18.2. Rate Laws http://www.ck12.org
Lesson Summary
- The concentration dependence of a reaction rate is shown in an equation called a rate law. The rate of a
reaction is equal to a specific rate constant multiplied by the concentration of each reactant raised to some
power. - The specific rate constant is unique for every reaction and is dependent upon temperature. A large rate constant
indicates a relatively fast reaction, while a small rate constant indicates a relatively slow reaction. - Rate laws must be determined experimentally. The order of the reaction with respect to each reactant and the
value of the specific rate constant can be determined by a set of experiments in which the concentrations of
various reactants are systematically varied.
Lesson Review Questions
Reviewing Concepts
- What is the general relationship between the rate of a reaction and the concentrations of the reactants? Explain.
- How does the size of the specific rate constant (k) for a reaction relate to the general speed of the reaction?
- A certain reaction is found to depend only upon the concentration of reactant A. How will the rate of the
reaction be affected when the initial concentration of A is doubled, given each of the three possibilities for the
order of the reaction?
a. 1st order
b. 2nd order
c. zero order - What are the units of the specific rate constant when the reaction is zero order? 1st order? 2nd order? 3rd
order? - When performing a set of experiments to determine a rate law, why are initial rates of reaction measured and
compared? - For the reaction A + B→C, the reaction is first-order with respect to A and second-order overall. Write the
rate law for the reaction.
Problems
- For the decomposition reaction X→Y + Z, the reaction is first-order with respect to X, and the value of
the specific rate constant is 0.0296 s−^1. Calculate the initial reaction rate when starting with the following
concentrations of X.
(a) [X] = 0.410 M
(b) [X] = 0.0223 M - The reaction between the peroxydisulfate ion (S 2 O 82 −) and the iodide ion (I−) is: S 2 O^28 −(aq)+3I−(aq)→2SO^24 −(aq)+I− 3 (aq).
(a) From the following data (Table18.2), which were collected at a set temperature, determine the rate law
and calculate the specific rate constant.
TABLE18.2:Rate Law data
Experiment [S 2 O 82 −] (M) [I−] (M) Initial Rate (M/s)
1 0.080 0.034 2.2× 10 −^4