u2013Cl<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">255<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013C<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">415<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">C\u2013Cl<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">330<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">F\u2013Br<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">235<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013N<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">390.<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">C\u2013Br<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">275<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013Si<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">230.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013O<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">464<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">C\u2013I<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">240<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013P<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013F<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">569<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013N<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">160<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013S<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">225<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013Si<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">395<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N=N<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">418<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013Cl<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">359<\/td>\n<\/tr>\n<tr style=\"height: 18px\" valign=\"top\">\n<td style=\"height: 18px;width: 135.4px\" data-align=\"left\">H\u2013P<\/td>\n<td style=\"height: 18px;width: 71.3px\" data-align=\"left\">320.<\/td>\n<td style=\"height: 18px;width: 36.25px\" data-align=\"left\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-1494\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5a.png\" alt=\"\" width=\"36\" height=\"16\" srcset=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5a.png 107w, https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5a-65x29.png 65w\" sizes=\"auto, (max-width: 36px) 100vw, 36px\" \/><\/td>\n<td style=\"height: 18px;width: 71.3px\" data-align=\"left\">946<\/td>\n<td style=\"height: 18px;width: 37.3667px\" data-align=\"left\">Si\u2013Br<\/td>\n<td style=\"height: 18px;width: 70.45px\" data-align=\"left\">290.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013S<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">340.<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013O<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">200<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Si\u2013I<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013Cl<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">432<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013F<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">270<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013P<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013Br<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">370.<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013P<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">210<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013S<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">230.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">H\u2013I<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">295<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013Cl<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">200<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013Cl<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">330.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013C<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">345<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">N\u2013Br<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">245<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013Br<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">270.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C=C<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">611<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013O<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">140<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">P\u2013I<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\n<\/tr>\n<tr style=\"height: 19px\" valign=\"top\">\n<td style=\"height: 19px;width: 135.4px\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-1495\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5b.png\" alt=\"\" width=\"40\" height=\"17\" srcset=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5b.png 102w, https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5b-65x27.png 65w\" sizes=\"auto, (max-width: 40px) 100vw, 40px\" \/><\/td>\n<td style=\"height: 19px;width: 71.3px\" data-align=\"left\">837<\/td>\n<td style=\"height: 19px;width: 36.25px\" data-align=\"left\">O=O<\/td>\n<td style=\"height: 19px;width: 71.3px\" data-align=\"left\">498<\/td>\n<td style=\"height: 19px;width: 37.3667px\" data-align=\"left\">S\u2013S<\/td>\n<td style=\"height: 19px;width: 70.45px\" data-align=\"left\">215<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013N<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">290.