Equilibrium Constant & Temperature (Edexcel IAL Chemistry)

Changing Conditions - Temperature

Spec point — 13.6

• Changes in temperature change the equilibrium constants K_c* *and K_p
• For an endothermic reaction such as:

2HI (g) ⇌ H₂ (g) + I₂ (g)

K_c = (left square bracket H _ 2 right square bracket left square bracket I _ 2 right square bracket)/(left square bracket HI right square bracket ^2)

• With an increase in temperature:

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

• For an exothermic reaction such as:

2SO₂ (g) + O₂ (g) ⇌ 2SO₃ (g)

K_c = (left square bracket SO _ 3 right square bracket ^2)/(left square bracket SO _ 2 right square bracket ^2 left square bracket O _ 2 right square bracket)

• With an increase in temperature:

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

Worked Example

Increasing K_p

What will increase the value of K_p for the following equilibrium?

2A (g) + B (g) ⇌ 2C (g)

ΔH = +6.5 kJ mol^-1

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.

How the equilibrium shifts with temperature changes:

Effect on the value of the equilibrium constant

• For a reaction that is exothermic in the forward direction, increasing the temperature pushes the equilibrium from right to left
• Therefore, the value of the equilibrium constant_{* *}will decrease as the ratio of [ products ] to [ reactants ] decreases
• Conversely, if the temperature is raised in an endothermic reaction, the value of the equilibrium constant_{* *}will increase