Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 13.5 Independence of K from concentration/pressure-Study Notes - New Syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 13.5 Independence of K from concentration/pressure- Study Notes- New syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 13.5 Independence of K from concentration/pressure- Study Notes -International A Level (IAL) Chemistry (YCH11) – per latest Syllabus.

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Edexcel International A Level (IAL) Chemistry (YCH11) -Concise Summary Notes- All Topics

13.5 Independence of Equilibrium Constant (\( \mathrm{K} \))

The equilibrium constant (\( \mathrm{K_c} \) or \( \mathrm{K_p} \)) depends only on temperature. It is not affected by changes in concentration, pressure, or the presence of a catalyst. These factors may change the position of equilibrium temporarily, but the value of \( \mathrm{K} \) remains constant at a given temperature.

Key Idea

At constant temperature, the value of \( \mathrm{K} \) remains constant regardless of external changes.

(i) Effect of Concentration

  • Changing concentration disturbs the equilibrium position.
  • The system shifts to restore equilibrium (Le Chatelier’s Principle).
  • New equilibrium concentrations are established.
  • However, the ratio in the \( \mathrm{K_c} \) expression remains unchanged.

Conclusion: \( \mathrm{K} \) does not change.

(ii) Effect of Pressure

  • Changing pressure affects gaseous equilibria.
  • The equilibrium position shifts towards fewer or more moles of gas.
  • Partial pressures or concentrations change accordingly.
  • The value of \( \mathrm{K_p} \) or \( \mathrm{K_c} \) remains constant.

Conclusion: Pressure does not affect \( \mathrm{K} \).

(iii) Effect of a Catalyst

  • A catalyst lowers the activation energy of both forward and backward reactions equally.
  • It increases the rate at which equilibrium is reached.
  • It does not change equilibrium concentrations.

Conclusion: A catalyst does not affect \( \mathrm{K} \).

Important Exception: Temperature

  • Only temperature changes the value of \( \mathrm{K} \).
  • Increasing temperature:
    • Increases \( \mathrm{K} \) for endothermic reactions.
    • Decreases \( \mathrm{K} \) for exothermic reactions.

Key Features

  • \( \mathrm{K} \) depends only on temperature.
  • Concentration and pressure change equilibrium position but not \( \mathrm{K} \).
  • Catalysts affect rate, not equilibrium constant.
  • A new equilibrium always re-establishes the same value of \( \mathrm{K} \).

Example 1:

Explain why adding more reactant does not change the value of \( \mathrm{K_c} \).

▶️ Answer/Explanation

Adding reactant increases its concentration and disturbs equilibrium.

The system shifts to the right to use up the added reactant.

New equilibrium concentrations are established.

However, the ratio of concentrations in the \( \mathrm{K_c} \) expression remains constant.

Therefore, \( \mathrm{K_c} \) does not change.

Example 2:

Explain why increasing pressure does not change \( \mathrm{K_p} \) for a gaseous equilibrium.

▶️ Answer/Explanation

Increasing pressure changes the partial pressures of gases and shifts equilibrium.

However, equilibrium is re-established with new partial pressures.

The ratio of partial pressures in the \( \mathrm{K_p} \) expression remains constant.

Therefore, \( \mathrm{K_p} \) is unchanged at constant temperature.

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