12 Reaction Rate Law
Learning Objectives
By the end of this section, you should be able to:
Define reaction rate law and reaction rate constant (k)
Reaction rate law Definition: The relationship between the rate of reaction and the concentration of reactants.
The rate law is usually proportional to the concentrations of reactants raised to a certain power:
Take the reaction we used as an example before:
The general form for reaction rate law is
For gas cases, we can use partial pressure
The rate constant
For example, let’s look at the rate of the gas-phase decomposition of dinitrogen pentoxide,
Say the rate law is found to be directly proportional to the concentration of
Reaction rate laws can be complicated and may tell us about the mechanism of the reactions. For example, consider the reaction between hydrogen and bromine:
Simple stoichiometry:
Complicated rate law:
Rate Law vs. Equilibrium Constant
Be careful not to confuse equilibrium constant expressions with rate law expressions. The expression for
In contrast, the expression for the rate law generally bears no relation to the reaction equation and must be determined experimentally.
Reaction Rate Law Units
Reaction rate (r) is generally expressed in units of concentration over time (e.g.
This means the rate constant
Exercise: Rate Constant Units
For the following example, what are the units for the reaction rate constant (
with p in Pa and time in seconds
Solution
Since r is expressed in concentration over time, the units of r are
References
[1] Chemistry LibreTexts. 2020. The Rate Law. [online] Available at: <https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Kinetics/Rate_Laws/The_Rate_Law> [Accessed 23 April 2020].
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