PHYSICAL CHEMISTRY IN BRIEF

CHAP. 9: CHEMICAL KINETICS [CONTENTS] 265

Note: Note that the rates in equations (9.2) through (9.5) are defined at constant volume.

The concentration of componenti,ci=ni/V, can change in consequence of a change in

the amount of substance or in the volume. Here, however, we are interested in changes

concerning the amount of substance. We will assume again constant volume and point out

only those cases in which it changes. The rate of reaction also depends on temperature. If

not otherwise stated, we will assume that the reaction proceeds at constant temperature.

9.1.2 Kinetic equation

The kinetic equation is a differential equation representing the relation between the concentra-

tions of substances and time. For the reaction (9.1) it may be written in the form

rA=−

dcA

dτ

=f(cA, cB,... , cS, cT,.. .) (9.8)

or in similar forms, with the rate of formation of the product or the rate of reaction appearing

on the left side of the equation. By solving kinetic equations we obtain the time dependence of

the reactants concentrations. These solutions [see9.1.6] will be referred to as theintegrated

forms of the kinetic equation.

9.1.3 Simple reactions, order of reaction, rate constant

We will use the term “simple” for such a chemical reaction whose kinetic equation has the form

rA=−

dcA

dτ

=kAcαAcβB... , (9.9)

where A,B,... are thereactants. The exponentαis called theorder of reaction with respect
to componentA,βis the order of reaction with respect to B, etc. The sum of the exponents
n=α+β+... is called the(overall) order of reaction.
The constantkAin the kinetic equation (9.9) is called therate constant. It is the function
of temperature [see9.159]. The dimension of the rate constant depends on the reaction order.
U Main unit: s−^1 (mol m−^3 )^1 −n, wherenis the order of reaction.