Consider the exothermic reaction

A(g) + B(g) ⇌ C(g)          ΔH < 0

The following table illustrates the changes expected.

Change	Rate of Reaction	Position of Equilibrium	Change to K
[A] / PA increases	Forward rate increases	Shifts to the right	None
[A] / PA decreases	Forward rate  decreases	Shifts to the left	None
[C] / PC increases	Reverse rate increases	Shifts to the left	None

As we can see, the position of equilibrium shifts to try and counteract the change. The explanation for this is due to the change in rate of forward or backward reactions, causing the position of equilibrium to shift.

Take note, however, that while the position of equilibrium shifts, the value of K does NOT shift. This is because the new established equilibrium has concentrations which give the same result for K as the concentrations before the change was introduced.

Similarly, if we have an endothermic reaction

A(g) + B(g) ⇌ C(g)          ΔH > 0

Change	Rate of Reaction	Position of Equilibrium	Change to K
[A] / PA increases	Forward rate increases	Shifts to the right	None
[A] / PA decreases	Forward rate decreases	Shifts to the left	None
[C] / PC increases	Reverse rate increases	Shifts to the left	None

This table is exactly the same as the table given above! The reason for this is because we haven’t done anything to the temperature, so there is nothing to counteract energy-wise! This is the subject of the next discussion, on changes in temperature.