Chemistry › Chemical equilibria
Chemical equilibria
Unit C06 · 3 lessons.
Most reactions in industry stop short of completion. Where they stop, and how to move that point, starts from dynamic equilibrium and Le Chatelier's principle applied to the Haber and Contact processes. Kc for solutions and Kp for gases then put a number on the position, from concentrations and from partial pressures.
Most marks go on one claim: that a catalyst or a change in concentration changes the constant. Only temperature changes Kc or Kp. A change in concentration or pressure moves the position until the ratio returns to the same value, and a catalyst speeds up the forward and backward reactions equally, so equilibrium is reached sooner and nothing else changes.
The papers that set it are AQA Papers 1, 2 and 3, OCR A Papers 1 and 3, Edexcel Papers 1 and 3 and Cambridge Papers 1, 2 and 4.
Specification sections: AQA 3.1.6.1, 3.1.6.2, 3.1.10 · OCR A 3.2.3, 5.1.2 · Edexcel 10.1 to 10.4, 11.1 to 11.5 · CAIE 7.1, 25.2
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Dynamic equilibrium, Le Chatelier's principle and industrial compromise
Reversible reactions, dynamic equilibrium and the closed system, the position of equilibrium, Le Chatelier's principle for changes in concentration, pressure and temperature, investigating a shift with cobalt(II) chloride, why a catalyst leaves the position unchanged, and the compromise between yield, rate and cost in the Haber process, the Contact process, the hydration of ethene and the synthesis of methanol.
- The equilibrium constant Kc
Writing Kc for homogeneous equilibria and working out its units, calculating Kc from equilibrium amounts and from initial amounts with an ICE table, using Kc to find an equilibrium amount, Kc for a reversed equation, heterogeneous equilibria and the solvent left out of Kc, quenching a sample for titration, the reaction quotient Q, the effect of concentration, pressure, catalysts and temperature on Kc, and the partition coefficient Kpc for a solute between two solvents.
- Kp: equilibria in the gas phase
Mole fraction and partial pressure, writing Kp for gas-phase equilibria and working out its units, calculating Kp from partial pressures and from initial amounts and the total pressure, the degree of dissociation, using Kp to find the composition at a new pressure, Kp for heterogeneous equilibria with a solid present, pressures in atmospheres and converting Kp between units, and the effects of temperature, pressure and catalysts on Kp. A-level only for AQA.
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