Edexcel International A Level Chemistry

Revision Notes

4.4.4 Equilibrium Constant & Temperature

Changing Conditions - Temperature

  • Changes in temperature change the equilibrium constants Kc and Kp
  • For an endothermic reaction such as:

2HI (g) ⇌ H2 (g) + I2 (g)

Kcfraction numerator left square bracket straight H subscript 2 right square bracket space left square bracket straight I subscript 2 right square bracket over denominator left square bracket HI right square bracket squared end fraction

  • 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)

Kcfraction numerator left square bracket SO subscript 3 right square bracket squared over denominator left square bracket SO subscript 2 right square bracket squared space left square bracket straight O subscript 2 right square bracket end fraction

  • With an increase in temperature:
    • [SO3] decreases
    • [SO2] and [O2increases
    • Because [SO3] decreases and [SO2] and [O2] increases the equilibrium constant  decreases

Worked example

Increasing Kp

What will increase the value of Kp 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 Kp

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

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

How the equilibrium shifts with temperature changes:

Effects of temperature table, IGCSE & GCSE Chemistry revision notes

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

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