The equilibrium constant KcAQA A-Level Chemistry: Subtopic test

1. 1
   Nitrogen and hydrogen are sealed in a vessel at 500 K and form ammonia: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). When equilibrium is reached, the concentrations are [N₂] = 0.40 mol dm⁻³, [H₂] = 0.20 mol dm⁻³ and [NH₃] = 0.16 mol dm⁻³.

   (a)

   Which is the correct expression for Kc for this reaction?

   [1 mark]

   • AKc = [NH₃]² / ([N₂][H₂]³)
   • BKc = [N₂][H₂]³ / [NH₃]²
   • CKc = [NH₃] / ([N₂][H₂])
   • DKc = 2[NH₃] / ([N₂] + 3[H₂])

   (b)

   What are the units of Kc for this reaction?

   [1 mark]

   • Amol dm⁻³
   • Bmol⁻¹ dm³
   • Cmol² dm⁻⁶
   • Ddm⁶ mol⁻²

   (c)

   Calculate the value of Kc for this reaction at 500 K, including its units.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Ethanoic acid and ethanol react in the presence of a small amount of sulfuric acid catalyst: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). At room temperature the equilibrium constant Kc for this reaction is 4.0.

   (a)

   What are the units of Kc for this reaction?

   [1 mark]

   • Amol dm⁻³
   • Bmol⁻¹ dm³
   • CThere are no units
   • Dmol² dm⁻⁶

   (b)

   The student doubles the concentration of ethanol in an equilibrium mixture at constant temperature and waits for equilibrium to be re-established. What happens to the value of Kc?

   [1 mark]

   • AIt doubles
   • BIt stays the same
   • CIt halves
   • DIt increases because the equilibrium shifts to the right

   (c)

   A mixture of 1.00 mol of ethanoic acid and 1.00 mol of ethanol is left to reach equilibrium at room temperature. Calculate the amount, in mol, of ethyl ethanoate present at equilibrium.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Dinitrogen tetraoxide, a colourless gas, is in equilibrium with nitrogen dioxide, a brown gas: N₂O₄(g) ⇌ 2NO₂(g), ΔH = +57 kJ mol⁻¹. A sealed 2.00 dm³ vessel initially contains 0.0600 mol of N₂O₄ only. It is held at a fixed temperature until equilibrium is reached, when 0.0800 mol of NO₂ is present.

   (a)

   State and explain the effect of increasing the temperature on the value of Kc, and state the effect on Kc of increasing the pressure by compressing the vessel.

   [3 marks]

   (b)

   Calculate the value of Kc at this temperature, including its units.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Hydrogen and iodine react to form hydrogen iodide: H₂(g) + I₂(g) ⇌ 2HI(g). At 718 K the equilibrium constant Kc is 49. A sealed 3.00 dm³ flask initially contains 0.900 mol of H₂ and 0.900 mol of I₂ and no HI, and is held at 718 K until equilibrium is reached. When the experiment is repeated at a higher temperature, the value of Kc is found to be lower.

   (a)

   Calculate the amounts, in mol, of H₂, I₂ and HI present at equilibrium at 718 K.

   [6 marks]

   (b)

   The chemist makes three separate changes to the equilibrium mixture: (i) the volume of the flask is halved at constant temperature, (ii) a catalyst is added, and (iii) the temperature is raised. For each change, deduce the effect on the equilibrium amount of HI and on the value of Kc.

   [6 marks]

   Total for question 4: 12 marks

End of questions