CCEA A Level Chemistry
CCEA A Level Chemistry 4.4 Equilibrium Exam Questions
Free CCEA A Level Chemistry revision for 4.4 Equilibrium: what you need to know, common mistakes and exam-style questions with answers. Covers calculating equilibrium concentrations and amounts, Kc calculations with units for homogeneous systems and the effect of temperature on Kc.
Practise 4.4 Equilibrium with past papers
Once you have learnt this topic, find the 4.4 Equilibrium questions in real CCEA A Level Chemistry past papers and mark them with the mark scheme. Doing lots of real questions is the quickest way to learn how examiners ask about each idea.
StemUp mark scheme flashcards
A-Level Chemistry Flashcards for 2027 Onwards
Every answer is written in mark scheme wording, so you learn exactly what examiners give marks for. Then test yourself with the questions on this page.
From £10.99View flashcards
About CCEA A Level Chemistry 4.4 Equilibrium
Section 4.4 of Unit A2 1 extends the qualitative equilibrium work of AS 2 (2.10) into calculations. You need to calculate equilibrium concentrations from initial amounts and the change at equilibrium, and calculate Kc with units, for homogeneous systems only. Typical examples are esterification of ethanoic acid with ethanol and the hydrogen iodide equilibrium. You also need to explain that only temperature changes Kc, and to predict the direction of change using the sign of delta H. Examiners usually give a multi-step calculation, so a clear table of initial, change and equilibrium amounts is the best approach. These ideas lead straight into acid-base equilibria (4.5).
What you need to know
- ICE table Write initial moles, the change (using the mole ratio) and the equilibrium moles for every species. Divide by volume to get concentrations, unless volume cancels.
- Kc expression Kc = products raised to their coefficients over reactants raised to their coefficients, using equilibrium concentrations. Only homogeneous systems are required.
- Units Substitute mol dm-3 for each term and cancel. When the total moles on each side are equal, volume cancels and Kc has no units.
- Temperature and Kc For an exothermic forward reaction, raising temperature decreases Kc. For an endothermic forward reaction, raising temperature increases Kc. Changing concentration or pressure does not change Kc.
- Esterification example CH3COOH + C2H5OH in equilibrium with CH3COOC2H5 + H2O. Kc is about 4 at room temperature and has no units.
Common mistakes in 4.4 Equilibrium
- Using initial amounts in the Kc expression. Use equilibrium amounts only.
- Forgetting to divide by volume when moles do not cancel. Convert to concentrations when the total moles of reactants and products differ.
- Forgetting to apply the mole ratio in the change row. If 2HI forms from H2 + I2, the change in HI is twice the change in H2.
- Saying adding a catalyst changes Kc. It does not change Kc or the equilibrium position.
- Giving units when none are needed. Check by cancelling before writing units.
Try three 4.4 Equilibrium exam-style questions
1. 1.00 mol ethanoic acid and 1.00 mol ethanol reach equilibrium with 0.67 mol ethyl ethanoate formed. Calculate Kc. (3 marks)
Show the mark scheme answer
At equilibrium: acid 0.33, ethanol 0.33, ester 0.67, water 0.67; Kc = (0.67 x 0.67) / (0.33 x 0.33); = 4.1 (no units).
2. 0.50 mol H2 and 0.50 mol I2 are heated in a sealed vessel. At equilibrium 0.78 mol HI is present. Calculate Kc for H2 + I2 in equilibrium with 2HI. (3 marks)
Show the mark scheme answer
H2 and I2 used = 0.39 each, so 0.11 mol each remain; Kc = 0.78^2 / (0.11 x 0.11); = 0.6084 / 0.0121 = 50 (no units).
3. The forward reaction of an equilibrium is endothermic. State and explain the effect of increasing temperature on Kc. (2 marks)
Show the mark scheme answer
Kc increases; equilibrium shifts in the endothermic (forward) direction so more products form.
4.4 Equilibrium FAQs
What does CCEA A Level Chemistry 4.4 Equilibrium cover?
It covers calculating equilibrium amounts and Kc with units for homogeneous systems, and the effect of temperature on Kc.
Which unit is 4.4 Equilibrium examined in?
It is examined in Unit A2 1: Further Physical and Organic Chemistry, a 2 hour paper with a multiple-choice section and structured questions. Related practical skills can also be assessed in Unit A2 3.
What is the best method for Kc calculations?
Use a table of initial, change and equilibrium moles, convert to concentrations if needed, substitute into Kc, and work out units by cancelling.
