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3.3 Chemical Kinetics and Chemical Equilibrium 71

3.3.2 Chemical Equilibrium

Now consider a general reversible reaction aA þ bB $ cC þ dD that starts with a
mixture of A and B without C and D. A and B are thus identified as the reactants and
C + D are the products. At any moment t [ 0, once C and D present, they also react
and produce A and B reversing the process. At microscopic level, reactions proceed
both ways, but at macro level, the concentrations of A and B decrease and those of
C and D increase until they all remain stable, or reached chemical equilibrium state.
The equilibrium state is a dynamic one in which reactant and product concentra-
tions remain constant, not because the reaction stops but because the rates of the
forward and reverse reactions are equal.
Chemical reactions are dynamic in the sense that at any instant changes continue
in a system on a microscopic scale, but everything appears to be constant from a
macro scale point of view. This is called an equilibrium state. A combustion system
has a tendency to reach an equilibrium state. When the combustion conditions such
as pressure and temperature change, extra time is required for the system to rees-
tablish the new equilibrium state.
For the chemical reaction

k f
aA þ bB $ cC þ dD ð3:26Þ
k b
where k f ; k b are the rate constants for the forward and backward/reverse reactions,
respectively.
By considering both forward and backward reactions, the net consumption rate
of species A becomes
a b c d
r A ¼ k f A½Š B½Š þk b ½CŠ ½DŠ ð3:27Þ

When the reaction is at equilibrium, the reaction rate is zero ðr A ¼ 0Þ. Solving
this equation one can get the equilibrium constant based on concentration, which is
the ratio of the reaction constants
c d
k f ½CŠ ½DŠ
K C ¼ ¼ a b ð3:28Þ
k b ½AŠ ½BŠ

The unit of K C depends on the unit of concentration and the mole difference
ðcþdÞ ðaþbÞ
Dn ¼ c þ dÞ a þ bÞ;itis ½Š .
ð
ð
Example 3.6: Chemical equilibrium constant
At certain temperature, the equilibrium constant is 4.0 for the reaction. All com-
pounds are in gas phase