KineticsKinetics

Jack WangJack Wang

Chemistry APChemistry AP

AP Chemistry 2002 FRQAP Chemistry 2002 FRQ

An environmental concern is the depletion An environmental concern is the depletion of Oof O33 in Earth’s upper atmosphere, where in Earth’s upper atmosphere, where OO33 is normally in equilibrium with O is normally in equilibrium with O22 and and O. A proposed mechanism for the O. A proposed mechanism for the depletion of Odepletion of O33 in the upper atmosphere is in the upper atmosphere is shown below.shown below.

Step IStep I OO33 + Cl + Cl O O22 + ClO + ClO Step IIStep II ClO + O ClO + O Cl + O Cl + O22

A) Write a balanced equation for the A) Write a balanced equation for the overall reaction represented by Step overall reaction represented by Step I and Step II above.I and Step II above.

Step IStep I OO33 + Cl + Cl O O22 + ClO + ClO

Step IIStep II ClO + O ClO + O Cl + O Cl + O22

If we simply add these two reactions If we simply add these two reactions together and cancel out equal together and cancel out equal occurrences on the products and occurrences on the products and reactants sides, we get…reactants sides, we get…

OO33 + O + O 2O 2O22

B) Clearly identify the catalyst in the B) Clearly identify the catalyst in the mechanism above. Justify your mechanism above. Justify your answer.answer.

Step IStep I OO33 + Cl + Cl O O22 + ClO + ClO

Step IIStep II ClO + O ClO + O Cl + O Cl + O22

A catalyst by definition is a A catalyst by definition is a substance that speeds up a chemical substance that speeds up a chemical process without actually changing process without actually changing the products of reaction. the products of reaction.

Overall Reaction: O3 + O 2O2

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 C) Clearly identify the intermediate in the mechanism above. Justify your the mechanism above. Justify your answer.answer.

Step IStep I OO33 + Cl + Cl O O22 + ClO + ClO

Step IIStep II ClO + O ClO + O Cl + O Cl + O22

An intermediate is a substance that An intermediate is a substance that is is produced by one reactionproduced by one reaction and and thenthen used as a reactant in used as a reactant in anotheranother. The intermediate is not . The intermediate is not present in the overall reaction.present in the overall reaction.

Overall Reaction:Overall Reaction: OO33 + O + O 2O2O22

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 D) If the rate law for the overall reaction is found to be reaction is found to be rate rate = = kk[O[O33]][Cl], determine the following.[Cl], determine the following.

i.i. The overall order of the reaction.The overall order of the reaction.

ii.ii. Appropriate units for the rate Appropriate units for the rate constant, constant, k.k.

iii.iii. The rate-determining step of the The rate-determining step of the reaction, along with justification for reaction, along with justification for your answer.your answer.

D) If the rate law for the overall D) If the rate law for the overall reaction is found to be reaction is found to be rate rate = = kk[O[O33]][Cl], determine the following.[Cl], determine the following.

i.i. The overall order of the reaction.The overall order of the reaction. The overall order of a reaction is The overall order of a reaction is

found by adding the individual found by adding the individual orders. Since [Oorders. Since [O33] and [Cl] are both ] and [Cl] are both first order reactions according to first order reactions according to the rate law… 1+1 = 2 the rate law… 1+1 = 2

The rate of this reaction has an The rate of this reaction has an overall order of 2.overall order of 2.

D) If the rate law for the overall reaction D) If the rate law for the overall reaction is found to be is found to be rate rate = = kk[O[O33][Cl], ][Cl], determine the following.determine the following.

ii.ii. Appropriate units for the rate Appropriate units for the rate constant, constant, k.k.

Rate= kRate= k[O[O33][Cl] ][Cl] K=Rate/K=Rate/[O[O33][Cl]][Cl] Since [X] is the concentration, it is Since [X] is the concentration, it is

measured in M; Rate is measured in measured in M; Rate is measured in M/sM/s

K = (M/s)/(M*M)K = (M/s)/(M*M) K= MK= M-1-1ss-1-1

D) If the rate law for the overall reaction is D) If the rate law for the overall reaction is found to be found to be rate rate = = kk[O[O33][Cl], determine ][Cl], determine the following.the following.

iii.iii. The rate-determining step of the The rate-determining step of the reaction, along with justification for reaction, along with justification for your answer.your answer.

The rate law of the reaction is The rate law of the reaction is

rate rate = = kk[O[O33][Cl]][Cl]

and the rate law applies to the and the rate law applies to the concentrations of the materials in the concentrations of the materials in the slowest step, the rate-determining slowest step, the rate-determining step.step.

The concentrations of O3 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 IStep I OO33 + Cl + Cl O O22 + ClO + ClOStep IIStep II ClO + O ClO + O Cl + O Cl + O22

Rate determining step

NoteNote

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

The EndThe End