Edexcel A Level Chemistry
Edexcel A Level Chemistry Topic 11: Equilibrium II Exam Questions
Free Edexcel A Level Chemistry exam questions for Topic 11: Equilibrium II, with a mark scheme for every pack. Covers equilibrium constants Kc and Kp, heterogeneous equilibria, partial pressures, units of K and the effect of temperature on K.
Topic 11: Equilibrium II 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 A Level Chemistry Topic 11: Equilibrium II
Topic 11 of Edexcel A Level Chemistry, Equilibrium II, makes equilibrium quantitative. You write and calculate Kc for homogeneous and heterogeneous equilibria and Kp using mole fractions and partial pressures, and work out the units of each constant. A central idea is that only temperature changes the value of K, and the position of equilibrium shifts because the concentration or pressure terms must change to restore K. The topic also links K to entropy through delta S total = R ln K. It is assessed in Paper 1, builds on Topic 10 and leads to the acid-base equilibria in Topic 12. Examiners often set multi-step Kc and Kp calculations.
What you need to know
- Kc Kc uses equilibrium concentrations in mol dm-3. For heterogeneous equilibria, pure solids and liquids are left out of the expression.
- Kp Kp uses equilibrium partial pressures of gases. Partial pressure = mole fraction x total pressure, and mole fraction = moles of gas/total moles of gas.
- Equilibrium calculations Start with initial moles, use the equation ratio to find changes, calculate equilibrium moles, then convert to concentrations (divide by volume) or partial pressures.
- Units of K Substitute units into the expression and cancel. For N2O4 ⇌ 2NO2, Kc has units mol dm-3 and Kp has units kPa or atm.
- Effect of temperature on K For an exothermic forward reaction, K decreases as temperature increases; for an endothermic forward reaction, K increases. The equilibrium position shifts as a result.
- Effect of concentration, pressure and catalysts These do not change K. A change in pressure or concentration alters the quotient so the system shifts until it again equals K; a catalyst affects neither.
- K and entropy delta S total = R ln K, so a large positive total entropy change corresponds to a large K and a reaction that goes nearly to completion.
Common mistakes in Topic 11: Equilibrium II
- Including solids in a Kc or Kp expression for a heterogeneous equilibrium.
- Using moles instead of concentrations in Kc when the number of moles on each side is different.
- Saying increasing pressure increases Kp; Kp only changes with temperature.
- Using total moles including solids when calculating mole fractions; only gaseous moles count.
- Leaving units off or working them out from the balanced equation instead of from the expression.
Try three Topic 11: Equilibrium II exam-style questions
1. For N2O4(g) ⇌ 2NO2(g) at equilibrium, the total pressure is 100 kPa and the mole fraction of NO2 is 0.400. Calculate Kp and give its units. (3 marks)
Show the mark scheme answer
p(NO2) = 0.400 x 100 = 40.0 kPa, p(N2O4) = 0.600 x 100 = 60.0 kPa; Kp = 40.0^2/60.0 = 26.7; units kPa.
2. Write the Kp expression for CaCO3(s) ⇌ CaO(s) + CO2(g). (1 mark)
Show the mark scheme answer
Kp = p(CO2).
3. The forward reaction in an equilibrium is endothermic. Explain the effect of increasing temperature on Kc and on the position of equilibrium. (2 marks)
Show the mark scheme answer
Kc increases; the concentration of products must increase relative to reactants to match the new Kc, so equilibrium shifts to the right.
Topic 11: Equilibrium II FAQs
What does Edexcel A Level Chemistry Topic 11 Equilibrium II cover?
It covers calculating Kc and Kp, heterogeneous equilibria, partial pressures, units of K and how temperature changes the value of K.
Which paper is Equilibrium II examined in?
Topic 11 is assessed in Paper 1, Advanced Inorganic and Physical Chemistry, and in Paper 3.
How do I set out Kc and Kp calculations?
Use a table with initial moles, change and equilibrium moles, then convert to concentrations or partial pressures before substituting into the expression.
