Equilibrium I
Notes for every section, eight new exam-style questions with full worked solutions, and online recall cards for your review days. Nothing here is copied from the book.
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10.1 Reversible reactions and dynamic equilibrium
A reversible reaction can go in both directions, shown by ⇌.
Dynamic equilibrium: the forward and reverse reactions happen at the same rate, so the concentrations of reactants and products stay constant.
Equilibrium is only reached in a closed system, where nothing can enter or leave. "Dynamic" means both reactions are still happening; the concentrations are constant, not necessarily equal.
Check yourself: At equilibrium, are the concentrations of reactants and products equal?
Not necessarily; they are constant
10.2 Le Chatelier's principle and concentration
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10.3 Changing the pressure
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10.4 Changing the temperature
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10.5 Catalysts and equilibrium
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10.6 Equilibria you can see in the lab
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10.7 Industry: the compromise between yield and rate
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10.8 The equilibrium constant, Kc
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Extra worked questions
Cover the solution, answer on paper, then mark yourself against each step. Each step shows the mark it earns.
All 8 worked solutions are free for members. The first one is open to everyone.
Writing Kc for the Haber process
Write the expression for Kc for N2(g) + 3H2(g) ⇌ 2NH3(g), and give its units.
Try it first, then show the worked solution
| Expression | Kc = [NH3]2 ÷ [N2][H2]3 | 1 |
|---|---|---|
| Units | (mol dm−3)2 ÷ (mol dm−3)4 = mol−2 dm6 | 1 |
Answer: [NH3]2 / [N2][H2]3; mol−2 dm6
Examiner tip: Show the cancelling of units. It earns the mark even if your final arrangement has a slip.
A catalyst and equilibrium
An iron catalyst is used in the Haber process. State its effect on (a) the position of equilibrium and (b) the time taken to reach equilibrium. Explain (a).
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Pressure and sulfur trioxide
2SO2(g) + O2(g) ⇌ 2SO3(g). State and explain the effect of increasing the pressure on the yield of SO3.
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Chromate and dichromate
2CrO42−(aq) + 2H+(aq) ⇌ Cr2O72−(aq) + H2O(l). Yellow chromate solution is acidified, then sodium hydroxide is added. Describe and explain the colour changes.
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Temperature and ammonia
For N2 + 3H2 ⇌ 2NH3, ΔH = −92 kJ mol−1. Explain the effect on the yield of ammonia of raising the temperature, and the effect on Kc.
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Heterogeneous Kc expressions
Write Kc expressions for (a) C(s) + H2O(g) ⇌ CO(g) + H2(g) and (b) 3Fe(s) + 4H2O(g) ⇌ Fe3O4(s) + 4H2(g). (c) Explain why the solids are left out.
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Conditions for making ethanol
C2H4(g) + H2O(g) ⇌ C2H5OH(g), ΔH = −45 kJ mol−1. The process uses about 300 °C, 60–70 atm and a phosphoric acid catalyst. Explain the choice of temperature and pressure.
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Choosing conditions for ammonia
Ammonia is made at about 450 °C and 200 atm with an iron catalyst. N2 + 3H2 ⇌ 2NH3, ΔH = −92 kJ mol−1. Explain why these conditions are used.
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Video walkthroughs
Narrated walkthroughs of exam-style questions from this topic of the book, showing where each mark comes from. Try the question first, then watch. Captions are available in the player.
Le Chatelier: the iron thiocyanate equilibrium
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The contact process: yield and pressure
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Making methanol: yield and compromise
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The Haber process: reading the yield graph
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Kc for dissociation in terms of alpha
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Recall cards
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