Temperature, pressure and catalysts at equilibrium

Three more ways to push an equilibrium

Making ammonia is exothermic: N₂ + 3H₂ ⇌ 2NH₃ has ΔH of −92.4 kJ per mole, so the reverse reaction is endothermic.

Raise the temperature and the equilibrium is pushed backward, towards the reactants: the heat-absorbing direction wins. Lower the temperature and it is driven forward.

Pressure matters only for gas reactions where the moles of gas change. Here 1 mole of N₂ and 3 moles of H₂ (4 moles) give 2 moles of NH₃. Raising the pressure moves the equilibrium to the side with fewer moles, forward, so high pressure favours ammonia.

A catalyst speeds up the forward and backward reactions equally. It does not shift the equilibrium; it only shortens the time taken to reach it.

Heat counts like a substance: for an exothermic reaction heat is a product, so adding heat pushes it back.

Key terms

Temperature rise

Predicting the shift when heating.

Pressure rise

Gas reactions with unequal moles.

Moles of gas

Deciding if pressure matters.

Catalyst at equilibrium

Reaching equilibrium sooner.

Short questions with model answers

1. Q1. What conditions give the most ammonia from N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92.4 kJ/mole? (Investigate ii, p. 92)

   • Exothermic forward: lower temperature
   • 4 mol gas → 2 mol: high pressure
   • Remove NH₃ as liquid; use Fe

   High pressure, a lower temperature, remove ammonia, and an iron catalyst for speed.

2. Q2. PCl₅ ⇌ PCl₃ + Cl₂ is endothermic. What happens if the temperature is raised? (Exercise iii, p. 88)

   • Forward absorbs heat
   • Heating favours the endothermic side

   It shifts forward: more PCl₃ and Cl₂ form.

Common mistakes

• ✗ Heating always pushes a reaction forward.

  ✓ Heating favours the endothermic direction; for ammonia that is backward.

• ✗ Higher pressure favours the side with more moles of gas.

  ✓ It favours the side with fewer moles (p. 88).

• ✗ A catalyst shifts the equilibrium towards the products.

  ✓ It speeds both directions equally; it only saves time.

MCQs

1. 1. For N₂ + 3H₂ ⇌ 2NH₃ (ΔH negative), raising the temperature:

   1. (a) Shifts it forward
   2. (b) Shifts it backward
   3. (c) Has no effect
   4. (d) Stops the reaction
   Show answer

   (b) p. 87.

2. 2. Increasing the pressure on N₂ + 3H₂ ⇌ 2NH₃:

   1. (a) Favours ammonia
   2. (b) Favours N₂ and H₂
   3. (c) Has no effect
   4. (d) Makes it endothermic
   Show answer

   (a) 4 moles → 2 moles; fewer moles win (p. 88).

3. 3. CaCO₃(s) ⇌ CaO(s) + CO₂(g) in a closed vessel. Increasing the pressure favours:

   1. (a) The forward reaction
   2. (b) The backward reaction
   3. (c) Neither
   4. (d) Only CO₂ escaping
   Show answer

   (b) Gas only on the right; pressure pushes towards no gas (MCQ, p. 91).

4. 4. Adding a catalyst to a reversible reaction:

   1. (a) Increases only the forward rate
   2. (b) Shifts it to the products
   3. (c) Reduces the time to reach equilibrium
   4. (d) Stops the backward reaction
   Show answer

   (c) p. 88: both rates rise, so equilibrium comes sooner.

5. 5. PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). Compressing the mixture shifts it:

   1. (a) Forward
   2. (b) Backward
   3. (c) Nowhere
   4. (d) To completion
   Show answer

   (b) 1 mole on the left, 2 on the right: fewer moles on the left.

Quick revision

• Raising temperature favours the endothermic direction; for ammonia, backward.
• Raising pressure favours the side with fewer moles of gas, if the moles differ.
• A catalyst does not shift equilibrium; it only reaches it faster.