Flashcards · 9620 · AS Topic 7

Chemical equilibria, Le Chatelier’s principle and Kc (AS Level) flashcards

11 cards on AS Topic 7, subtopics 3.1.7.1 to 3.1.7.2. Pick a subtopic or study them all.

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Conditions for dynamic equilibrium: In a closed system, the rates of the forward and reverse reactions are equal and the concentrations of all species remain constant.
At equilibrium, are the concentrations of reactants and products equal?: Not usually. They are constant, not equal. The reactions have not stopped; they continue at equal rates.
Le Chatelier’s principle: When a change is applied to a system at equilibrium, the position of equilibrium shifts to oppose the change.
Effect on equilibrium position of increasing pressure, and of increasing temperature: Pressure: shifts to the side with fewer moles of gas (no effect if equal). Temperature: shifts in the endothermic direction.
Effect of a catalyst on an equilibrium: No shift in position. It increases the rates of the forward and reverse reactions equally, so equilibrium is reached sooner.
Haber process conditions, and why the temperature is a compromise: N₂ + 3H₂ ⇌ 2NH₃: about 450 °C, 200 atm, iron catalyst. A lower temperature would give a higher yield (exothermic) but an uneconomically slow rate.

Kc expression for aA + bB ⇌ cC + dD: Kc = [C]ᶜ[D]ᵈ ÷ ([A]ᵃ[B]ᵇ), using equilibrium concentrations in mol dm⁻³.
Which change alters the value of Kc?: Only temperature. Changes in concentration, pressure or adding a catalyst move the position (or speed) but not Kc.
For an exothermic forward reaction, what happens to Kc when the temperature rises?: Kc decreases, because the endothermic (reverse) direction is favoured.
Common error when calculating Kc from equilibrium amounts: Using moles directly. Divide each amount by the volume to get concentrations before substituting.
When does Kc have no units?: When the powers in the expression cancel, for example when the total moles on each side of the equation are equal. Otherwise work the units out each time.