Equilibrium — A-Level Chemistry Revision
Revise Equilibrium for A-Level Chemistry. Step-by-step explanation, worked examples, common mistakes and exam-style practice aligned to AQA, Edexcel and OCR.
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Go to Atomic Structure (A-Level)What is Equilibrium?
Dynamic equilibrium occurs in reversible reactions when the rate of the forward reaction equals the rate of the backward reaction in a closed system. Le Chatelier's principle is used to predict how the position of equilibrium shifts in response to changes in concentration, pressure, or temperature. The equilibrium constant, Kc (for concentrations) or Kp (for partial pressures), gives a quantitative measure of the position of equilibrium at a specific temperature.
Board notes: All boards thoroughly test Le Chatelier's principle and Kc calculations. AQA and Edexcel often include Kp calculations, requiring students to be comfortable with partial pressures and mole fractions. OCR may place more emphasis on the industrial applications of equilibrium, such as the Haber process for ammonia or the Contact process for sulfuric acid, and the compromises made between rate, yield, and cost.
Step-by-step explanationWorked example
For the reaction N2(g) + 3H2(g) <=> 2NH3(g), the initial concentrations are [N2]=1.0 M, [H2]=2.0 M, and [NH3]=0 M. At equilibrium, [N2]=0.8 M. Calculate Kc. Step 1: Use stoichiometry to find equilibrium concentrations. Change in [N2] = 1.0 - 0.8 = 0.2 M. Change in [H2] = 3 * 0.2 = 0.6 M. Change in [NH3] = 2 * 0.2 = 0.4 M. Equilibrium [H2] = 2.0 - 0.6 = 1.4 M. Equilibrium [NH3] = 0.4 M. Step 2: Write the Kc expression: Kc = [NH3]^2 / ([N2][H2]^3). Step 3: Substitute equilibrium concentrations: Kc = (0.4)^2 / (0.8 * (1.4)^3) = 0.16 / (0.8 * 2.744) = 0.073 dm^6 mol^-2.
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Common mistakes
- 1Stating that the concentrations of reactants and products are equal at equilibrium. At equilibrium, it is the rates of the forward and reverse reactions that are equal, not necessarily the concentrations.
- 2Incorrectly applying Le Chatelier's principle, especially with temperature. An increase in temperature favours the endothermic direction, not necessarily the forward or backward reaction.
- 3Forgetting that catalysts do not affect the position of equilibrium. Catalysts increase the rate of both the forward and backward reactions equally, so the equilibrium is reached faster, but the yield of products remains unchanged.
Equilibrium exam questions
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Step-by-step method
Step-by-step explanation
4 steps · Worked method for Equilibrium
Core concept
Dynamic equilibrium occurs in reversible reactions when the rate of the forward reaction equals the rate of the backward reaction in a closed system. Le Chatelier's principle is used to predict how th…
Frequently asked questions
What is the difference between Kc and Kp?
Kc is the equilibrium constant in terms of molar concentrations of reactants and products. Kp is the equilibrium constant in terms of the partial pressures of gaseous reactants and products. Kp is only used for reactions involving gases.
How does changing pressure affect the equilibrium position?
Changing the pressure only affects equilibria involving gases where there is a different number of moles of gas on each side of the equation. An increase in pressure will shift the equilibrium to the side with fewer moles of gas to counteract the change.