Equilibrium constants Kc and KpEdexcel International A Level Chemistry: Subtopic test

1. 1
   In the Haber process nitrogen and hydrogen react reversibly: N₂(g) + 3H₂(g) ⇌ 2NH₃(g). A sealed vessel is held at 700 K until equilibrium is reached. The equilibrium concentrations are: N₂ 0.40 mol dm⁻³, H₂ 1.20 mol dm⁻³ and NH₃ 0.60 mol dm⁻³.

   (a)

   Which expression gives Kc for this equilibrium?

   [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 equilibrium?

   [1 mark]

   • Ano units
   • Bmol² dm⁻⁶
   • Cmol⁻² dm⁶
   • Dmol⁻¹ dm³

   (c)

   Calculate the value of Kc at 700 K, including units.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Carbon dioxide reacts with hot carbon: C(s) + CO₂(g) ⇌ 2CO(g). Excess solid carbon is held in a closed vessel at 1000 K and the total pressure at equilibrium is 2.0 atm. The gas mixture at equilibrium contains 0.30 mol of CO₂ and 0.90 mol of CO.

   (a)

   Which expression gives Kp for this equilibrium?

   [1 mark]

   • AKp = p(CO)² / (p(C) × p(CO₂))
   • BKp = p(CO)² / p(CO₂)
   • CKp = p(CO₂) / p(CO)²
   • DKp = 2p(CO) / p(CO₂)

   (b)

   What is the partial pressure of CO at equilibrium?

   [1 mark]

   • A0.75 atm
   • B1.8 atm
   • C0.50 atm
   • D1.5 atm

   (c)

   Calculate Kp at 1000 K, including units.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Dinitrogen tetraoxide dissociates in a sealed flask: N₂O₄(g) ⇌ 2NO₂(g). A student places 0.250 mol of N₂O₄ in a 5.00 dm³ flask at 373 K. At equilibrium 0.050 mol of N₂O₄ remains and the total pressure in the flask is 2.75 atm.

   (a)

   Calculate Kc at 373 K, including units.

   [3 marks]

   (b)

   Calculate Kp at 373 K, including units.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In the Contact process sulfur dioxide is oxidised: 2SO₂(g) + O₂(g) ⇌ 2SO₃(g). In one experiment 4.00 mol of SO₂ and 2.00 mol of O₂ are sealed in a 2.00 dm³ vessel. At equilibrium 3.00 mol of SO₃ is present.

   (a)

   Calculate Kc for this reaction, including units.

   [6 marks]

   (b)

   The experiment is repeated at a higher temperature. The equilibrium mixture contains 2.00 mol of SO₂, 1.00 mol of O₂ and 2.00 mol of SO₃ at a total pressure of 5.0 atm. Calculate Kp, including units.

   [6 marks]

   Total for question 4: 12 marks

End of questions