The shape of the lesson
By the end of the lesson, learners can
- Reversible reactions: Two colour changes appear in every syllabus.
- Dynamic equilibrium: Extended.
- Changing the conditions: Extended.
- Hydrated CuSO4: blue; anhydrous: white.
- Dynamic equilibrium: equal rates, constant concentrations, closed system.
- Hotter: endothermic direction. Higher pressure: fewer gas molecules.
- Catalyst: faster to equilibrium, no shift.
One picture every idea returns to
A reversible reaction can go in either direction, shown by the symbol ⇌. Heating hydrated copper(II) sulfate (blue) drives off water to leave anhydrous copper(II) sulfate (white); adding water reverses it. Extended: in a closed system a reversible reaction reaches dynamic equilibrium, where the forward and reverse reactions happen at the same rate and the concentrations of reactants and products stay constant. Changing the conditions moves the position of equilibrium to oppose the change.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, Reactions that go both ways.. Heat blue copper(II) sulfate crystals and they turn white; add water and they turn blue again. Put the figure on the board: it is the one picture every idea in the lesson returns to. | Whole class, then pairs |
| 5 to 13 min | Teach and check: Reversible reactions | Two colour changes appear in every syllabus. | Whole class, then pairs |
| 13 to 21 min | Teach and check: Dynamic equilibrium | Extended. Project tab 1 of the lab (Equilibria lab, dynamic equilibrium) and let a student drive it while the class predicts. | Pairs |
| 21 to 30 min | Teach and check: Changing the conditions | Extended. | Rally Robin |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (6 questions, 15 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Changing the conditions, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Rally Robin
Pairs take turns to say one item each, back and forth: a definition, a unit, a step of the method. Keep it fast and oral; the point is retrieval, not writing.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| At equilibrium the concentrations are constant, not equal. | There may be far more product than reactant, or the other way round. And a catalyst never moves the position of equilibrium: it speeds up the forward and reverse reactions equally. |
Reversible reactions and equilibrium: worksheet
6 questions, 15 marks. Written in the style of the papers; no past paper question is reproduced.
Describe what is seen when blue hydrated copper(II) sulfate is heated, and when water is then added to the product. Write the equation, using the reversible symbol.
Describe a chemical test for water, giving the result.
(Extended) State three features of a reaction at dynamic equilibrium.
(Extended) For H2(g) + I2(g) ⇌ 2HI(g), predict the effect of increasing the pressure on the position of equilibrium. Explain your answer.
(Extended) For CO(g) + 2H2(g) ⇌ CH3OH(g) the forward reaction is exothermic. State and explain the effect on the yield of methanol of (a) increasing the temperature and (b) increasing the pressure.
(Extended) Explain why adding a catalyst to a reaction at equilibrium does not change the amount of product.
Answer key and marking notes
- On heating, the blue crystals turn to a white powder (and steam is given off).
- When water is added, the white powder turns blue again and gets hot.
- CuSO4·5H2O ⇌ CuSO4 + 5H2O.
- Add the liquid to anhydrous copper(II) sulfate (or blue cobalt(II) chloride paper).
- It turns from white to blue (or the cobalt(II) chloride turns from blue to pink).
- The forward and reverse reactions happen at the same rate.
- The concentrations of reactants and products remain constant.
- It occurs in a closed system (nothing is added or removed).
- No effect on the position of equilibrium.
- There are two gas molecules on each side, so neither side is favoured by a change in pressure.
- (a) The yield decreases: increasing the temperature moves the equilibrium in the endothermic direction, which is the reverse reaction.
- (b) The yield increases: there are 3 gas molecules on the left and 1 on the right,
- so higher pressure moves the equilibrium to the right, the side with fewer gas molecules.
- A catalyst increases the rates of the forward and reverse reactions equally.
- So the position of equilibrium does not change; equilibrium is only reached more quickly.
8 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
A strip of blue cobalt(II) chloride paper is dipped into a colourless liquid and turns pink. What can be concluded?
- AThe liquid contains water
- BThe liquid is pure water
- CThe liquid is an acid
- DThe liquid is an alkali
How could a student show that a sample of colourless liquid is pure water?
- AShow that it boils at exactly 100 °C
- BShow that it turns white anhydrous copper(II) sulfate blue
- CShow that it has no colour or smell
- DShow that it turns blue cobalt(II) chloride paper pink
Removing the water from hydrated copper(II) sulfate is endothermic. What can you say about adding water to anhydrous copper(II) sulfate?
