Equilibrium Constant & Temperature (Edexcel International A Level Chemistry) : Revision Note

Stewart Hird

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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 [I2increases

    • [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:

    • [SO3decreases

    • [SO2] and [O2increases

    • Because [SO3decreases and [SO2] and [O2increases 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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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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