Changes Affecting the Equilibrium Constant (CIE AS Chemistry)

Changes in concentration

• If all other conditions stay the same, the equilibrium constant K_c is not affected by any changes in concentration of the reactants or products
• For example, the decomposition of hydrogen iodide:

2HI ⇌ H₂ + I₂

The equilibrium expression is:

Adding more HI makes the ratio of [ products ] to [ reactants ] smaller

To restore equilibrium, [H₂] and [I₂] increases and [HI] decreases

Changes in pressure

• A change in pressure only changes the position of the equilibrium (see Le Chatelier’s principle)
• If all other conditions stay the same, the equilibrium constant K_c is not affected by any changes in pressure of the reactants and products

Changes in temperature

• Changes in temperature change the equilibrium constant K_c
• For an endothermic reaction such as:

An increase in temperature:

[H₂] and [I₂] increases

Because [H₂] and [I₂] are increasing and [HI] is decreasing, the equilibrium constant K_c increases

• For an exothermic reaction such as:

An increase in temperature:

[SO₃] decreases

Because [SO₃] decreases and [SO₂] and [O₂] increases the equilibrium constant K_c decreases

Presence of a catalyst

• If all other conditions stay the same, the equilibrium constant K_c is not affected by the presence of a catalyst
• A catalyst speeds up both the forward and reverse reactions at the same rate so the ratio of [ products ] to [ reactants ] remains unchanged

Worked example: Factors affecting K_c

Answer

Only a change in temperature will affect the value of K_c and any other changes in conditions would result in the position of the equilibrium moving in such way to oppose this change.

Adding a catalyst will increase the rate of reaction meaning the state of equilibrium will be reached faster but will have no effect on the position of the equilibrium and therefore K_c is unchanged.

Worked example: Factors which increase K_p value

Answer

Only temperature changes permanently affect the value of K_p

An increase in temperature shifts the reaction in favour of the products.

The [ products ] increases and [ reactants ] decreases, therefore, the K_p value increases.