Edexcel International A Level Chemistry
Edexcel International A Level Chemistry Chemical Equilibria Exam Questions
Free Edexcel International A Level Chemistry exam questions for Chemical Equilibria, with a mark scheme for every pack. Covers Kc and Kp expressions, heterogeneous equilibria, partial pressures, equilibrium calculations and how conditions affect K.
Chemical Equilibria exam questions
1 question pack of real past exam questions, each with its mark scheme. Do not stop after a few: answer as many as you can without notes, then mark every answer with the mark scheme. The more questions you do, the more quickly you will spot how examiners ask about each idea.
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About Edexcel International A Level Chemistry Chemical Equilibria
Chemical equilibria in Unit 4 of Edexcel International A Level Chemistry turns the qualitative ideas of Unit 2 Equilibria into calculations. You write expressions for Kc and Kp, including heterogeneous systems where solids are omitted, calculate equilibrium amounts from initial amounts, and work out units. A key idea is that only temperature changes the value of K, and the position of equilibrium shifts because the concentration ratio no longer equals K. The topic links to Entropy (feasibility) and Acid-base equilibria. Examiners set multi-step Kc and Kp calculations and ask you to explain changes using the equilibrium constant.
What you need to know
- Kc For aA + bB and cC + dD, Kc = [C]^c[D]^d / [A]^a[B]^b using equilibrium concentrations in mol dm-3. Units depend on the powers and may cancel completely.
- Equilibrium calculations Use an initial, change, equilibrium table in moles, apply the mole ratio, then divide by volume to get concentrations before substituting.
- Kp For gaseous equilibria, partial pressure = mole fraction x total pressure, and Kp uses partial pressures. Mole fraction = moles of gas / total moles of gas.
- Heterogeneous equilibria Pure solids and liquids are left out of Kc and Kp expressions, for example Kp = p(CO2) for CaCO3(s) and CaO(s) + CO2(g).
- Effect of temperature on K For an exothermic forward reaction K decreases as temperature increases; for an endothermic forward reaction K increases. This explains the shift in equilibrium position.
- Concentration, pressure and catalysts These do not change K. Changing concentration or pressure changes the quotient, so the composition shifts until the ratio equals K again. A catalyst affects only the rate.
Common mistakes in Chemical Equilibria
- Using moles instead of concentrations in Kc when the number of moles differs on each side. Divide by volume first.
- Including solids or water (as solvent) in heterogeneous expressions.
- Saying increasing pressure changes Kp. Kp depends only on temperature.
- Forgetting units or getting them upside down. Substitute the units into the expression and cancel.
- Using the initial amounts in the expression instead of equilibrium amounts.
Try three Chemical Equilibria exam-style questions
1. 1.00 mol of H2 and 1.00 mol of I2 are heated in a 1.00 dm3 sealed flask. At equilibrium 1.56 mol of HI is present. Calculate Kc for H2 + I2 and 2HI. (3 marks)
Show the mark scheme answer
Moles H2 and I2 reacting = 1.56 / 2 = 0.78, so equilibrium H2 = I2 = 0.22 mol (0.22 mol dm-3); Kc = 1.56^2 / (0.22 x 0.22); = 50.3 (no units).
2. For N2O4(g) and 2NO2(g), the partial pressures at equilibrium are 60 kPa N2O4 and 40 kPa NO2. Calculate Kp and give its units. (2 marks)
Show the mark scheme answer
Kp = p(NO2)^2 / p(N2O4) = 40^2 / 60 = 26.7; units kPa.
3. The forward reaction in an equilibrium is exothermic. Explain, in terms of K, why increasing the temperature reduces the yield. (3 marks)
Show the mark scheme answer
K decreases when temperature increases for an exothermic reaction; the ratio of products to reactants is now greater than the new K; so the position shifts to the left, reducing the yield until the ratio equals K.
Chemical Equilibria FAQs
What does Edexcel International A Level Chemistry Chemical equilibria cover?
It covers writing Kc and Kp expressions, heterogeneous equilibria, mole fractions and partial pressures, equilibrium calculations with units, and how temperature, pressure, concentration and catalysts affect K and equilibrium position.
Which unit is Chemical equilibria examined in?
Chemical equilibria is part of Unit 4, building on the qualitative Equilibria topic in Unit 2.
How do I avoid mistakes in Kc calculations?
Always set out an initial, change and equilibrium table, use the mole ratios carefully, convert to concentrations and substitute units alongside numbers.