- AIt is exothermic, releasing the same amount of energy as was taken in
- BIt is endothermic, taking in the same amount of energy
- CIt is exothermic, releasing much less energy than was taken in
- DThere is no energy change because it is only a colour change
Ammonium chloride is heated at the bottom of a test tube. It forms ammonia and hydrogen chloride, which re-form white ammonium chloride on the cool upper part of the tube. How is this best described?
- Aa reversible reaction: NH4Cl ⇌ NH3 + HCl
- Ba physical change, because the same solid appears again
- Can irreversible thermal decomposition
- Da reversible physical change of state
The graph shows how the rates of the forward and reverse reactions change after reactants alone are sealed in a flask. Which statement describes the mixture after time T?
- ABoth reactions continue at equal rates, so the concentrations of reactants and products stay constant
- BBoth reactions have stopped, so the concentrations stay constant
- CThe concentrations of the reactants and the products are equal
- DOnly the reverse reaction takes place, so the amount of product falls
Brown nitrogen dioxide and colourless dinitrogen tetroxide form an equilibrium: 2NO2(g) ⇌ N2O4(g). The forward reaction is exothermic. A sealed syringe of the equilibrium mixture is placed in ice. What is seen?
- AThe mixture becomes paler, because the equilibrium moves to the right
- BThe mixture becomes darker brown, because the equilibrium moves to the left
- CThe mixture becomes paler, because the equilibrium moves to the left
- DNo change, because cooling only slows both reactions
Hydrogen is made industrially by CH4(g) + H2O(g) ⇌ CO(g) + 3H2(g). What is the effect of increasing the pressure on the position of equilibrium?
- AIt moves to the left, the side with fewer gas molecules
- BIt moves to the right, the side with fewer gas molecules
- CIt moves to the right, because more product molecules form
- DIt does not move, because all the substances are gases
Yellow chromate ions and orange dichromate ions are in equilibrium: 2CrO42−(aq) + 2H+(aq) ⇌ Cr2O72−(aq) + H2O(l). A few drops of dilute acid are added. What happens?
- AThe solution turns more orange, because the equilibrium moves to the right
- BThe solution turns more yellow, because the equilibrium moves to the left
- CThe solution turns more orange, because the acid is a catalyst
- DNo change, because adding acid does not affect an equilibrium
Quiz key
Blue anhydrous cobalt(II) chloride turns pink when it gains water, so the liquid contains water. The test cannot show the water is pure: a salt solution or a mixture of water and ethanol would give the same result. The colour change is caused by hydration, not by acidity or alkalinity.
A pure substance has a sharp, fixed boiling point, and pure water boils at 100 °C (at normal atmospheric pressure); impurities raise it. Turning anhydrous copper(II) sulfate blue or cobalt(II) chloride paper pink only shows that water is present, not that it is pure. Many impure liquids are also colourless and odourless.
If one direction of a reversible reaction is endothermic, the reverse is exothermic by exactly the same amount, so hydrating anhydrous copper(II) sulfate gives out heat equal to the energy taken in to dehydrate it. It cannot also be endothermic, the amounts are equal rather than smaller, and the white powder getting noticeably warm shows there is an energy change.
At the hot end ammonium chloride decomposes into two new substances, ammonia and hydrogen chloride, so this is a chemical change. On cooling they react to re-form ammonium chloride, so the reaction goes in both directions: a reversible reaction. Calling it physical or a change of state ignores the new gases, and it is not irreversible because the original solid is re-formed.
After time T the two rates are equal and neither is zero, so this is dynamic equilibrium: reactants form products as fast as products turn back into reactants, and the concentrations stay constant. The reactions have not stopped, because the rates are above zero. Constant concentrations need not be equal. Both reactions still happen, so it is wrong to say only the reverse reaction occurs.
Cooling moves the position of equilibrium in the exothermic direction, which is the forward reaction, so more colourless N2O4 forms and less brown NO2 remains: the mixture turns paler. Moving to the left would make it darker, and a shift to the left cannot make it paler. Cooling does slow both reactions, but it also changes the position of equilibrium, so there is a visible change.
There are 2 gas molecules on the left (1 CH4 + 1 H2O) and 4 on the right (1 CO + 3 H2). Increasing the pressure favours the side with fewer gas molecules, so the position moves to the left and less hydrogen is made. The right-hand side has more gas molecules, not fewer. Every substance being a gas is exactly why pressure has an effect here, because the numbers of gas molecules on the two sides are different.
Adding acid increases the concentration of H+, a reactant, so the position of equilibrium moves to the right to use some of it up, forming more orange dichromate. Moving to the left would do the opposite. The acid is a reactant that appears in the equation, not a catalyst (a catalyst would not shift the position at all), so a change of concentration does affect this equilibrium.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.