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Open Kc & Qc
Kc expressions & equilibrium response
Practise Kc expressions, then disturb a Haber equilibrium and watch Qc move back towards Kc.
AQA & OCR A · Kc, Kp and equilibrium response
Equilibrium
Build the expression first, keep the equilibrium amounts connected to the equation, then choose the quantity the question actually needs: concentration for Kc or partial pressure for Kp.
Choose the bit you need
Calculations
Practise Kc
Kc calculations
Turn equilibrium moles into concentrations, use stoichiometric changes and calculate Kc with the right powers and units.
Year 13
Practise Kp
Kp & partial pressure
Work from equilibrium moles through mole fractions and partial pressures before calculating Kp.
Reasoning
Practise equilibrium changes
What changes K?
Separate a shift in equilibrium position from a change in the value of Kc or Kp.
Build the quantity before you substitute
Kc uses equilibrium concentrations. Kp uses equilibrium partial pressures. If the question starts with moles, convert them to the quantity required by the constant before substituting.
- 1Balance the equationThe coefficients control both equilibrium changes and powers in the expression.
- 2Find equilibrium amountsApply the stoichiometric change to each species.
- 3Convert to the right quantityKc: concentration. Kp: mole fraction, then partial pressure.
- 4Calculate K and its unitsWrite every power before using the calculator.
Where AQA and OCR A differ
- Kc work uses homogeneous equilibria. Year 12 includes Kc expressions, calculations and the effect of temperature on Kc.
- Year 13 adds Kp for homogeneous gaseous equilibria, including mole fractions, partial pressures and quantitative calculations.
- Changing concentration or pressure can change equilibrium composition, but at fixed temperature it does not change Kc or Kp. A catalyst changes the rate of reaching equilibrium, not the equilibrium constant.
- Year 12 includes homogeneous Kc expressions/calculations and interpreting the magnitude of Kc.
- Year 13 extends Kc and Kp to equilibria where pure solids and pure liquids are omitted from the equilibrium expression.
- OCR A expects mole fraction and partial-pressure work, and can require equilibrium amounts to be linked through stoichiometric changes. Quadratic equations are not required here.