<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013F<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">160<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">S\u2013Cl<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">250.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C=N<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">615<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013Si<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">370<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">S\u2013Br<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">215<\/td>\n<\/tr>\n<tr style=\"height: 31px\" valign=\"top\">\n<td style=\"height: 31px;width: 135.4px\" data-align=\"left\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-1497\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5d.png\" alt=\"\" width=\"44\" height=\"16\" srcset=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5d.png 105w, https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5d-65x24.png 65w\" sizes=\"auto, (max-width: 44px) 100vw, 44px\" \/><\/td>\n<td style=\"height: 31px;width: 71.3px\" data-align=\"left\">891<\/td>\n<td style=\"height: 31px;width: 36.25px\" data-align=\"left\">O\u2013P<\/td>\n<td style=\"height: 31px;width: 71.3px\" data-align=\"left\">350<\/td>\n<td style=\"height: 31px;width: 37.3667px\" data-align=\"left\">Cl\u2013Cl<\/td>\n<td style=\"height: 31px;width: 70.45px\" data-align=\"left\">243<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013O<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">350.<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013Cl<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">205<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Cl\u2013Br<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">220.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C=O<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">741<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">O\u2013I<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">200<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Cl\u2013I<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">210.<\/td>\n<\/tr>\n<tr style=\"height: 31px\" valign=\"top\">\n<td style=\"height: 31px;width: 135.4px\" data-align=\"left\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-1496\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png\" alt=\"\" width=\"34\" height=\"13\" srcset=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png 104w, https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c-65x25.png 65w\" sizes=\"auto, (max-width: 34px) 100vw, 34px\" \/><\/td>\n<td style=\"height: 31px;width: 71.3px\" data-align=\"left\">1080.<\/td>\n<td style=\"height: 31px;width: 36.25px\" data-align=\"left\">F\u2013F<\/td>\n<td style=\"height: 31px;width: 71.3px\" data-align=\"left\">160<\/td>\n<td style=\"height: 31px;width: 37.3667px\" data-align=\"left\">Br\u2013Br<\/td>\n<td style=\"height: 31px;width: 70.45px\" data-align=\"left\">190.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013F<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">439<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">F\u2013Si<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">540<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">Br\u2013I<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">180.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013Si<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">360.<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">F\u2013P<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">489<\/td>\n<td style=\"height: 15px;width: 37.3667px\" data-align=\"left\">I\u2013I<\/td>\n<td style=\"height: 15px;width: 70.45px\" data-align=\"left\">150.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 135.4px\" data-align=\"left\">C\u2013P<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">265<\/td>\n<td style=\"height: 15px;width: 36.25px\" data-align=\"left\">F\u2013S<\/td>\n<td style=\"height: 15px;width: 71.3px\" data-align=\"left\">285<\/td>\n<td style=\"height: 15px;width: 37.3667px\"><\/td>\n<td style=\"height: 15px;width: 70.45px\"><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<table id=\"fs-idm44464336\" class=\"top-titled\" style=\"height: 165px\" summary=\"This table has three columns and ten rows. The first row is a header row that labels the columns: \u201cBond,\u201d \u201cBond Length in angstroms,\u201d and, \u201cBond Energy in k J \/ mol.\u201d Under the column \u201cBond\u201d are the values: C bonds to C with a single bond; C bonds to C with a double bond; C bonds to C with a triple bond; C bonds to N with a single bond; C bonds to N with a double bond; C bonds to N with a triple bond; C bonds to O with a single bond; C bonds to O with a double bond; and C bonds to O with a triple bond. Under the column \u201cBond Length in angstroms\u201d are the values: 1.54; 1.34; 1.20; 1.43; 1.38; 1.16; 1.43; 1.23; and 1.13. Under the column \u201cBond Energy in k J \/ mol\u201d are the values: 345; 611; 837; 290; 615; 891; 350; 741; and 1080.