PHYSICAL CHEMISTRY IN BRIEF

CHAP. 8: CHEMICAL EQUILIBRIUM [CONTENTS] 229

Solution

Subtracting the left side yields

0 =SO 3 (g)−SO 2 (g)−

1

2

O 2 (g).

If we denote sulphur trioxide with the subscript 1 , sulphur dioxide with the subscript 2 and oxygen

with the subscript 3 , then

R 1 =SO 3 , ν 1 = +1,

R 2 =SO 2 , ν 2 =− 1 ,

R 3 =O 2 , ν 3 =−^12.

8.2.2 Material balance

The record of a chemical reaction reflects the mass conservation law in a closed system. Ma-

terial balance allows for the changes in the amount of substance of individual components

participating in a given reaction to be expressed using a single variableξwhich is called the

extent of reaction

ξ=

ni−ni, 0

νi

, (8.3)

whereni, 0 denotes the initial amount of thei-th substance andξis the extent of reaction.

Note:The extent of reaction is an extensive quantity.

U Main unit:mol.
For the amount of thei-th component in any stage of reaction it holds that

ni=ni, 0 +νiξ. (8.4)

Sometimes also thedegree of conversionα

α=

ni, 0 −ni

ni, 0

=−

ξνi

ni, 0

(8.5)

is used.