Question

Chemical species A, B, C, D and E are involved in a reaction. They may represent reactants, products, intermediates, catalysts or inhibitors. The rate of change in the concentration for each of them is given below:

d[A]/dt = -k1[A][B] + k-1[C][B] + k2[C][D] - k-2[A][E]

d[B]/dt = 0

d[C]/dt = k1[A][B] - k-1[C][B] - k2[C][D] + k-2[A][E]

d[D]/dt = -k2[C][D] + k-2[A][E]

d[E]/dt = k2[C][D] - k-2[A][E]

a) Write down the elementary steps in the proposed reaction mechanism that are consistent with the above equations. Clearly indicate which of the rate constants is associated with each step. State the molecularity of each step in the proposed mechanism.

b) Write down the overall net reaction that takes place (i.e. without intermediates present).

c) Identify each of A, B, C, D and E as a reactant, product, intermediate, catalyst or inhibitor.

Answer 1

a) rate law : Rate=K *product of the concentration of reactants raised to the power of their coefficients

example: nA+mB --->pC+qD k=rate constant

rate=-1/n d[A]/dt=-1/m d[B]/dt=1/p d[C]/dt=1/qd[C]/dt=k*[A]^n [B]^m

A+B <--->C+B with rate constant of forward reaction ,k1 and rate constant of reverse reaction K-1

molecularity is the number of molecules in the elementary reaction

moleculrity=2

C+D<--->A+E with rate constant of forward reaction ,k2 and rate constant of reverse reaction K-2

moleculrity=2

net rxn is obtained by adding the above written equation :and cancelling the common terms.

A+B+C+D <--->C+B+A+E

D <--->E (net equation)

But, A is a reactant and a product so net equation

A+D<--->E+A

c) reactant : A,D

product:A, E

intermediates:C (as it is transiently formed and used up in the reaction,Its rate of formation=rate of consumption(steady state approximation)

Catalyst :B (remains unchanged and is retained after the reaction goes to completion)