\">\n<thead>\n<tr style=\"height: 15px\">\n<th style=\"height: 15px;width: 475.35px\" colspan=\"3\" data-align=\"center\">Average Bond Lengths and Bond Energies for Some Common Bonds<\/th>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<th style=\"height: 15px;width: 188.233px\" data-align=\"left\">Bond<\/th>\n<th style=\"height: 15px;width: 109.317px\" data-align=\"left\">Bond Length (\u00c5)<\/th>\n<th style=\"height: 15px;width: 150.2px\" data-align=\"left\">Bond Energy (kJ\/mol)<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C\u2013C<\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.54<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">345<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C=C<\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.34<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">611<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-1495\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5b.png\" alt=\"\" width=\"43\" height=\"18\" srcset=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5b.png 102w, https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5b-65x27.png 65w\" sizes=\"auto, (max-width: 43px) 100vw, 43px\" \/><\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.20<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">837<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C\u2013N<\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.43<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">290.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C=N<\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.38<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">615<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-1497\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5d.png\" alt=\"\" width=\"44\" height=\"16\" srcset=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5d.png 105w, https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5d-65x24.png 65w\" sizes=\"auto, (max-width: 44px) 100vw, 44px\" \/><\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.16<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">891<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C\u2013O<\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.43<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">350.<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\">C=O<\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.23<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">741<\/td>\n<\/tr>\n<tr style=\"height: 15px\" valign=\"top\">\n<td style=\"height: 15px;width: 189.067px\" data-align=\"left\"><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-1496\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png\" alt=\"\" width=\"42\" height=\"16\" srcset=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png 104w, https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c-65x25.png 65w\" sizes=\"auto, (max-width: 42px) 100vw, 42px\" \/><\/td>\n<td style=\"height: 15px;width: 110.15px\" data-align=\"left\">1.13<\/td>\n<td style=\"height: 15px;width: 150.2px\" data-align=\"left\">1080.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<p id=\"fs-idp29959728\">We can use bond energies to calculate approximate enthalpy changes for reactions where enthalpies of formation are not available. Calculations of this type will also tell us whether a reaction is exothermic or endothermic. An exothermic reaction (\u0394<em data-effect=\"italics\">H<\/em> negative, heat produced) results when the bonds in the products are stronger than the bonds in the reactants. An endothermic reaction (\u0394<em data-effect=\"italics\">H<\/em> positive, heat absorbed) results when the bonds in the products are weaker than those in the reactants.<\/p>\n<p id=\"fs-idp234028832\">The enthalpy change, \u0394<em data-effect=\"italics\">H<\/em>, for a chemical reaction is approximately equal to the sum of the energy required to break all bonds in the reactants (energy \u201cin\u201d, positive sign) plus the energy released when all bonds are formed in the products (energy \u201cout,\u201d negative sign). This can be expressed mathematically in the following way:<\/p>\n<div id=\"fs-idm40243760\" style=\"text-align: center\" data-type=\"equation\">\u0394<em>H<\/em> = \u03a3D<sub>bonds broken<\/sub> &#8211; \u03a3D<sub>bonds formed<\/sub><\/div>\n<p id=\"fs-idp118053616\">In this expression, the symbol \u01a9 means \u201cthe sum of\u201d and D represents the bond energy in kilojoules per mole, which is always a positive number. The bond energy is obtained from a table (like <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>) and will depend on whether the particular bond is a single, double, or triple bond. Thus, in calculating enthalpies in this manner, it is important that we consider the bonding in all reactants and products. Because D values are typically averages for one type of bond in many different molecules, this calculation provides a rough estimate, not an exact value, for the enthalpy of reaction.