Question
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 I O3 + Cl → O2 + ClO
Step II ClO + O → Cl + O2
(a) Write a balanced equation for the overall reaction represented by Step I and Step II above.
(b) Clearly identify the catalyst in the mechanism above. Justify your answer.
(c) Clearly identify the intermediate in the mechanism above. Justify your answer.
(d) If the rate law for the overall reaction is found to be rate = k[O3][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
▶️Answer/Explanation
Ans:
(a)O3 + O → 2 O2
(b)Cl is the catalyst in the reaction. It is a reactant in Step I and reappears as a product in Step II.
(c) ClO is the intermediate in the reaction. It is a product in Step I and reappears as a reactant in Step II.
(d)(i) overall order is 1 + 1 = 2
(d)(ii)\(k = \frac{rate}{[O_{3}][Cl]}=\frac{M time^{-1}}{M^{2}}=M^{-1}time^{-1}\)
(d)(iii)Step I is the rate-determining step in the mechanism. The coefficients of the reactants in Step I correspond to the exponents of the species concentrations in the rate law equation.
OR
The reaction rate is affected by the concentrations of [O3] and [Cl], both appearing only in Step I.