Disturbing an equilibrium: adding or removing substances

Push it and it pushes back

An equilibrium stays put forever unless something disturbs it. The book lists five disturbances: adding or removing a reactant, adding or removing a product, changing the temperature, adding a catalyst, and changing the pressure when gases are involved.

Take N₂ + 3H₂ ⇌ 2NH₃ at equilibrium and add more nitrogen. Its concentration jumps, so the mixture is no longer at equilibrium. Nitrogen reacts with hydrogen to make more ammonia until a new equilibrium is reached.

Now remove some ammonia. Its concentration drops, so more nitrogen and hydrogen react to replace it, again until the concentrations settle.

The new equilibrium has constant concentrations again, but they are different from the old ones. Adding a reactant or removing a product shifts it forward; adding a product or removing a reactant shifts it backward.

The reaction always moves to use up what you added, or to replace what you took away.

Key terms

Disturb

Starting a shift.

Shift forward

After adding reactant or removing product.

Shift backward

After adding product or removing reactant.

New equilibrium

Where the shift ends.

Short questions with model answers

1. Q1. C₂H₅OH + CH₃COOH ⇌ CH₃COOC₂H₅ + H₂O. How can the yield of ethyl acetate be improved? (Exercise, p. 89)

   • Water is a product
   • Remove water

   Withdraw water from the mixture; more ethyl acetate forms.

2. Q2. PCl₅ ⇌ PCl₃ + Cl₂ is at equilibrium. Cl₂ gas is added. What happens? (Exercise ii, p. 88)

   • Cl₂ is a product
   • Cl₂ reacts with PCl₃

   The equilibrium shifts backward: more PCl₅ forms.

Common mistakes

• ✗ After the shift, concentrations return to their old values.

  ✓ They become constant again, but at new values (p. 87).

• ✗ Adding a product makes even more product.

  ✓ Extra product reacts back: the shift is backward.

• ✗ Removing a product does nothing to the equilibrium.

  ✓ More reactants react to replace it: the shift is forward.

MCQs

1. 1. N₂ is added to N₂ + 3H₂ ⇌ 2NH₃ at equilibrium. More:

   1. (a) NH₃ forms
   2. (b) H₂ forms
   3. (c) N₂ forms
   4. (d) Nothing changes
   Show answer

   (a) p. 87.

2. 2. Some NH₃ is withdrawn from the equilibrium mixture. The equilibrium:

   1. (a) Shifts forward
   2. (b) Shifts backward
   3. (c) Stops
   4. (d) Is not affected
   Show answer

   (a) p. 87: more N₂ and H₂ react to make NH₃.

3. 3. After a disturbance, the new equilibrium concentrations are:

   1. (a) The same as before
   2. (b) Constant but different
   3. (c) Always zero
   4. (d) Always changing
   Show answer

   (b) p. 87.

4. 4. Which of these is NOT one of the book's ways to disturb an equilibrium?

   1. (a) Changing temperature
   2. (b) Adding a product
   3. (c) Painting the container
   4. (d) Changing pressure of gases
   Show answer

   (c) p. 86 lists five ways.

5. 5. Adding a product to an equilibrium mixture shifts it:

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

   (b) The extra product reacts back to reactants.

Quick revision

• An equilibrium can be disturbed in five ways: reactants, products, temperature, catalyst, pressure.
• Add reactant or remove product: shift forward. Add product or remove reactant: shift backward.
• The new equilibrium has constant but different concentrations.