PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 266

The kinetic equation in the relation (9.9) is written for the rate of consumption of substance
A. Similarly we may write kinetic equations for the rate of consumption of substance B, the
rate of formation of substance S, or for the rate of reaction, e.g.

rB=−

dcB

dτ

=kBcαAcβB....

The partial orders of reaction and the overall order of reaction remain the same, but the
numerical values of the rate constantskA, kBare different and depend on the way in which the
reaction is recorded. From relation (9.5) it follows that

kB=

b

a

kA.

Example

The simple reaction

A + 2B→C

is of the half-order with respect to substanceAand of the second-order with respect to substance

B. Determine the overall order of the reaction, and write the kinetic equation for the rate of

consumption of substanceA, for the rate of formation of substanceC, and the relation between

the respective rate constants.

Solution

The overall order of the reaction isn =^12 + 2 =^52. The kinetic equation for the rate of

consumption of substanceAis

−

dcA

dτ

=kAc^1 A/^2 c^2 B.

The kinetic equation for the rate of formation of substanceCis

dcC

dτ

=kCc^1 A/^2 c^2 B.

The rate constants are identical in this case,kC=kA.