<\/p>\n<p id=\"fs-idp17129504\">Consider the following reaction:<\/p>\n<div id=\"fs-idp51383312\" style=\"text-align: center\" data-type=\"equation\">H<sub>2<\/sub>(<em>g<\/em>) + Cl<sub>2<\/sub>(<em>g<\/em>) \u27f6 2HCl(<em>g<\/em>)<\/div>\n<p id=\"fs-idp77710320\">or<\/p>\n<div id=\"fs-idp56299152\" style=\"text-align: center\" data-type=\"equation\">H-H(<em>g<\/em>) + Cl-Cl(<em>g<\/em>) \u27f6 2H-Cl(<em>g<\/em>)<\/div>\n<p id=\"fs-idp201091440\">To form two moles of HCl, one mole of H\u2013H bonds and one mole of Cl\u2013Cl bonds must be broken. The energy required to break these bonds is the sum of the bond energy of the H\u2013H bond (436 kJ\/mol) and the Cl\u2013Cl bond (243 kJ\/mol). During the reaction, two moles of H\u2013Cl bonds are formed (bond energy = 432 kJ\/mol), releasing 2 \u00d7 432 kJ; or 864 kJ. Because the bonds in the products are stronger than those in the reactants, the reaction releases more energy than it consumes:<\/p>\n<div id=\"fs-idp14860448\" data-type=\"equation\">\u0394<em>H<\/em> = \u03a3D<sub>bonds broken<\/sub> &#8211; \u03a3D<sub>bonds formed<\/sub><\/div>\n<div data-type=\"equation\">\u0394<em>H<\/em> = (D<sub>H-H<\/sub> + D<sub>Cl-Cl)<\/sub> &#8211; 2D<sub>H-Cl<\/sub><\/div>\n<div data-type=\"equation\">\u00a0\u00a0\u00a0\u00a0\u00a0 = (436 kJ + 243 kJ) &#8211; 2(432 kJ) = -185 kJ<\/div>\n<div data-type=\"equation\"><\/div>\n<p id=\"fs-idp29639024\">This excess energy is released as heat, so the reaction is exothermic. <a href=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/back-matter\/standard-thermodynamic-properties-for-selected-substances\/\">Appendix G<\/a> gives a value for the standard molar enthalpy of formation of HCl(g), \u0394<em>H<\/em><sub>f<\/sub>\u00b0, of \u201392.307 kJ\/mol. Twice that value is \u2013184.6 kJ, which agrees well with the answer obtained earlier for the formation of two moles of HCl.<\/p>\n<div id=\"fs-idm33428592\" class=\"textbox textbox--examples\" data-type=\"example\">\n<p id=\"fs-idp71590928\"><strong>Using Bond Energies to Calculate Approximate Enthalpy Changes:<\/strong><\/p>\n<p>Methanol, CH<sub>3<\/sub>OH, may be an excellent alternative fuel. The high-temperature reaction of steam and carbon produces a mixture of the gases carbon monoxide, CO, and hydrogen, H<sub>2<\/sub>, from which methanol can be produced. Using the bond energies in <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>, calculate the approximate enthalpy change, \u0394<em data-effect=\"italics\">H<\/em>, for the reaction here:<\/p>\n<div id=\"fs-idp93467344\" style=\"text-align: center\" data-type=\"equation\">CO(<em>g<\/em>) + 2H<sub>2<\/sub>(<em>g<\/em>) \u27f6CH<sub>3<\/sub>OH(<em>g<\/em>)<\/div>\n<div data-type=\"equation\"><\/div>\n<p id=\"fs-idp41458304\"><strong>Solution:<\/strong><\/p>\n<p>First, we need to write the Lewis structures of the reactants and the products:<\/p>\n<p><span id=\"fs-idp525280\" class=\"scaled-down\" data-type=\"media\" data-alt=\"A set of Lewis diagrams show a chemical reaction. The first structure shows a carbon atom with a lone pair of electrons triple bonded to an oxygen with a lone pair of electrons. To the right of this structure is a plus sign, then the number 2 followed by a hydrogen atom single bonded to a hydrogen atom. To the right of this structure is a right-facing arrow followed by a hydrogen atom single bonded to a carbon atom that is single bonded to two hydrogen atoms and an oxygen atom with two lone pairs of electrons. The oxygen atom is also single bonded to a hydrogen atom.\"><img decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/CNX_Chem_07_05_CH3OHLew_img-1.jpg\" alt=\"A set of Lewis diagrams show a chemical reaction. The first structure shows a carbon atom with a lone pair of electrons triple bonded to an oxygen with a lone pair of electrons. To the right of this structure is a plus sign, then the number 2 followed by a hydrogen atom single bonded to a hydrogen atom. To the right of this structure is a right-facing arrow followed by a hydrogen atom single bonded to a carbon atom that is single bonded to two hydrogen atoms and an oxygen atom with two lone pairs of electrons. The oxygen atom is also single bonded to a hydrogen atom.\" data-media-type=\"image\/jpeg\" \/><\/span><\/p>\n<p id=\"fs-idp202067232\">From this, we see that \u0394<em data-effect=\"italics\">H<\/em> for this reaction involves the energy required to break a C\u2013O triple bond and two H\u2013H single bonds, as well as the energy produced by the formation of three C\u2013H single bonds, a C\u2013O single bond, and an O\u2013H single bond. We can express this as follows:<\/p>\n<div id=\"fs-idp76020096\" data-type=\"equation\">\n<div id=\"fs-idp14860448\" data-type=\"equation\">\u0394<em>H<\/em> = \u03a3D<sub>bonds broken<\/sub> &#8211; \u03a3D<sub>bonds formed<\/sub><\/div>\n<div data-type=\"equation\">\u0394<em>H<\/em> = (D<sub><img loading=\"lazy\" decoding=\"async\" class=\"alignnone wp-image-1496\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png\" alt=\"\" width=\"23\" height=\"9\" srcset=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c.png 104w, https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/7.5c-65x25.png 65w\" sizes=\"auto, (max-width: 23px) 100vw, 23px\" \/><\/sub> + 2D<sub>H-H)<\/sub> &#8211; (3D<sub>C-H<\/sub> + D<sub>C-O<\/sub> + D<sub>O-H<\/sub>)<\/div>\n<div data-type=\"equation\"><\/div>\n<div data-type=\"equation\"><\/div>\n<\/div>\n<p id=\"fs-idp53734416\">Using the bond energy values in <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>, we obtain:<\/p>\n<div