PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 309

From the form of the kinetic equation it is obvious that the reaction A→B is not a simple
reaction [see9.1.3]. At low concentrations of the substance A, whenk 2 cA k 3 , equation
(9.145) rearranges to a second-order equation

dcB

dτ

=

k 1 k 3

k 2

cA.

while at high concentrations or high values ofk 2 , whenk 2 cAk 3 , equation (9.145) rearranges
to a first-order equation
dcB
dτ

=

k 1 k 3

k 2

cA.

9.5.7 Pre-equilibrium principle

Let us consider the reaction
A + B→C

and its mechanism as a sequence of reactions

A + B

→k^1

←

k 2

X,

X

k 3

→ C,

where X is an unstable intermediate of the reaction. The rate of formation of substance C is
determined by the kinetic equation
dcC
dτ

=k 3 cX, (9.146)

wherecXis the concentration of the unstable intermediate, which mostly cannot be obtained
experimentally. If the rate of X decomposition into the product C is much lower than the rate
of formation of X and its decomposition into the reactants A and B, i.e. ifk 3 k 2 andk 3 k 1 ,
we may approximate the concentrationcXusing the relation

K=

cX

cAcB

, (9.147)