Page 259 - Advanced thermodynamics for engineers

CHAPTER
12
CHEMICAL EQUILIBRIUM

AND DISSOCIATION

Up to now, this book has concentrated on combustion problems which can be solved by methods based
on equilibrium but which do not require an explicit statement of the fact, e.g. complete combustion of a
hydrocarbon fuel in air can be analysed by assuming that the products consist only of H 2 O and CO 2 .
These methods are not completely correct and a more rigorous analysis is necessary to obtain greater
accuracy.
Consider the combustion of carbon monoxide (CO) with oxygen (O 2 ); up till now the reaction has
been described by the equation
1
CO þ O 2 /CO 2 (12.1)
2
It is implied in this equation that carbon monoxide combines with oxygen to form carbon dioxide,
and as soon as that has happened, the reaction ceases. This is not a true description of what happens in
practice. The real process is one of dynamic equilibrium with some of the carbon dioxide breaking
down into carbon monoxide and oxygen (or even more esoteric components) again, which might then
recombine to form carbon dioxide. The breakdown of the CO 2 molecule is known as dissociation.To
evaluate the amount of dissociation that occurs (the degree of dissociation), it is necessary to evolve
new techniques.
It will be seen that the calculation of combustion with a number of degrees of freedom is quite
tedious, but easily formalised into an iterative technique; this is ideal for developing into a computer
program. EQUIL2 is a program that does this, and it incorporates the enthalpy coefficients used in this
text; the program is available on http://booksite.elsevier.com/9780444633736.

12.1 GIBBS ENERGY
The concept of Gibbs energy, G, was introduced in Chapter 2. The change in the specific Gibbs energy,
g, for a system of fixed composition was defined in terms of other properties as

dg ¼ vdp sdT: (12.2)
It was also shown that for a closed system at constant temperature and pressure, performing only
mechanical work, to be in equilibrium

dGÞ ¼ 0 (12.3)
p;T
Advanced Thermodynamics for Engineers. http://dx.doi.org/10.1016/B978-0-444-63373-6.00012-5 247
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