The effect of change on equilibrium composition
For a reaction system at equilibrium, when equilibrium concentrations are substituted into the reaction quotient expression, the value calculated is K, the equilibrium constant. K is constant for a reaction at a particular temperature.
The reaction quotient is shown for the general reaction:
If, for a system at equilibrium, the concentration of a substance appearing in the reaction quotient is changed, the reaction quotient no longer equals K.
To predict the direction of both the reaction to restore equilibrium and the changes in the concentrations, first establish how a change in concentration affects magnitude of the reaction quotient relative to the equilibrium constant K.
Increase concentration of reactant A above
Increase concentration of product C above.
Directly after the increase, reaction quotient > K (C appears in the numerator)
Thus reaction in the reverse direction consumes D and some of the added C until equilibrium is restored. The new equilibrium composition has higher [A] and [B] as well as higher [C]. [D] is lower.
The arrows in the equilibrium expression at the right show the direction (not the magnitude) of changes in concentrations.
Note that the concentrations in the new equilibrium composition still combine to make the reaction quotient equal to K.
If the amount of catalyst is increased
The reaction quotient is shown for the general reaction:
aA + bB 
cC + dD
cC + dD| Kc = | [C]c[D]d |
| [A]a[B]b | |
| constant | reaction quotient |
If, for a system at equilibrium, the concentration of a substance appearing in the reaction quotient is changed, the reaction quotient no longer equals K.
The natural tendency is to restore equilibrium by net reaction in either the forward or reverse direction until reaction quotient again equals K. When equilibrium is restored, each of [A], [B], [C] and [D] is changed.
To predict the direction of both the reaction to restore equilibrium and the changes in the concentrations, first establish how a change in concentration affects magnitude of the reaction quotient relative to the equilibrium constant K.
Increase concentration of reactant A above
Directly after the increase, reaction quotient <K (A appears in the denominator).
Thus reaction in the forward direction consumes B and some of the added A until equilibrium is restored. The new equilibrium composition has higher [C] and [D] as well as [A]. [B] is decreased
The arrows in the equilibrium expression at the right show the direction (not the magnitude) of changes in concentration.
Note that the concentrations in the new equilibrium composition still combine to make the reaction quotient equal to K.
Thus reaction in the forward direction consumes B and some of the added A until equilibrium is restored. The new equilibrium composition has higher [C] and [D] as well as [A]. [B] is decreased
| Kc = no change | [C]c [D]d | |
[A]a[B]b | ||
| direction of changes in concentrations if A is added | ||
The arrows in the equilibrium expression at the right show the direction (not the magnitude) of changes in concentration.
Note that the concentrations in the new equilibrium composition still combine to make the reaction quotient equal to K.
Increase concentration of product C above.
Directly after the increase, reaction quotient > K (C appears in the numerator)
Thus reaction in the reverse direction consumes D and some of the added C until equilibrium is restored. The new equilibrium composition has higher [A] and [B] as well as higher [C]. [D] is lower.
| Kc = no change | [C]c[D]d | |
| [A]a[B]b | ||
| direction of changes in concentrations if C is added | ||
The arrows in the equilibrium expression at the right show the direction (not the magnitude) of changes in concentrations.
Note that the concentrations in the new equilibrium composition still combine to make the reaction quotient equal to K.
reaction quotient = K (catalysts do not appear in the expression)
The system is still at equilibrium so there is no change in the equilibrium composition.
The system is still at equilibrium so there is no change in the equilibrium composition.