Question. When steam is passed over hot iron in a closed container, this equilibrium is set up:3Fe(s) + 4H2O(g) ⇌ Fe3O4(s) + 4H2(g) (a) State whether this is a homogeneous or heterogeneous equilibrium. Give a reason. (1)
(b) Write the expression for Kc and state its units. (2)
(c) At equilibrium, [H2O] = 0.020 mol dm−3 and [H2] = 0.040 mol dm−3. Calculate Kc. (1)
(d) State the effect on the position of equilibrium of (i) increasing the pressure, (ii) adding more iron. (2)Model answer.(a) Heterogeneous, because the species are in more than one phase (solids and gases).(b) Kc = [H2]4 / [H2O]4. The solids Fe and Fe3O4 are left out. The units cancel, so there are no units.(c) Kc = (0.040)4 / (0.020)4 = (0.040 / 0.020)4 = 24 = 16(d)(i) No effect: there are 4 gas molecules on each side.
(d)(ii) No effect: iron is a solid, so its concentration is constant and it does not appear in Kc.Examiner's comments. Many candidates include [Fe] and [Fe3O4] in the expression; always check the state symbols first. In (d), count only gas molecules. Answers such as 'shifts right because there is more reactant' for adding iron score zero.
(b) Write the expression for Kc and state its units. (2)
(c) At equilibrium, [H2O] = 0.020 mol dm−3 and [H2] = 0.040 mol dm−3. Calculate Kc. (1)
(d) State the effect on the position of equilibrium of (i) increasing the pressure, (ii) adding more iron. (2)Model answer.(a) Heterogeneous, because the species are in more than one phase (solids and gases).(b) Kc = [H2]4 / [H2O]4. The solids Fe and Fe3O4 are left out. The units cancel, so there are no units.(c) Kc = (0.040)4 / (0.020)4 = (0.040 / 0.020)4 = 24 = 16(d)(i) No effect: there are 4 gas molecules on each side.
(d)(ii) No effect: iron is a solid, so its concentration is constant and it does not appear in Kc.Examiner's comments. Many candidates include [Fe] and [Fe3O4] in the expression; always check the state symbols first. In (d), count only gas molecules. Answers such as 'shifts right because there is more reactant' for adding iron score zero.