Equilibrium Constants

Chemical Equilibria|Le Chatelier's Principle|Factors Affecting Chemical equilibria| The Haber Process|The Contact Process|Equilibrium Constants

Equilibrium Constant Kc
Let us consider the following general equation:

aA + bB <=> cC + dD

c  d       a   b                         Kc = [C] [D]   / [A] [B]
 * Expression for Kc gives:

Kc is the ratio of the concentration of the prodcuts raised to their coefficients in the balanced stoichiometric equation to the concentration of the reactants raised to their coefficients in the balanced stoichiometric equation.

Kc will be considered in 2 different situations:


 * In Homogeneous Equilibria
 * In Heterogeneous Equilibria

Kc in Homogeneous equilibria
Homogeneous equilibria implies that all species are in the same phase, that is they are either all aqueous or all gaseous.

For Example:

2SO2(g) + O2(g) <=> 2SO3 (g) 2       1    2                               Kc = [SO3]   / [O2][SO2]

Kc in Heterogeneous equilibria
Heterogeneous equilibria implies that not all species are in the same state, that is there might be some species in gaseous state, others in aqueous and others in solid.

For example:

H2O (g) + C (s) <=> H2 (g) + CO (g) 1   1         1                               Kc = [H2] [CO]   / [H2O]


 * Units of Kc

Calculating units of Kc is very simple. All that has to be done is replace the species by the units mol/dm3 and evaluate the units as shown below:

CH3COOH + C2H5OH <=> CH3COOC2H5 + H2O

1   1            1         1                         Kc = [CH3COOC2H5] [H2O]  / [CH3COOH] [C2H5OH]

Units :( mol/dm3 x mol/dm3 )/ (mol/dm3 )x(mol/dm3 ) : No units

A large value of Kc implies that the equilibrium lies more to the right, that is there is a large amount of products formed.
 * Significance of Kc

Kc is said to be temperature dependant. That is the value of Kc changes ONLY when temperature changes.
 * Kc - Temperature dependance