This set has twelve original questions, ordered from easier to harder, covering all five lessons in reversible changes and equilibrium. Questions 1 to 4 cover equations and meaning, 5 to 9 cover shifts and rate, and 10 to 12 mix skills, including a calculation and some invented data.
Attempt each question on paper first and write your reasoning as you would in an exam. Only then open the answer. Use the routing list at the end for anything you missed, and log the error type with the mistake log and retest queue.
Questions
1. Ammonium chloride solid breaks down into ammonia gas and hydrogen chloride gas, and the two gases can recombine. Write the reversible equation with state symbols.
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Formulas: NH₄Cl, NH₃, HCl. Atoms: 1 N, 4 H, 1 Cl on the left; 1 N, 3 + 1 = 4 H, 1 Cl on the right. Already balanced.
NH₄Cl(s) ⇌ NH₃(g) + HCl(g)
2. Balance: SO₂(g) + O₂(g) ⇌ SO₃(g)
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Right side needs to match oxygen. Start with 2SO₃: that gives 2 S and 6 O. Then 2SO₂ gives 2 S and 4 O, and O₂ adds 2 O, making 6.
2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
Check: S 2 = 2; O 4 + 2 = 6 = 6.
3. A student writes: “When the mixture reaches equilibrium, the reactions stop and the amounts of reactants and products are equal.” Identify and correct both errors.
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Error 1: the reactions do not stop. They continue, which is why equilibrium is dynamic. Error 2: the amounts are not necessarily equal. They are constant.
Correct statement: “At equilibrium the forward and reverse reactions occur at the same rate, so the concentrations of reactants and products stay constant.”
4. Calcium carbonate decomposes: CaCO₃(s) ⇌ CaO(s) + CO₂(g). Explain why this reaction can reach equilibrium in a sealed container but not in an open one.
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In a sealed container the CO₂ gas cannot escape, so it can react with CaO and the reverse reaction keeps pace with the forward one. In an open container the CO₂ escapes, so the reverse reaction cannot match the forward rate and the amounts keep changing.
5. H₂(g) + I₂(g) ⇌ 2HI(g). More iodine is added at constant temperature. State the direction of shift and the effect on the amount of HI.
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Adding I₂ raises a reactant concentration. The system uses it up, so the position shifts towards the products and the amount of HI increases.
6. N₂O₄(g) ⇌ 2NO₂(g). Pressure is increased at constant temperature. Predict the shift, giving the gas count on each side.
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Left: 1 gas molecule. Right: 2 gas molecules. Higher pressure favours the side with fewer gas molecules, so the position shifts towards N₂O₄ (the left) and the amount of NO₂ decreases.
7. 2SO₂(g) + O₂(g) ⇌ 2SO₃(g); the forward reaction is exothermic. The temperature is increased. Predict the shift and the effect on the amount of SO₃.
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Higher temperature favours the endothermic direction. The reverse reaction is endothermic, so the position shifts towards the reactants and the amount of SO₃ decreases.
8. A catalyst is added to a mixture that is already at equilibrium. State the effect on the position of equilibrium and explain why.
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There is no change to the position. The catalyst speeds up the forward and reverse reactions equally, so the rates stay equal and the amounts stay constant. If the system had not yet reached equilibrium, the catalyst would get it there sooner.
9. For N₂(g) + 3H₂(g) ⇌ 2NH₃(g), with an exothermic forward reaction, copy and complete the table for each change: effect on rate, and effect on equilibrium yield of NH₃.
| Change | Rate | Yield of NH₃ |
|---|---|---|
| Add a catalyst | ? | ? |
| Increase temperature | ? | ? |
| Increase pressure | ? | ? |
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| Change | Rate | Yield of NH₃ |
|---|---|---|
| Add a catalyst | faster | no change |
| Increase temperature | faster | decreases |
| Increase pressure | faster | increases (4 gas molecules on left, 2 on right) |
10. For CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g), explain the effect of increasing pressure on the position of equilibrium and on the rate.
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Gas molecules: left 1 + 1 = 2; right 1 + 1 = 2. The counts are equal, so pressure does not shift the position. The rate does increase, because the gas particles are closer together and collide more often.
11. The table below contains invented data for practice only, not real plant data, for an exothermic reversible reaction.
| Temperature (°C) | Relative rate (arbitrary units) | Equilibrium yield (%) |
|---|---|---|
| 250 | 2 | 80 |
| 400 | 40 | 45 |
| 550 | 500 | 12 |
(a) Which temperature gives the highest yield? (b) Which gives the fastest reaction? (c) Explain why a temperature near 400 °C might be chosen.
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(a) 250 °C, with 80% yield. (b) 550 °C, with a relative rate of 500 units.
(c) The two goals conflict. At 250 °C the yield is high but the rate is very low, so little product is made in a given time. At 550 °C the rate is high but only 12% of the mixture is product. Around 400 °C gives a moderate yield (45%) at a much more useful rate (40 units). That is a compromise.
12. In the reaction N₂ + 3H₂ ⇌ 2NH₃, 28 g of nitrogen reacts in the ideal case (Ar: N = 14, H = 1). (a) Calculate the maximum mass of ammonia. (b) A plant obtains 13.6 g in one pass. Calculate the percentage yield.
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(a) Moles of N₂ = 28 ÷ 28 = 1.0 mol. The ratio N₂ : NH₃ is 1 : 2, so 2.0 mol NH₃. Mr of NH₃ = 14 + 3 = 17. Mass = 2.0 × 17 = 34 g.
(b) Percentage yield = 13.6 ÷ 34 × 100 = 40%.
Check: 34 × 0.40 = 13.6. The low yield per pass is why unreacted gases are recycled.
If you got these wrong
| Error type | Questions | Go to |
|---|---|---|
| Writing or balancing the equation | 1, 2 | Represent a reversible reaction |
| Saying reactions stop or amounts are equal; open versus closed | 3, 4 | Explain a dynamic state |
| Wrong direction of shift; equal gas counts | 5, 6, 7, 10 | Predict a shift under a stated change |
| Mixing up rate and position; catalyst | 8, 9 | Separate rate change from equilibrium position |
| Compromise wording or yield calculation | 11, 12 | Interpret a yield-rate compromise |
The mole and equation-ratio tutor and equation balance reasoning trainer are useful for questions 2 and 12.
If your errors sit in a particular habit rather than a particular lesson, our teachers in online one-to-one Chemistry tuition can go through your written working and find it.