Chemistry · Lesson 11

Chemical equilibrium (Le Chatelier)

JAMBWAECNECO

What the syllabus asks

JAMB: Equilibrium: reversible reactions, factors affecting equilibrium, Le Chatelier's principle, equilibrium constant

WAEC: Chemical equilibrium: reversible reactions, dynamic equilibrium, factors affecting equilibrium (concentration, pressure, temperature, catalyst), Le Chatelier's principle, equilibrium constant expressions, industrial applications (Haber and Contact processes)

NECO: Chemical equilibrium: reversible reactions, dynamic equilibrium, factors affecting equilibrium (concentration, pressure, temperature, catalyst), Le Chatelier's principle, equilibrium constant expressions, industrial applications (Haber and Contact processes)

What you will learn

Explain reversible reactions and dynamic equilibrium.
State Le Chatelier's principle and apply it to changes in concentration, pressure, temperature and catalyst.
Write equilibrium constant expressions and calculate K from equilibrium concentrations.
Relate the size of K to the position of equilibrium.
Explain the choice of conditions in the Haber and Contact processes.
Solve exam-style equilibrium problems.

Key points

Reversible reactions go both ways (⇌); reversing changes the sign of ΔH.
Dynamic equilibrium: closed system, equal forward and backward rates, constant concentrations.
Le Chatelier: the equilibrium shifts to oppose any imposed change.
More reactant or less product shifts right.
Higher pressure shifts to fewer gas moles; no effect if gas moles are equal.
Higher temperature shifts in the endothermic direction and changes K.
A catalyst speeds up both directions equally; no change in position or K.
Kc = [products]^coeff/[reactants]^coeff; omit solids and pure liquids.
Haber: N₂ + 3H₂ ⇌ 2NH₃, ~450 °C, ~200 atm, iron. Contact: 2SO₂ + O₂ ⇌ 2SO₃, ~450 °C, 1–2 atm, V₂O₅.

Start here: Reversible reactions

Most reactions you have met go one way. When you burn kerosene in a lantern, you do not expect the smoke and water to rebuild the kerosene. But some reactions can go both ways. These are reversible reactions, shown with the double arrow ⇌.

Everyday and lab examples

Heating blue copper(II) sulphate crystals drives off water and leaves white anhydrous copper(II) sulphate. Add water to the white powder and it turns blue again and gets hot.
CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l)
Heating ammonium chloride in a test tube: the white solid seems to vanish at the bottom and reappear near the cool mouth of the tube. It splits into ammonia and hydrogen chloride when hot, and they recombine when cool.
NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
Dinitrogen tetroxide: N₂O₄(g) ⇌ 2NO₂(g). N₂O₄ is colourless; NO₂ is brown.
Making ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g).

The reaction from left to right is the forward reaction; from right to left is the backward (reverse) reaction.

If the forward reaction is exothermic, the backward reaction is endothermic by exactly the same amount, and vice versa.

Worked example 11.1

For N₂ + 3H₂ ⇌ 2NH₃, ΔH = −92 kJ/mol. What is ΔH for the decomposition of 2 mol of ammonia?

Decomposition is the backward reaction.
Reversing a reaction reverses the sign of ΔH, because the energy given out going forward must be taken in going back.
ΔH = +92 kJ/mol.

+92 kJ/mol (endothermic)

Try it yourself. Why does white anhydrous copper(II) sulphate get warm when water is added?

Show solution

Dehydration is endothermic, so the reverse (hydration) is exothermic.

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Practice questions

1. For N₂ + 3H₂ ⇌ 2NH₃ (exothermic), increasing pressure will

shift equilibrium to the right
shift equilibrium to the left
have no effect
stop the reaction

Show answer

A: shift equilibrium to the right
Le Chatelier: higher pressure favours fewer gas moles (4 → 2).

2. A reaction that can go in both directions is

irreversible
exothermic only
a precipitation
reversible

Show answer

D: reversible
Shown with ⇌.

3. At dynamic equilibrium,

only products are present
the concentrations are equal
forward and backward rates are equal
all reactions stop

Show answer

C: forward and backward rates are equal
Both reactions continue at the same rate.

4. Le Chatelier's principle states that a system at equilibrium

is unaffected by changes
shifts to oppose an imposed change
stops reacting
always favours products

Show answer

B: shifts to oppose an imposed change
By definition.

5. For N₂ + 3H₂ ⇌ 2NH₃ (exothermic), increasing pressure

increases the yield of NH₃
decreases the yield
has no effect
stops the reaction

Show answer

A: increases the yield of NH₃
Fewer gas molecules on the right (2 vs 4).

Key words

Reversible reaction: A reaction that can proceed in both directions.
Dynamic equilibrium: State in a closed system where forward and backward rates are equal so concentrations remain constant.
Closed system: One in which no substance can enter or leave.
Position of equilibrium: The relative amounts of reactants and products at equilibrium.
Le Chatelier's principle: A system at equilibrium shifts to oppose any change imposed on it.
Equilibrium constant (K or Kc): Ratio of product to reactant concentrations, each raised to its coefficient, at equilibrium.
Exothermic: Releases heat; ΔH negative.
Endothermic: Absorbs heat; ΔH positive.
Catalyst: A substance that speeds up a reaction without being used up.
Homogeneous equilibrium: All species in the same phase.
Heterogeneous equilibrium: Species in different phases.
Yield: Amount of product obtained compared with the maximum possible.

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