CIE AS Chemistry (9701) 2019-2021

Revision Notes

1.7.3 Changes Affecting the Equilibrium Constant

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:

Changes that Affect the Equilibrium Constant equation 1

Adding more HI makes the ratio of [ products ] to [ reactants ] smaller
To restore equilibrium, [H2] and [I2] increases 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 pressure of the reactants and products

Changes in temperature

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

Changes that Affect the Equilibrium Constant equation 2

An increase in temperature:
[H2] and [I2] increases
[HI] decreases

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

  • For an exothermic reaction such as:

Changes that Affect the Equilibrium Constant equation 3

An increase in temperature:
[SO3] decreases
[SO2] and [O2] increases

Because [SO3] decreases and [SO2] and [O2] increases 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

Equilibria Worked example - Factors affecting Kc, downloadable AS & A Level Chemistry revision notes

Answer

Only a change in temperature will affect the value of Kc 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 Kc is unchanged.

Worked example: Factors which increase Kp value

Equilibria Worked example - Factors which increase Kp value, downloadable AS & A Level Chemistry revision notes

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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