PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 263

Therate of reaction(9.1) is

r=−

1

a

dcA

dτ

=−

1

b

dcB

dτ

=···

1

s

dcS

dτ

=

1

t

dcT

dτ

=···, [V, T], (9.4)

wherecistands for the reactants concentrations and τ for time. If we re-write the general
chemical reaction (9.1) to the form

0 =

∑n

i=1

νiRi,

we may write equations (9.4) in the form:

r=

1

νi

dcRi

dτ

, [V, T]. (9.5)

The rate of reaction does not depend on the choice of substance, but it does depend on the
way in which the chemical reaction (9.1) is written. The rates of consumption and formation
are different for individual substances; they do not, however, depend on the form of reaction
record. Since the rates are easy to convert to one another, it is not so important which one we
use.
The following relations apply between the rate of reaction and the rates of consumption and
formation of the components:

rA=a r , rB=b r , rS=s r , rT=t r. (9.6)

The rate of consumption of substance A is determined experimentally by approximating the
derivative in relation (9.2) by the ratio of (sufficiently small) differences

dcA

dτ

≈

∆cA

∆τ

. (9.7)

The rates of formation of products are determined in a similar way.