Cliffs AP Chemistry, 3rd Edition

9. Given the following reaction mechanism, express the rate of the overall reaction.

2 A + 3 C →4 D + 5 E slow

A + 2 D ?2 A + 2 E fast

2 E + C →3 D + 2 C fast

A k⋅2[A]^2 3[C]^3

B k⋅[D]^4 [E]^5 / [A]^2 [C]^3

C k⋅[D]^4 [E]^5

D k⋅[A]^3 [C]^2

E k⋅[A]^2 [C]^3

Answer: E

The rate is determined by the slowest step. Only when given the complete mechanisms in steps
can you identify the slowest step and use the coefficients of the reactants as the orders in the
rate equation.

10. Referring to example 9, express the overall reaction.

A. 3 C + 2 D →2 A + 4 E

B. 3 A + 3 C →3 D + 4 E

C. 2 A + 3 C →A + 4 E

D. A + 2 C →5 D + 5 E

E. 2 A + C →D + 2 E

Answer: D

To find the overall equation when given the reaction mechanism, simply add the reactions.

2 A + 3 C →4 D + 5 E slow

A + 2 D ?2 A + 2 E fast

2 E + C →3 D + 2 C fast

There is a total of 3 A’s on the left and 2 A’s on the right: balance of 1 A on the left. There is a
total of 4 C’s on the left and 2 C’s on the right: balance of 2 C’s on the left. There is a total of
2 D’s on the left and 7 D’s on the right: balance of 5 D’s on the right. There is a total of 2 E’s
on the left and 7 E’s on the right: balance of 5 E’s on the right.

A + 2 C →5 D + 5 E

Part II: Specific Topics