Kinetics Jack Wang Chemistry AP

AP Chemistry 2002 FRQ An environmental concern is the depletion of O3 in Earth’s upper atmosphere, where O3 is normally in equilibrium with O2 and O. A proposed mechanism for the depletion of O3 in the upper atmosphere is shown below. Step IO3 + Cl  O2 + ClO Step IIClO + O  Cl + O2

A) Write a balanced equation for the overall reaction represented by Step I and Step II above. Step IO3 + Cl  O2 + ClO Step IIClO + O  Cl + O2 If we simply add these two reactions together and cancel out equal occurrences on the products and reactants sides, we get… O3 + O  2O2

B) Clearly identify the catalyst in the mechanism above. Justify your answer. Step IO 3 + Cl  O 2 + ClO Step IIClO + O  Cl + O 2 A catalyst by definition is a substance that speeds up a chemical process without actually changing the products of reaction. Overall Reaction: O 3 + O  2O 2 The substance that acts as a catalyst in this equation is Cl. In Step I, 1 mol of Cl acts as the reactant that will serve as the catalyst. In Step II, 1 mol of Cl is the product. Therefore, in the overall reaction Cl is not present and follows the definition where the catalyst does not change the products of the reaction.

C) Clearly identify the intermediate in the mechanism above. Justify your answer. Step IO 3 + Cl  O 2 + ClO Step IIClO + O  Cl + O 2 An intermediate is a substance that is produced by one reaction and then used as a reactant in another. The intermediate is not present in the overall reaction. Overall Reaction: O 3 + O  2O 2 In this reaction, we can see that in step I, ClO is produced and in Step II, ClO is a reactant. If we look at the overall reaction, ClO is not present. ClO is the intermediate in this reaction.

D) If the rate law for the overall reaction is found to be rate = k[O 3 ][Cl], determine the following. i. The overall order of the reaction. ii. Appropriate units for the rate constant, k. iii. The rate-determining step of the reaction, along with justification for your answer.

D) If the rate law for the overall reaction is found to be rate = k[O 3 ][Cl], determine the following. i. The overall order of the reaction. The overall order of a reaction is found by adding the individual orders. Since [O 3 ] and [Cl] are both first order reactions according to the rate law… 1+1 = 2 The overall order of a reaction is found by adding the individual orders. Since [O 3 ] and [Cl] are both first order reactions according to the rate law… 1+1 = 2 The rate of this reaction has an overall order of 2. The rate of this reaction has an overall order of 2.

D) If the rate law for the overall reaction is found to be rate = k[O 3 ][Cl], determine the following. ii. Appropriate units for the rate constant, k. Rate= k[O 3 ][Cl] Rate= k[O 3 ][Cl] K=Rate/[O 3 ][Cl] K=Rate/[O 3 ][Cl] Since [X] is the concentration, it is measured in M; Rate is measured in M/s Since [X] is the concentration, it is measured in M; Rate is measured in M/s K = (M/s)/(M*M) K = (M/s)/(M*M) K= M -1 s -1 K= M -1 s -1

D) If the rate law for the overall reaction is found to be rate = k[O 3 ][Cl], determine the following. iii. The rate-determining step of the reaction, along with justification for your answer. The rate law of the reaction is The rate law of the reaction is rate = k[O 3 ][Cl] and the rate law applies to the concentrations of the materials in the slowest step, the rate-determining step. The concentrations of O 3 and Cl are present in the reactants of the reaction found in Step I, thus also following the rate-law. Therefore, the rate-determining step, the slow step, of the reaction is Step I. Step IO 3 + Cl  O 2 + ClO Step IIClO + O  Cl + O 2  Rate determining step

Note In the previous problem, the slow step is defined as the rate determining step because in a series of reactions, the rate of the slow step reaction determines the rate of change in concentrations of reactants and products, and significant affects the rate at which the overall reaction occurs. The fast step reactions are not considered because they do not significantly affect the rate at which the overall reaction occurs. In the previous problem, the slow step is defined as the rate determining step because in a series of reactions, the rate of the slow step reaction determines the rate of change in concentrations of reactants and products, and significant affects the rate at which the overall reaction occurs. The fast step reactions are not considered because they do not significantly affect the rate at which the overall reaction occurs.

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