The equilibrium constant KcEdexcel A-Level Chemistry: Subtopic test

1. 1
   Iron(III) ions react with thiocyanate ions in aqueous solution to form a blood-red complex ion: Fe³⁺(aq) + SCN⁻(aq) ⇌ [Fe(SCN)]²⁺(aq). A student studies the equilibrium mixture.

   (a)

   Which is the correct expression for Kc for this equilibrium?

   [1 mark]

   • A$K_c = \frac{[\mathrm{Fe}^{3+}][\mathrm{SCN}^-]}{[\mathrm{Fe(SCN)}^{2+}]}$
   • B$K_c = \frac{[\mathrm{Fe(SCN)}^{2+}]}{[\mathrm{Fe}^{3+}]+[\mathrm{SCN}^-]}$
   • C$K_c = \frac{[\mathrm{Fe(SCN)}^{2+}]}{[\mathrm{Fe}^{3+}][\mathrm{SCN}^-]}$
   • D$K_c = [\mathrm{Fe}^{3+}][\mathrm{SCN}^-][\mathrm{Fe(SCN)}^{2+}]$

   (b)

   The student writes the equilibrium in reverse: [Fe(SCN)]²⁺(aq) ⇌ Fe³⁺(aq) + SCN⁻(aq). What is the expression for Kc for this equation?

   [1 mark]

   • A$K_c = \frac{[\mathrm{Fe(SCN)}^{2+}]}{[\mathrm{Fe}^{3+}][\mathrm{SCN}^-]}$
   • B$K_c = \frac{[\mathrm{Fe}^{3+}]+[\mathrm{SCN}^-]}{[\mathrm{Fe(SCN)}^{2+}]}$
   • C$K_c = \frac{[\mathrm{Fe}^{3+}]^2[\mathrm{SCN}^-]^2}{[\mathrm{Fe(SCN)}^{2+}]^2}$
   • D$K_c = \frac{[\mathrm{Fe}^{3+}][\mathrm{SCN}^-]}{[\mathrm{Fe(SCN)}^{2+}]}$

   (c)

   The equation is rewritten as 2Fe³⁺(aq) + 2SCN⁻(aq) ⇌ 2[Fe(SCN)]²⁺(aq). Deduce the expression for Kc for this equation and state how it is related to Kc for the original equation.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Calcium carbonate decomposes when heated strongly in a sealed container: CaCO₃(s) ⇌ CaO(s) + CO₂(g). A student investigates this heterogeneous equilibrium.

   (a)

   Which is the correct expression for Kc for this equilibrium?

   [1 mark]

   • A$K_c = [\mathrm{CO_2}]$
   • B$K_c = \frac{[\mathrm{CaO}][\mathrm{CO_2}]}{[\mathrm{CaCO_3}]}$
   • C$K_c = \frac{[\mathrm{CaCO_3}]}{[\mathrm{CaO}][\mathrm{CO_2}]}$
   • D$K_c = [\mathrm{CaO}][\mathrm{CO_2}]$

   (b)

   Why do CaCO₃ and CaO not appear in the expression for Kc?

   [1 mark]

   • ASolids do not take part in the reaction
   • BThe concentration of a pure solid is constant, so it is incorporated into the value of Kc
   • CThe concentration of a solid is zero
   • DSolids cannot be present in a system at equilibrium

   (c)

   Deduce the expression for Kc for the equilibrium 3Fe(s) + 4H₂O(g) ⇌ Fe₃O₄(s) + 4H₂(g).

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Methanol is manufactured from carbon monoxide and hydrogen: CO(g) + 2H₂(g) ⇌ CH₃OH(g). All three substances are gases.

   (a)

   Deduce the expression for Kc for the formation of methanol. Deduce the expression for Kc' for the decomposition of methanol, CH₃OH(g) ⇌ CO(g) + 2H₂(g), and state how Kc' is related to Kc.

   [3 marks]

   (b)

   The equation is rewritten as ½CO(g) + H₂(g) ⇌ ½CH₃OH(g). Deduce the expression for Kc'' for this equation, state how it is related to Kc for the original equation, and explain why a value of Kc must be quoted together with the equation it refers to.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Three equilibria are studied: (1) H₂(g) + I₂(g) ⇌ 2HI(g); (2) C(s) + H₂O(g) ⇌ CO(g) + H₂(g); (3) 2SO₂(g) + O₂(g) ⇌ 2SO₃(g).

   (a)

   Deduce the expression for Kc for equilibria (1), (2) and (3), and explain why the expression for equilibrium (2) is not simply products over reactants.

   [6 marks]

   (b)

   Equation (3) is rewritten as SO₂(g) + ½O₂(g) ⇌ SO₃(g) and then as 2SO₃(g) ⇌ 2SO₂(g) + O₂(g). Deduce the expression for Kc for each new equation, and explain how each is related to Kc for equation (3).

   [6 marks]

   Total for question 4: 12 marks

End of questions