Equilibrium IIEdexcel A-Level Chemistry: Topic test

1. 1
   Phosphorus(V) chloride dissociates when heated in a sealed vessel: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). At equilibrium at 500 K the partial pressures are 0.50 atm for PCl₅ and 0.95 atm for each of PCl₃ and Cl₂.

   (a)

   Which is the correct expression for Kp for this equilibrium?

   [1 mark]

   • A$\frac{p(\mathrm{PCl_5})}{p(\mathrm{PCl_3})\,p(\mathrm{Cl_2})}$
   • B$\frac{p(\mathrm{PCl_3})\,p(\mathrm{Cl_2})}{p(\mathrm{PCl_5})}$
   • C$\frac{p(\mathrm{PCl_3}) + p(\mathrm{Cl_2})}{p(\mathrm{PCl_5})}$
   • D$\frac{p(\mathrm{PCl_3})\,p(\mathrm{Cl_2})}{p(\mathrm{PCl_5})^2}$

   (b)

   What is the value of Kp at 500 K, with its units?

   [1 mark]

   • A1.8 atm
   • B0.55 atm⁻¹
   • C1.8 atm⁻¹
   • D0.90 atm

   (c)

   A catalyst is added to the equilibrium mixture at 500 K. State and explain the effect on the value of Kp.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Limestone is heated in a closed lime kiln: CaCO₃(s) ⇌ CaO(s) + CO₂(g), ΔH = +178 kJ mol⁻¹. At 1000 K the value of Kp is 0.045 atm. Some of each solid is always present at equilibrium.

   (a)

   The temperature of the kiln is raised to 1100 K. Which row shows the effect on the value of Kp and on the amount of CaO at equilibrium?

   [1 mark]

   • AKp decreases and the amount of CaO decreases
   • BKp is unchanged and the amount of CaO increases
   • CKp increases and the amount of CaO increases
   • DKp increases and the amount of CaO decreases

   (b)

   At constant temperature the volume of the closed kiln is halved and equilibrium is re-established. What is the new equilibrium partial pressure of carbon dioxide?

   [1 mark]

   • A0.0225 atm
   • B0.090 atm
   • C0.0020 atm
   • D0.045 atm

   (c)

   Write the expression for Kp for this equilibrium. Explain why the equilibrium partial pressure of carbon dioxide at 1000 K is 0.045 atm whatever the masses of CaCO₃ and CaO present.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Ethanol reacts with butanoic acid in the presence of a few drops of concentrated sulfuric acid: C₃H₇COOH(l) + C₂H₅OH(l) ⇌ C₃H₇COOC₂H₅(l) + H₂O(l). A mixture of 1.50 mol of butanoic acid and 1.50 mol of ethanol is left at constant temperature until equilibrium is reached, when 1.00 mol of ethyl butanoate is present.

   (a)

   Calculate the value of Kc for this equilibrium, including units if appropriate.

   [3 marks]

   (b)

   A further 1.50 mol of ethanol is added to the equilibrium mixture at constant temperature. Explain what happens to the position of equilibrium and to the value of Kc. State the effect of adding more concentrated sulfuric acid as a catalyst on the value of Kc.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Styrene is made by the dehydrogenation of ethylbenzene vapour over an iron(III) oxide catalyst: C₆H₅C₂H₅(g) ⇌ C₆H₅CHCH₂(g) + H₂(g), ΔH = +124 kJ mol⁻¹. In a laboratory experiment, 1.00 mol of ethylbenzene is heated in a sealed vessel at 900 K. At equilibrium the mixture contains 0.80 mol of ethylbenzene, 0.20 mol of styrene and 0.20 mol of hydrogen, and the total pressure is 1.50 atm. The value of Kp at 1000 K is 0.33 atm.

   (a)

   Calculate the value of Kp at 900 K, giving your answer with units.

