Equilibrium (2) Effect of temperature change on Kp and Kc

Edexcel A level Chemistry (2017)

Topic 11: Equilibrium II:

Here is the third learning objective:

11/II/3. To know the effect of changing temperature on the equilibrium constant (Kc and Kp), for both exothermic and endothermic reactions

If    N₂O₄(g)    ⇌   2NO₂(g)  ΔH= +57.2kJmol^–1

Then the position of equilibrium moves left if the system is cooled.

Also if the system is warmed the position of equilibrium moves to the right forming more NO₂ and the colour of the gas mixture darkens.

The reaction left to right is endothermic so Le Chatelier’s principle would suggest that the reaction is moving in such away to evolve heat when the temperature is lowered and to absorb heat when the temperature is raised.

But what is the theoretical basis for this conclusion?

Experimental values show there is an approximate relationship between temperature and K_p.

The data in the table below reflects this relationship.

A plot of log₁₀K_p vs 1/T  gives a straight line as the graph shows.

If it is assumed (and this is where the relationship becomes approximate) that ΔH is constant over the range of temperatures used then the graph shows that:

The slope of the graph is – ΔH^⦵/2.303 R.

This equation confirms quantitatively Le Chatelier’s qualitative principle.

If ΔH^⦵   is positive the forward reaction is endothermic, then as temperature increases, 1/T decreases so Kp increases and so the position of equilibrium must be moving to the right to favour products.  As was said above when the temperature is raised the system absorbs heat and moves to favour products.

A summary is given in the table below:

ΔH⦵ of forward reaction	Temperature change applied	Effect on value of K	Change in equilibrium position
Endothermic	Increase	Increase	Favours products
Endothermic	Decrease	Decrease	Favours reactants
Exothermic	Increase	Decrease	Favours reactants
Exothermic	Decrease	Increase	Favours products