Chemical EquilibriaEdexcel International A Level Chemistry: Topic test

1. 1
   Hydrogen and iodine react reversibly in a sealed 2.00 dm³ flask at 700 K: H₂(g) + I₂(g) ⇌ 2HI(g). A mixture of 0.500 mol H₂ and 0.500 mol I₂ is allowed to reach equilibrium, at which 0.786 mol of HI is present.

   (a)

   Which is the correct expression for $K_c$ for this reaction?

   [1 mark]

   • A$K_c = \frac{[H_2][I_2]}{[HI]^2}$
   • B$K_c = \frac{[HI]^2}{[H_2][I_2]}$
   • C$K_c = \frac{[HI]}{[H_2][I_2]}$
   • D$K_c = \frac{[HI]^2}{[H_2]+[I_2]}$

   (b)

   The pressure in the flask is doubled by halving its volume at constant temperature. Which statement is correct?

   [1 mark]

   • A$K_c$ doubles because the concentrations double.
   • BThe position of equilibrium shifts to the right, giving more HI.
   • CThe position of equilibrium shifts to the left, giving less HI.
   • DNeither the position of equilibrium nor $K_c$ changes

   (c)

   Calculate the value of $K_c$ for this equilibrium at 700 K.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Phosphorus(V) chloride dissociates on heating: PCl₅(g) ⇌ PCl₃(g) + Cl₂(g). At 500 K an equilibrium mixture at a total pressure of 2.00 atm contains 0.400 mol PCl₅, 0.300 mol PCl₃ and 0.300 mol Cl₂.

   (a)

   What is the partial pressure of PCl₃ in the equilibrium mixture?

   [1 mark]

   • A0.600 atm
   • B0.300 atm
   • C0.750 atm
   • D0.800 atm

   (b)

   $K_p$ for this reaction increases as the temperature is raised. Which statement is correct?

   [1 mark]

   • AThe forward reaction is exothermic.
   • BThe forward reaction is exothermic, because a higher temperature increases the forward rate more than the reverse rate.
   • CThe forward reaction is endothermic, so the position of equilibrium moves to the right as temperature rises
   • DThe forward reaction is endothermic, but the position is unchanged because $K_p$ is a constant.

   (c)

   Calculate the value of $K_p$ at 500 K, including its units.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Steam is passed over red-hot iron in a closed vessel at 1000 K and the system reaches equilibrium: 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g). At equilibrium the partial pressure of steam is 0.200 atm and that of hydrogen is 0.600 atm.

   (a)

   Write the expression for $K_p$ for this equilibrium and calculate its value, including units if any.

   [3 marks]

   (b)

   In a second experiment at 1000 K the equilibrium partial pressure of hydrogen is 1.20 atm. Calculate the equilibrium partial pressure of steam and the total pressure of gas.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Methanol is made industrially at about 523 K and 90 atm over a copper-based catalyst: CO(g) + 2H₂(g) ⇌ CH₃OH(g), ΔH = −91 kJ mol⁻¹. In a laboratory experiment 1.00 mol of CO and 2.00 mol of H₂ were allowed to reach equilibrium at 523 K and a constant total pressure of 90.0 atm. The equilibrium mixture contained 0.600 mol of CH₃OH.

   (a)

   Calculate the value of $K_p$ for this equilibrium at 523 K, including its units. Show how you find each partial pressure.

   [6 marks]

   (b)

   Explain how raising the temperature, increasing the pressure and adding a catalyst each affect the equilibrium yield of methanol and the value of $K_p$. Suggest why 523 K is used rather than a much lower temperature.

   [6 marks]

   Total for question 4: 12 marks

5. 5
   A gas-phase reaction has $\Delta S_{total}$ = +57.6 J K⁻¹ mol⁻¹ at 298 K, and the forward reaction is endothermic. Use R = 8.31 J K⁻¹ mol⁻¹.

   (a)

   Which value of $\Delta S_{total}$ corresponds to a reaction with an equilibrium constant $K$ less than 1?

   [1 mark]

   • A+12.0 J K⁻¹ mol⁻¹
   • B0 J K⁻¹ mol⁻¹
   • C−12.0 J K⁻¹ mol⁻¹
   • D+120 J K⁻¹ mol⁻¹

   (b)

   The temperature is raised. What happens to $\Delta S_{total}$ and to $K$ for this endothermic reaction?

   [1 mark]

   • ABoth increase
   • B$\Delta S_{total}$ increases but $K$ decreases.
   • CBoth decrease.
   • D$\Delta S_{total}$ is unchanged, so $K$ is unchanged.

   (c)

   Calculate the equilibrium constant $K$ for this reaction at 298 K.

   [2 marks]

   Total for question 5: 4 marks

6. 6
   Ethanoic acid and ethanol react in a liquid mixture at 298 K: CH₃COOH(l) + C₂H₅OH(l) ⇌ CH₃COOC₂H₅(l) + H₂O(l). The equilibrium constant $K_c$ is 4.00 at this temperature. A mixture of 1.00 mol of ethanoic acid and 1.00 mol of ethanol is left to reach equilibrium.

   (a)

   What are the units of $K_c$ for this reaction?

   [1 mark]

   • Amol dm⁻³
   • BNo units
   • Cmol⁻¹ dm³
   • Dmol² dm⁻⁶

   (b)

   An acid catalyst is added to the mixture. Assuming it acts only as a catalyst, what is its effect?

   [1 mark]

   • AMore ester forms at equilibrium and $K_c$ increases.
   • BMore ester forms at equilibrium but $K_c$ is unchanged.
   • CLess ester forms at equilibrium and $K_c$ decreases.
   • DThe equilibrium is reached faster but the amount of ester and $K_c$ are unchanged

   (c)

   Calculate the amount, in mol, of ethyl ethanoate present at equilibrium.

   [2 marks]

   Total for question 6: 4 marks

7. 7
   The water–gas shift reaction is used to make hydrogen: CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g). The equilibrium constant $K_c$ is 9.0 at 700 K and 1.00 at 1100 K.

   (a)

   Deduce whether the forward reaction is exothermic or endothermic. Explain how the position of equilibrium changes when the temperature is raised from 700 K to 1100 K.

   [3 marks]

   (b)

   At 1100 K a mixture of 1.00 mol of CO and 3.00 mol of H₂O is allowed to reach equilibrium in a closed vessel. Calculate the amount, in mol, of hydrogen at equilibrium.

   [4 marks]

   Total for question 7: 7 marks

8. 8
   Hydrogen for industry is made by steam reforming: CH₄(g) + H₂O(g) ⇌ CO(g) + 3H₂(g), ΔH = +206 kJ mol⁻¹ and $\Delta S_{system}$ = +211 J K⁻¹ mol⁻¹. In an experiment at 1100 K, 0.300 mol of CH₄ and 0.300 mol of H₂O were heated in a closed vessel at a constant total pressure of 2.00 atm. At equilibrium the vessel contained 0.200 mol of CO. Use R = 8.31 J K⁻¹ mol⁻¹.

   (a)

   Calculate the value of $K_p$ at 1100 K, including its units.

   [6 marks]

   (b)

   Calculate $\Delta S_{total}$ for the reaction at 1100 K and use it to calculate $K$. Comment on the agreement with your answer to (a) and explain how $K$ changes if the temperature is raised.

   [6 marks]

   Total for question 8: 12 marks

End of questions