Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 13.4 Effects of temperature, pressure, catalyst-Study Notes - New Syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 13.4 Effects of temperature, pressure, catalyst- Study Notes- New syllabus

Edexcel International A Level (IAL) Chemistry (YCH11) -Unit 4 – 13.4 Effects of temperature, pressure, catalyst- 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

13.4 Effect of Temperature, Pressure and Catalysts on Equilibrium Composition

The position of equilibrium (equilibrium composition) can be affected by changes in temperature and pressure, but not by the presence of a catalyst. These effects can be explained using Le Chatelier’s Principle.

Le Chatelier’s Principle

When a system at equilibrium is disturbed, it responds in a way that opposes the change.

(i) Effect of Temperature

Temperature changes affect equilibrium because they change the balance between forward and backward reactions.

  • Exothermic reaction (\( \mathrm{\Delta H < 0} \)):
    • Increasing temperature shifts equilibrium to the left (endothermic direction).
    • Decreasing temperature shifts equilibrium to the right.
  • Endothermic reaction (\( \mathrm{\Delta H > 0} \)):
    • Increasing temperature shifts equilibrium to the right.
    • Decreasing temperature shifts equilibrium to the left.

Key Point

  • Temperature changes do change the equilibrium composition.
  • Temperature also changes the value of \( \mathrm{K_c} \) and \( \mathrm{K_p} \).

(ii) Effect of Pressure (Gaseous Systems Only)

Pressure changes only affect equilibria involving gases.

  • Increasing pressure shifts equilibrium towards the side with fewer moles of gas.
  • Decreasing pressure shifts equilibrium towards the side with more moles of gas.

Example

\( \mathrm{N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)} \)

  • Left side: 4 moles of gas
  • Right side: 2 moles of gas
  • Increasing pressure shifts equilibrium to the right.

Important Notes

  • No effect if the number of gas moles is equal on both sides.
  • No effect on equilibria involving only solids/liquids.
  • Pressure does not change \( \mathrm{K_c} \) or \( \mathrm{K_p} \).

(iii) Effect of a Catalyst

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

Key Point

  • A catalyst affects kinetics, not thermodynamics.

Homogeneous vs Heterogeneous Systems

  • The same principles apply to both systems.
  • Pressure effects are only relevant when gases are involved.
  • Solids and liquids do not affect pressure changes.

Summary Table

  • Temperature → changes equilibrium position and \( \mathrm{K} \)
  • Pressure → changes equilibrium position (gases only), not \( \mathrm{K} \)
  • Catalyst → no effect on equilibrium position or \( \mathrm{K} \)

Example 1:

Explain the effect of increasing temperature on the equilibrium position of an exothermic reaction.

▶️ Answer/Explanation

Increasing temperature adds heat to the system.

In an exothermic reaction, heat is a product.

The system shifts in the endothermic direction to oppose the change.

Therefore, equilibrium shifts to the left.

Example 2:

Explain why adding a catalyst does not change the equilibrium composition.

▶️ Answer/Explanation

A catalyst lowers the activation energy for both forward and reverse reactions equally.

This increases the rate at which equilibrium is reached.

However, it does not change the relative stability of reactants and products.

Therefore, the equilibrium composition remains unchanged.

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