Lesson overview
Reversible reactions and equilibrium: Can a reaction run both ways at once?
Fill three tubes from one violet solution of cobalt(II) chloride, look inside the reference tube, then predict and test what heating, cooling, extra chloride ions and water do to the other two. Find out why a reaction at equilibrium keeps going both ways, how the position moves, and how the Haber and Contact processes choose their conditions.
Suggested sequence
- Lesson 1 · Water out, water back. Reversible reactions: hydrated and anhydrous copper(II) sulfate and cobalt(II) chloride, and their water tests. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- Lesson 2 · Pink or blue? Both ways at once. Dynamic equilibrium in a closed system, and how temperature, concentration, pressure and a catalyst move its position. Read together (10–15 min), show or explore the experiment (10 min), practise with self-checking (15–20 min).
- Lesson 3 · Ammonia and sulfuric acid. The Haber and Contact processes: raw materials, typical conditions and why they are a compromise. Read together and discuss (15–20 min), practise with self-checking (20–25 min).
- End of module · final check. 6 new questions, 20–30 min, without hints or notes; pupils check their answers once they have answered them all.
- One to two weeks later · return questions. 4 questions, 15 min, lesson closed.
Learning goals
Part 1: Water out, water back
- State that some reactions are reversible, shown by ⇌, and describe how heat on hydrated copper(II) sulfate and cobalt(II) chloride, and water added to the anhydrous compounds, change the direction; use them as tests for water (6.3.1, 6.3.2)
Part 2: Pink or blue? Both ways at once
- State that some reactions are reversible, shown by ⇌, and describe how heat on hydrated copper(II) sulfate and cobalt(II) chloride, and water added to the anhydrous compounds, change the direction; use them as tests for water (6.3.1, 6.3.2)
- Supplement: state that a reversible reaction in a closed system is at equilibrium when the forward and reverse rates are equal and the concentrations no longer change, and predict and explain, using information provided, how temperature, pressure, concentration and a catalyst affect the position of equilibrium (6.3.3, 6.3.4)
Part 3: Ammonia and sulfuric acid
- Supplement: state that a reversible reaction in a closed system is at equilibrium when the forward and reverse rates are equal and the concentrations no longer change, and predict and explain, using information provided, how temperature, pressure, concentration and a catalyst affect the position of equilibrium (6.3.3, 6.3.4)
- Supplement: state the equations, sources and typical conditions of the Haber and Contact processes, and explain those conditions in terms of rate, position of equilibrium, safety and economics (6.3.5–6.3.11)
Misconceptions to listen for
- “An equation with water in it is reversible.” — The arrow tells you, not the substances: ⇌ means the reaction can go both ways.
- “The crystals melt.” — They don't turn into a liquid: they lose their water and crumble to a white powder.
- “Both anhydrous compounds are white.” — Anhydrous copper(II) sulfate is white, but anhydrous cobalt(II) chloride is blue.
- “Adding water dissolves the powder into a blue solution, so it's a physical change.” — A few drops of water join the powder's crystals and make blue hydrated crystals, a new substance, giving out energy.
- “Heating blue copper(II) sulfate is a physical change, because water turns it back.” — A new substance forms each way: white anhydrous copper(II) sulfate has a different formula and colour from the blue crystals, and energy is taken in or given out. Both directions are chemical changes.
- “Hydrated cobalt(II) chloride is blue.” — It's the other way round: hydrated is pink, anhydrous is blue. Blue cobalt chloride paper turns pink with water.
- “The paper turns blue if there is water.” — Dry paper is already blue. Water turns it pink.
- “The crystals have gone off and must be thrown away.” — The change is reversible: heating drives the water out, and they work again.
- “Adding water needs heat.” — Adding water gives out heat: it is heating the hydrated crystals that takes heat in.
- “Blue copper(II) sulfate turning white shows water.” — That happens when water is driven out by heating. The test adds the liquid to the white powder.
Scope
Reversible reactions and equilibrium (6.3 Core 1 and 2; Supplement 3 to 11): the reversible sign, hydrated and anhydrous salts and the water tests, dynamic equilibrium in a closed system, how changing the conditions moves its position, and the Haber and Contact processes. Energy changes are in ENER01, collision theory and catalysts in RATE01, and fertilisers in FERT01.