Quick answer: Chemical equilibrium is reached when forward and reverse reactions occur at the same rate, so concentrations stay constant. This Chemistry 11-12 test on chemical equilibrium has 8 questions worth 20 marks, to do in 45 minutes: writing Kc, calculating Kc and butane isomerisation. The mark scheme is on a separate page; free printable PDF.
This test covers Kc and Kp expressions, calculations, Le Chatelier’s principle, the reaction quotient and the effect of temperature on K. Show your working and include units where needed.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: IB R2.3
Question 1 Writing Kc (2 marks)
For 2CO(g) + O₂(g) ⇌ 2CO₂(g):- Write the Kc expression. (1 mark)
- State the units of Kc. (1 mark)
Question 2 Calculating Kc (3 marks)
At equilibrium A(g) + 2B(g) ⇌ C(g) has [A] = 0.10, [B] = 0.20 and [C] = 0.60 mol/dm³. Calculate Kc with units.Question 3 Butane isomerisation (3 marks)
Butane ⇌ methylpropane has Kc = 2.5. Starting from 1.00 mol of butane only, calculate the moles of each isomer at equilibrium.Question 4 Nitrogen monoxide yield (3 marks)
N₂(g) + O₂(g) ⇌ 2NO(g) has ΔH = +180 kJ/mol. State and explain the effect on the equilibrium yield of NO of (a) raising the temperature and (b) raising the pressure.Question 5 Kc and temperature (3 marks)
- State whether the forward reaction is exothermic or endothermic. (1 mark)
- Explain your answer using Le Chatelier’s principle. (1 mark)
- Predict the trend of Kc above 700 K. (1 mark)
Question 6 Calculating Kp (2 marks)
At equilibrium N₂O₄(g) ⇌ 2NO₂(g) has p(N₂O₄) = 25 kPa and p(NO₂) = 50 kPa. Calculate Kp with units.Question 7 Role of a catalyst (2 marks)
Explain why a catalyst does not change the position of equilibrium but reduces the time needed to reach it.Question 8 Direction of change (2 marks)
A reaction has K = 4.0. A mixture has Q = 1.5. State the direction in which the mixture changes and explain.
