13622.06R 13 Some standard electrode potentials are given below. Half-equation for standard electrode potential EO — /V V3+(aq) + e- ⇌ V2+(aq) -0.26 SO2 4 -(aq) + 4H+(aq) + 2e- ⇌ SO2(g) + 2H2O(l) +0.17 VO2+(aq) + 2H+(aq) + e- ⇌ V3+(aq) + H2O(l) +0.32 Cu2+(aq) + 2e- ⇌ Cu(s) +0.34 I2(aq) + 2e- ⇌ 2I-(aq) +0.54 VO+ 2(aq) + 2H+ (aq) + e- ⇌ VO2+(aq) + H2O(l) +1.00 Cl2(g) + 2e- ⇌ 2Cl-(aq) +1.36 (a) VO+ 2 in acidic solution can be reduced to VO2+ by iodide ions. (i) Write an ionic equation for the reaction. [2] (ii) Calculate the emf for the reaction in (a)(i). Answer [1] 13622.06R [Turn over (b) A solution containing 131 mg of (VO2)2SO4 is acidified using an excess of sulfuric acid and mixed with a solution containing 441 mg of VSO4. The following reaction occurs. (VO2)2SO4(aq)  +  4H2SO4(aq)  +  4VSO4(aq)  →  3V2(SO4)3(aq)  +  4H2O(l) (i) Complete the table, giving the colour of the vanadium compounds in solution. Vanadium compound Colour (VO2)2SO4(aq) VSO4(aq) V2(SO4)3(aq) [3] (ii) Calculate the mass, in mg, of V2(SO4)3 which will be present in the solution when the reaction is complete. Answer mg [4] (iii) Identify an oxidising agent from the table of standard electrode potentials which would oxidise vanadium from the +2 to the +4 oxidation state but not to the +5 oxidation state. [1] 13622.06R (c) Describe how the standard electrode potential of the Cu2+/Cu half-cell could be measured using a standard hydrogen electrode. State the conditions needed. In this question you will be assessed on your written communication skills including the use of specialist scientific terms. [6] 13622.06R [Turn over (d) A hydrogen-oxygen fuel cell may operate in acidic or alkaline conditions. The standard electrode potentials involved in the alkaline hydrogen-oxygen fuel cell are: EO — / V Electrode 1: 2H2O(l)  +  2e-  ⇌  H2(g)  +  2OH-(aq) -0.83 Electrode 2: O2(g)  +  2H2O(l)  +  4e-  ⇌  4OH-(aq) +0.40 (i) State and explain which electrode (1 or 2) would be the negative electrode. [1] (ii) Calculate the emf of the alkaline hydrogen-oxygen fuel cell. Answer [1] (iii) Write an overall equation for the reaction occurring in the cell. [1] (iv) Complete the conventional cell representation of the alkaline hydrogen- oxygen fuel cell. Pt(s)    H2(g)    OH-(aq)    H2O(l)    O2(g)    H2O(l)    OH-(aq)    Pt(s) [2] (v) State one environmental advantage of using a hydrogen-oxygen fuel cell. [1] 13622.06R BLANK PAGE DO NOT WRITE ON THIS PAGE (Questions continue overleaf)