Page 262 - Advanced Thermodynamics for Engineers, Second Edition

12.4 DISSOCIATION 251

Equation (12.17) shows the stoichiometric proportions of the reactants and products. It shows that
1 mol CO and 1/2 mol O 2 could combine to form 1 mol CO 2 . If the reaction proceeded to completion,
no CO or O 2 would be left at the final condition.
This is the stoichiometric equation of the reaction and the amounts of substance in the equation
give the stoichiometric coefficients.
The general equation for a chemical reaction is

y a A þ y b B 5 y c C þ y d D (12.18)
where y is a stoichiometric coefficient and A, B, C and D are the substances involved in the reaction.
1
Applying Eqn (12.18) to the CO þ O 2 reaction gives
2
y CO ¼ 1

¼ 1 2 (12.19)
y O 2
¼ 1:
y CO 2
It is conventional in chemistry to assign negative values to the stoichiometric coefficients on the
left-hand side of the equation (nominally, the reactants) and positive signs to those on the right-hand
side (nominally, the products).

12.3.1 MIXTURES
Mixtures are not necessarily stoichiometric and the following terms were introduced in Chapter 10 to
describe the proportions of a mixture.
1. If the reactants occur in proportion to the stoichiometric coefficients then the mixture is said to
be chemically correct or stoichiometric.
2. If the reactants have a greater proportion of fuel than the correct mixture then it is said to be
rich.
3. If the reactants have a lesser proportion of fuel than the correct mixture then it is said to be
weak.
1
For example, the chemically correct mixture is CO þ O 2 /CO 2
2
Rich mixture (excess of fuel)
1
ð1 þ nÞCO þ O 2 /CO 2 þ nCO (12.20)
2
Weak mixture (excess of oxidant)

1
þ n O 2 /CO 2 þ nO 2 : (12.21)
CO þ
2
N.B. Equations (12.20) and (12.21) have been written neglecting dissociation.

12.4 DISSOCIATION
The basis of dissociation is the atomic model that all mixtures of gases are in a state of dynamic
equilibrium. Molecules of the compounds are being created whilst existing ones are breaking down
into simpler compounds or elements (dissociating). In the equilibrium situation the rates of creation