   [6 marks]

   (b)

   Explain how the equilibrium yield of styrene and the value of Kp are affected by raising the temperature to 1000 K, by doubling the total pressure at 900 K and by using a more active catalyst.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   Solid ammonium hydrogensulfide decomposes in a closed vessel at 298 K: NH₄HS(s) ⇌ NH₃(g) + H₂S(g). Only the solid is present at the start. At equilibrium the total pressure of the gases is 0.66 atm.

   (a)

   Which is the correct expression for Kp for this equilibrium?

   [1 mark]

   • A$\frac{p(\mathrm{NH_3})\,p(\mathrm{H_2S})}{p(\mathrm{NH_4HS})}$
   • B$p(\mathrm{NH_3}) + p(\mathrm{H_2S})$
   • C$\frac{p(\mathrm{NH_4HS})}{p(\mathrm{NH_3})\,p(\mathrm{H_2S})}$
   • D$p(\mathrm{NH_3})\,p(\mathrm{H_2S})$

   (b)

   What is the value of Kp at 298 K, with its units?

   [1 mark]

   • A0.44 atm²
   • B0.33 atm²
   • C0.11 atm²
   • D0.11 atm

   (c)

   The decomposition is endothermic. Predict and explain the effect on the value of Kp of raising the temperature.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Nitrogen monoxide reacts with oxygen in a closed vessel: 2NO(g) + O₂(g) ⇌ 2NO₂(g), ΔH = −114 kJ mol⁻¹. The value of Kp is measured at several temperatures.

   (a)

   The temperature is raised. Which row shows the effect on Kp and on the position of equilibrium?

   [1 mark]

   • AKp decreases and the position of equilibrium shifts to the left
   • BKp increases and the position of equilibrium shifts to the right
   • CKp is unchanged and the position of equilibrium shifts to the left
   • DKp decreases and the position of equilibrium is unchanged

   (b)

   The mixture is compressed at constant temperature and equilibrium is re-established. Which row is correct?

   [1 mark]

   • AThe position shifts to the right and Kp increases
   • BThe position shifts to the right and Kp is unchanged
   • CThe position shifts to the left and Kp is unchanged
   • DThe position is unchanged and Kp is unchanged

   (c)

   Write the expression for Kp for this equilibrium and deduce its units.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   Sulfuryl chloride decomposes on heating: SO₂Cl₂(g) ⇌ SO₂(g) + Cl₂(g), ΔH = +67 kJ mol⁻¹. 1.00 mol of SO₂Cl₂ is heated in a sealed vessel at 400 K. At equilibrium the vessel contains 0.30 mol of SO₂ and the total pressure is 2.0 atm.

   (a)

   Calculate the value of Kp at 400 K, with units.

   [3 marks]

   (b)

   Predict the effect on the value of Kp, and on the equilibrium amount of SO₂, of raising the temperature to 450 K. Predict the effect on the value of Kp and on the position of equilibrium of compressing the mixture at 400 K. Justify your answers.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Ethanal is made by the dehydrogenation of ethanol vapour over a copper catalyst: CH₃CH₂OH(g) ⇌ CH₃CHO(g) + H₂(g), ΔH = +68 kJ mol⁻¹. Only ethanol is present at the start. The value of Kp is 0.0200 atm at 500 K and 0.310 atm at 600 K. A student makes three claims. Claim 1: 'Doubling the total pressure at constant temperature doubles the value of Kp.' Claim 2: 'Using the copper catalyst increases the value of Kp.' Claim 3: 'Raising the temperature from 500 K to 600 K increases the equilibrium yield of ethanal.'

   (a)

   Write the expression for Kp for this reaction. At 600 K the equilibrium partial pressure of ethanol is 0.400 atm. Calculate the equilibrium partial pressure of ethanal and the total pressure.

   [6 marks]

   (b)

   Evaluate the student's three claims.

   [6 marks]

   Total for question 8: 12 marks

End of questions