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A LevelChemistryCIERevision Notes7. Equilibria7.1 Chemical Equilibria: Reversible Reactions & Dynamic EquilibriumChanges Affecting the Equilibrium Constant

Changes Affecting the Equilibrium Constant (CIE A Level Chemistry)

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Caroline

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Caroline

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Changes that Affect the Equilibrium Constant

Changes in concentration

  • If all other conditions stay the same, the equilibrium constant Kc 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:

Kc = fraction numerator stretchy left square bracket H subscript 2 stretchy right square bracket bold space stretchy left square bracket I subscript 2 stretchy right square bracket over denominator stretchy left square bracket HI stretchy right square bracket to the power of bold 2 end fraction= 6.25 x 10–3

  • Adding more HI makes the ratio of [ products ] to [ reactants ] smaller
  • To restore equilibrium, [H2] and [I2] increase and [HI] decreases
  • Equilibrium is restored when the ratio is 6.25 x 10-3 again

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 Kc is not affected by any changes in the pressure of the reactants and products

Changes in temperature

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

2HI (g) rightwards harpoon over leftwards harpoon H2 (g) + I2 (g)   Kc = fraction numerator open square brackets straight H subscript 2 close square brackets space open square brackets straight I subscript 2 close square brackets over denominator open square brackets HI close square brackets squared end fraction

  • With an increase in temperature:
    • [H2] and [I2] increases
    • [HI] decreases
    • Because [H2] and [I2] increase and [HI] decreases, the equilibrium constant Kc increases
  • For an exothermic reaction such as:

2SO2 (g) + O2 (g) rightwards harpoon over leftwards harpoon 2SO3 (g)   Kc = fraction numerator open square brackets SO subscript 3 close square brackets cubed over denominator open square brackets SO subscript 2 close square brackets squared space open square brackets straight O subscript 2 close square brackets end fraction

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

Presence of a catalyst

  • If all other conditions stay the same, the equilibrium constant Kc 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 Kc 

An equilibrium is established in the following reaction:

AB (aq) + CD (aq) rightwards harpoon over leftwards harpoon AC (aq) + Bd (aq)   ΔH = +180 kJ mol-1

Which factors would affect the value of Kc in this equilibrium?

Answer

  • Only a change in temperature will affect the value of Kc
  • Any other changes in conditions would result in the position of the equilibrium moving to oppose this change
  • Adding a catalyst increases the rate of reaction meaning the state of equilibrium will be reached faster but has no effect on the position of the equilibrium and, therefore, Kc is unchanged

Worked example

Factors which increase Kp value

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

2A (g) + B (g) rightwards harpoon over leftwards harpoon 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

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    Caroline

    Author: Caroline

    Expertise: Physics Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.