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Chapter 17 | Kinetics 927
This equation can be rearranged to give a one-step calculation to obtain an estimate for the activation
energy:
Using the experimental data presented here, we can simply select two data entries. For this example, we
select the first entry and the last entry:
After calculating and ln k, we can substitute into the equation:
and the result is Ea = 185,900 J/mol.
T (K)
k (L/mol/s)
ln k
555
3.52 10−7
1.80 10−3
−14.860
781
3.95 10−2
1.28 10−3
−3.231
This method is very effective, especially when a limited number of temperature-dependent rate constants are available for the reaction of interest.
Check Your Learning
The rate constant for the rate of decomposition of N2O5 to NO and O2 in the gas phase is 1.66 L/mol/s at 650 K and 7.39 L/mol/s at 700 K:
Assuming the kinetics of this reaction are consistent with the Arrhenius equation, calculate the activation energy for this decomposition.
Answer: 113,000 J/mol
17.6 Reaction Mechanisms
By the end of this section, you will be able to:
• Distinguish net reactions from elementary reactions (steps)
• Identify the molecularity of elementary reactions
• Write a balanced chemical equation for a process given its reaction mechanism
• Derive the rate law consistent with a given reaction mechanism
A balanced equation for a chemical reaction indicates what is reacting and what is produced, but it reveals nothing about how the reaction actually takes place. The reaction mechanism (or reaction path) is the process, or pathway, by which a reaction occurs.
A chemical reaction usually occurs in steps, although it may not always be obvious to an observer. The decomposition of ozone, for example, appears to follow a mechanism with two steps:
