C8.6–7 Reversible Reactions and Energy and Reversible Reactions

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: reactions that can go both ways, two classic examples, and how energy is transferred in each direction. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain what a reversible reaction is and use the ⇌ symbol
• Describe the reversible reactions when ammonium chloride and hydrated copper(II) sulfate are heated
• Explain that if a reversible reaction is exothermic in one direction it is endothermic in the other, by the same amount
• Use anhydrous copper(II) sulfate and cobalt(II) chloride to show that water is present, and do calculations about these reactions (masses from moles: Higher tier)
• Explain how an indicator such as litmus works by a reversible reaction

Key words

• Reversible reaction: A reaction in which the products can react to make the original reactants again.
• ⇌: The symbol for a reversible reaction: a forward arrow and a reverse arrow.
• Exothermic: A reaction that transfers energy to the surroundings, so the temperature of the surroundings goes up.
• Endothermic: A reaction that takes in energy from the surroundings, so the temperature of the surroundings goes down.
• Hydrated: Containing water chemically bound in the crystals (water of crystallisation).
• Anhydrous: Without water: the water of crystallisation has been removed.

Reactions that go both ways

Most reactions we meet go one way only: you cannot “unburn” a match. In a reversible reaction, the products can react to form the reactants again. We write:

A + B ⇌ C + D

Left to right is the forward reaction. Right to left is the reverse (backward) reaction. The conditions (such as temperature) decide which direction is favoured.

Watch out: “Reactants” always means the substances on the left of the equation as it is written. If the same reaction is written C + D ⇌ A + B, then C and D are the reactants. So always quote the equation you mean.

Indicators and colour tests

Many indicators work by a reversible reaction. Litmus is a weak acid that can be written as HLit. The molecule is red; when it loses an H⁺ ion it becomes the blue Lit− ion:

HLit(aq) ⇌ H⁺(aq) + Lit−(aq) (red ⇌ blue)

Adding alkali removes H⁺ ions, so more blue Lit− forms. Adding acid adds H⁺ ions, so more red HLit forms. You can switch the colour back and forth as many times as you like.

Heating ammonium chloride

When white ammonium chloride is heated it decomposes into two gases, ammonia and hydrogen chloride. In the cooler part of the tube the gases react again to re-form the white solid.

ammonium chloride ⇌ ammonia + hydrogen chloride

NH₄Cl(s) ⇌ NH₃(g) + HCl(g)

Key fact: Heating favours the forward (endothermic) reaction. Cooling favours the reverse (exothermic) reaction. Do this in a fume cupboard: ammonia and hydrogen chloride are both toxic and irritant.

Hydrated copper(II) sulfate

Blue copper(II) sulfate crystals contain water of crystallisation. Heating drives off the water, leaving white anhydrous copper(II) sulfate. Adding water back turns it blue again and the mixture gets hot.

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

CuSO₄·5H₂O(s) ⇌ CuSO₄(s) + 5H₂O(l) (blue ⇌ white)

Exam tip: White anhydrous copper(II) sulfate turns blue when water is added, so it is a test for water. It shows water is present, but not that the water is pure. Pure water boils at exactly 100 °C (and freezes at 0 °C). Blue cobalt chloride paper, which turns pink, is another test for water.

Worked example: mass of water driven off (Higher tier)

Mr of CuSO₄·5H₂O = 63.5 + 32 + (4 × 16) + 5 × (2 × 1 + 16) = 159.5 + 90 = 249.5
A student heats 1.996 g of the hydrated crystals until the mass stops changing. Moles = 1.996 ÷ 249.5 = 0.00800 mol.
Mass of anhydrous CuSO₄ left = 0.00800 × 159.5 = 1.276 g. Water lost = 1.996 − 1.276 = 0.720 g (= 0.0400 mol, 5 × 0.00800).

Answer: 1.276 g of white solid remains; 0.720 g of water is driven off

Energy changes in reversible reactions

If a reversible reaction is exothermic in one direction, it is endothermic in the other. The same amount of energy is transferred each way: energy is conserved.

• NH₄Cl ⇌ NH₃ + HCl; Forward endothermic (needs heating); Reverse exothermic
• CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O; Forward endothermic (needs heating); Reverse exothermic (gets hot)
• N₂ + 3H₂ ⇌ 2NH₃; Forward exothermic; Reverse endothermic

Worked example: energy in both directions

N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction gives out 92 kJ for every 2 mol of NH₃ made.
How much energy is taken in when 3 mol of NH₃ decomposes? 3 mol is 1.5 × 2 mol, so energy = 1.5 × 92 kJ.

Answer: 138 kJ taken in (the reverse reaction is endothermic)

Common mistakes

• Using → for a reversible reaction. Use ⇌.
• Saying the reverse reaction transfers a different amount of energy. It is the same amount, in the opposite direction.
• Saying anhydrous copper(II) sulfate is a test for pure water. It only shows water is present; use boiling point (100 °C) for purity.
• Mixing up the colours: hydrated copper(II) sulfate is blue; anhydrous is white.
• Forgetting state symbols: in the ammonium chloride reaction the products are both gases (g).

Revision questions

Try these, then check your answers below each one.

Question 1. What does the symbol ⇌ show in a chemical equation? (Choose from: The reaction is reversible, The reaction is very fast, The reaction needs a catalyst, The reaction is at room temperature.)

Answer: The reaction is reversible

Working: The ⇌ symbol means the reaction can go forwards and backwards: it is reversible.

Question 2. What colour is hydrated copper(II) sulfate?

Answer: blue

Working: Hydrated copper(II) sulfate is blue; anhydrous copper(II) sulfate is white.

Question 3. The diagram shows the energy change for the forward reaction of a reversible reaction. True or false? The reverse reaction, C + D → A + B, takes in 65 kJ.

Answer: True

Working: True. The forward reaction is exothermic (65 kJ given out), so the reverse reaction is endothermic and takes in exactly the same amount, 65 kJ.

Related lessons

• Previous lesson: C8.4–5 The Effect of Concentration, Pressure and The Effect of Catalysts
• Next lesson: C8.8–9 Dynamic Equilibrium and Altering Conditions
• In this chapter: C8.1 Rate of Reaction
• In this chapter: C8.2 Collision Theory and Surface Area
• In this chapter: C8.3 The Effect of Temperature

Continue in the full course

This lesson is section C8.6–7 (AGCSC2505) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.