Kinetics Solutions: #1

1.* (1997 F 11I) The elementary steps of a reaction (with their rate constants) are as follows:

A + B --k₁--> C
C --k₂--> D
D --k₃-->B + E

What is the reaction? What is the rate of appearance of product E?

The overall reaction is A --> E. Remember that we can't write rate laws in terms of the ephemeral concentrations of reaction intermediates. Instead, we use the steady-state approximation. The rate of disappearance of D (k₃[D]) equals the rate of appearance of D (k₂[C]). However, C is an intermediate too. The rate of appearance of C (k₁[A][B]) equals its rate of disappearance (k₂[C]). Therefore we can write:

k₃[D] = k₂[C] = k₁[A][B] = rate of appearance of E

Note that the steady-state approximation was made considerably easier because there were assumed to be no back reactions.