id=\"fs-idm103164880\" data-type=\"equation\">\n<div data-type=\"equation\">\u0394<em>H<\/em> = [1080. kJ + 2(436 kJ)] &#8211; [3(415 kJ) + 350. kJ + 464 kJ] = -107 kJ<\/div>\n<div data-type=\"equation\"><\/div>\n<div data-type=\"equation\"><\/div>\n<\/div>\n<p id=\"fs-idp126256512\">We can compare this value to the value calculated based on \u0394<em>H<\/em><sub>f<\/sub>\u00b0 data from Appendix G:<\/p>\n<div id=\"fs-idp158903840\" data-type=\"equation\">\u0394<em>H<\/em> = \u0394<em>H<\/em><sub>f<\/sub>\u00b0(CH<sub>3<\/sub>OH(<em>g<\/em>)) &#8211; [\u0394<em>H<\/em><sub>f<\/sub>\u00b0(CO(<em>g<\/em>)) + 2\u0394<em>H<\/em><sub>f<\/sub>\u00b0(H<sub>2<\/sub>(<em>g<\/em>))]<\/div>\n<div data-type=\"equation\">\u00a0\u00a0\u00a0\u00a0 = -201.0 kJ &#8211; [-110.52 kJ + 2(0)] = -90.5 kJ<\/div>\n<div data-type=\"equation\"><\/div>\n<div data-type=\"equation\"><\/div>\n<p id=\"fs-idp56832128\">Note that there is a fairly significant gap between the values calculated using the two different methods. This occurs because D values are the <em data-effect=\"italics\">average<\/em> of different bond strengths; therefore, they often give only rough agreement with other data.<\/p>\n<p id=\"fs-idp36454672\"><strong>Check Your Learning:<\/strong><\/p>\n<p>Ethyl alcohol, CH<sub>3<\/sub>CH<sub>2<\/sub>OH, was one of the first organic chemicals deliberately synthesized by humans. It has many uses in industry, and it is the alcohol contained in alcoholic beverages. It can be obtained by the fermentation of sugar or synthesized by the hydration of ethylene in the following reaction:<\/p>\n<p><span id=\"fs-idp144672\" class=\"scaled-down\" data-type=\"media\" data-alt=\"A set of Lewis structures show a chemical reaction. The first structure shows two carbon atoms that are double bonded together and are each single bonded to two hydrogen atoms. This structure is followed by a plus sign, then an oxygen atom with two lone pairs of electrons single bonded to two hydrogen atoms. A right-facing arrow leads to a carbon atom single bonded to three hydrogen atoms and a second carbon atom. The second carbon atom is single bonded to two hydrogen atoms and an oxygen atom with two lone pairs of electrons. The oxygen atom is single bonded to a hydrogen atom as well.\"><img decoding=\"async\" src=\"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-content\/uploads\/sites\/1463\/2021\/07\/CNX_Chem_07_05_Ethanol_img-1.jpg\" alt=\"A set of Lewis structures show a chemical reaction. The first structure shows two carbon atoms that are double bonded together and are each single bonded to two hydrogen atoms. This structure is followed by a plus sign, then an oxygen atom with two lone pairs of electrons single bonded to two hydrogen atoms. A right-facing arrow leads to a carbon atom single bonded to three hydrogen atoms and a second carbon atom. The second carbon atom is single bonded to two hydrogen atoms and an oxygen atom with two lone pairs of electrons. The oxygen atom is single bonded to a hydrogen atom as well.\" data-media-type=\"image\/jpeg\" \/><\/span><\/p>\n<p id=\"fs-idp27645248\">Using the bond energies in <a class=\"autogenerated-content\" href=\"#fs-idm44464336\">(Figure)<\/a>, calculate an approximate enthalpy change, \u0394<em data-effect=\"italics\">H<\/em>, for this reaction.<\/p>\n<p>&nbsp;<\/p>\n<div id=\"fs-idp88406224\" data-type=\"note\">\n<div data-type=\"title\"><strong>Answer:<\/strong><\/div>\n<p id=\"fs-idp24314032\">\u201335 kJ<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"fs-idm34679504\" class=\"bc-section section\" data-depth=\"1\"><\/div>\n<div id=\"fs-idp234492208\" class=\"summary\" data-depth=\"1\"><\/div>\n<div class=\"textbox shaded\" data-type=\"glossary\">\n<h3 data-type=\"glossary-title\"><strong>Glossary<\/strong><\/h3>\n<dl id=\"fs-idp10319376\">\n<dt>bond energy<\/dt>\n<dd id=\"fs-idm69489984\">(also, bond dissociation energy) energy required to break a covalent bond in a gaseous substance<\/dd>\n<\/dl>\n<dl id=\"fs-idp39711472\"><\/dl>\n<\/div>\n","protected":false},"author":1392,"menu_order":6,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[48],"contributor":[],"license":[],"class_list":["post-434","chapter","type-chapter","status-publish","hentry","chapter-type-numberless"],"part":301,"_links":{"self":[{"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/pressbooks\/v2\/chapters\/434","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/wp\/v2\/users\/1392"}],"version-history":[{"count":9,"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/pressbooks\/v2\/chapters\/434\/revisions"}],"predecessor-version":[{"id":2134,"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/pressbooks\/v2\/chapters\/434\/revisions\/2134"}],"part":[{"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/pressbooks\/v2\/parts\/301"}],"metadata":[{"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/pressbooks\/v2\/chapters\/434\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/wp\/v2\/media?parent=434"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/pressbooks\/v2\/chapter-type?post=434"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/wp\/v2\/contributor?post=434"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/pressbooks.bccampus.ca\/aperrott\/wp-json\/wp\/v2\/license?post=434"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}