Page 263 - Advanced Thermodynamics for Engineers, Second Edition

252 CHAPTER 12 CHEMICAL EQUILIBRIUM AND DISSOCIATION

and destruction of molecules of any compounds are equal. This means that macroscopic measuring
techniques do not sense the changes but give the impression that the system is in a state of ‘static’
equilibrium. The effect of this is that the reactions can no longer be said to be unidirectional but must
be shown as
1
CO þ O 2 5 CO 2 : (12.22)
2
On a molecular basis the above reaction can go either way. It is now necessary to consider a
technique which will define the net direction of change for a collection of molecules. First, consider the
1
general equation for the CO þ O 2 reaction, neglecting particularly esoteric and rare compounds.
2
1
CO þ O 2 5 a CO 2 þ b CO þ d O 2 þ e C þ f O (12.23)
2
where, the carbon (C) and atomic oxygen (O) are formed by the breakdown of the reactants. Expe-
rience shows that in the ranges normally encountered in practice, the C and O atoms have a negligible
effect. This allows the general reaction to be simplified to

1
CO þ O 2 5 a CO 2 þ b CO þ d O 2 : (12.24)
2
Itis possibletowrite this equationin a slightly differentform byconsidering the amount of CO 2 which
has dissociated. This can be defined as a ¼ (1 a), and the equation for the dissociation of CO 2 is
a
O 2 (12.25)
2
a CO 2 5 a CO þ
By considering the stoichiometric equation (Eqn (12.22)) and the dissociation equation (Eqn
(12.25)), a general equation may be constructed in terms of a:
1
CO þ O 2 /CO 2
2 (12.26)
a
O 2
2
a CO 2 5 a CO þ
Adding Eqn (12.26) gives
1 a
CO þ O 2 /ð1 aÞCO 2 þ a CO þ O 2 (12.27)
2 2
In Eqn (12.27), a is known as the degree of dissociation. This equation shows the effect of
dissociation on a chemically correct mixture. Before discussing methods of evaluating a, the effect of
dissociation on nonstoichiometric mixtures will be shown, first using the carbon monoxide reaction
and then a general hydrocarbon fuel.

12.4.1 WEAK MIXTURE WITH DISSOCIATION
Equation without dissociation

1
þ n O 2 /CO 2 þ nO 2 (12.28)
2
CO þ