Controlling Equilibrium Position

Concentration, pressure and catalysts do not affect K as they are quantities. However, temperature does affect K as it is a condition.

For exothermic reactions (in the forwards direction)

Increasing temperature:

Favours the endothermic direction
This shifts the equilibrium to the left.
This causes a decrease in K.

Decreasing temperature:

Favours the endothermic direction.
This causes the equilibrium to shift to the right.
This causes an increase in K.

For endothermic reactions (in the forwards direction)

Increasing temperature:

Favours the endothermic direction
This causes the equilibrium to shift to the right.
This causes an increase in K.

Decreasing temperature:

Favours the exothermic direction
This causes the equilibrium to shift to the left.
This causes a decrease in K.

"The shift in equilibrium position is a consequence of the value of K.

Kc = [B] / [A] - Concentration

Concentration does not affect the value of K. A change in [B] or [A] will disable the equilibrium because K is a constant value and the expression is now higher or lower than Kc. Therefore [B] or [A] must increase or decrease until Kc and the value for the expression are equal. When they become equal, equilibrium is restored. This is the same for pressure.

Temperature

It does affect the value of K.

Quantities [B] or [A] do not change.
Equilibrium position is affected by temperature.
To make Le Chatelier's Principle true, K must change to a new value.
This will temporarily disable the equilibrium as values of [B] and [A] in the expression will match the original value of K.
Therefore quantities of [B] and [A] must adjust.

An example:

N2 + O2 <==> 2NO (+180)

What effect does increasing temperature have on K?

This means Kp increases.
The system is no longer at equilibrium because the value of the expression is smaller than Kp. This disables the equilibrium.
Therefore the value of products should increase and the reactants should decrease as the equilibrium shifts to the right.
This restores equilibrium.

2SO2 + O2 <==> 2SO3 (-197)

What effect does decreasing pressure have on K?

No longer at equilibrium
Decrease p(SO3)2
p(SO2)2 + p(O2) increase by the equilibrium shifting to the left.
This restores equilibrium.

	Created by Chloe Drewery about 7 years ago

A level Chemistry (Chapter 19 - Equilibrium) Note on Controlling Equilibrium, created by Chloe Drewery on 14/09/2017.