Any disturbance to a system at equilibrium that results in a
change in the magnitude of either Q or K results in a
change in the equilibrium composition.The magnitude of K changes with temperature. The direction of the change depends on the sign of Δ
H for the reaction.
- For exothermic reactions (ΔH negative) have smaller K at higher temperatures. Thus reactants are present in relatively larger amounts when equilibrium is restored.
- For endothermic reactions (ΔH positive) have larger K at higher temperatures. Thus products are present in relatively larger amounts when equilibrium is restored.
Disturbances to an equilibrium system
at constant temperature change the magnitude of
Q, but NOT the magnitude of
K. Examples are
- changing the concentration of a component appearing in the Q expression
- changing external pressure or total volume for some reactions involving gases
The direction in which reaction must occur to reestablish equilibrium depends on the relative magnitude of
Q after the change has been applied and
K.
- If Q < K, reaction occurs in the forward direction until Q = K.
- If Q > K, reaction occurs in the reverse direction until Q = K.