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Chemical Reactivity · Part 2 of 3
9 exam-style questions with model answers, plus 12 quick multi-choice questions — every question on this part of the standard, grouped by the 3 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
State the two features of a system at dynamic equilibrium.
Explain why an equilibrium is described as 'dynamic' rather than 'static'.
Explain why an equilibrium can only be established in a closed system, using the decomposition of calcium carbonate (CaCO3 ⇌ CaO + CO2) as an example. Then explain why 'the concentrations are constant' does not mean 'the concentrations are equal'.
State Le Chatelier's principle.
For N2 + 3H2 ⇌ 2NH3 (ΔrH = −92 kJ mol−1), explain the effect of raising the temperature on the yield of ammonia.
The Haber process (N2 + 3H2 ⇌ 2NH3, ΔrH = −92 kJ mol−1) is run at about 450 °C and 200 atmospheres with an iron catalyst. Explain why these particular conditions are used, discussing the effect of each on both yield and rate, and evaluate the compromise involved.
Write the expression for Kc for the equilibrium N2 + 3H2 ⇌ 2NH3.
A reaction has Kc = 1.2 × 10−5 at 25 °C. Explain what this tells you about the position of equilibrium, and state whether it tells you anything about the rate.
For N2O4 ⇌ 2NO2 (ΔrH = +57 kJ mol−1), a 2.00 L flask at equilibrium contains 0.400 mol N2O4 and 0.200 mol NO2. Calculate Kc with units, then explain what would happen to both the position of equilibrium and the value of Kc if (i) the flask were compressed to 1.00 L and (ii) the temperature were raised.