Edexcel A Level Chemistry Topic 14: Redox II Exam Questions

About Edexcel A Level Chemistry Topic 14: Redox II

Topic 14 of Edexcel A Level Chemistry, Redox II, builds on Redox I to make redox quantitative. You learn how electrochemical cells are set up, measure standard electrode potentials against the standard hydrogen electrode, and calculate E cell to predict whether reactions are thermodynamically feasible. The limitations of predictions, such as slow reactions, are included. Fuel cells, such as the hydrogen-oxygen cell, are compared with other energy sources. The topic also covers redox titrations with potassium manganate(VII) and with sodium thiosulfate and iodine. It is assessed in Paper 1, and examiners commonly test cell diagrams and titration calculations.

What you need to know

• Electrochemical cells Two half-cells are joined by a salt bridge (usually saturated KNO3) and a high-resistance voltmeter. Metal/ion half-cells use the metal as electrode; others use platinum.
• Standard hydrogen electrode Hydrogen gas at 100 kPa bubbled over platinum in 1.00 mol dm-3 H+ at 298 K, with E = 0.00 V. Other standard electrode potentials are measured relative to it.
• Calculating E cell E cell = E(right, reduction) - E(left, oxidation). A positive E cell indicates the reaction is thermodynamically feasible.
• Limitations A feasible reaction may not happen if activation energy is high. Non-standard concentrations change electrode potentials, and E cell is proportional to delta S total and ln K.
• Fuel cells In a hydrogen-oxygen fuel cell, hydrogen is oxidised and oxygen reduced, producing water and electricity. They are efficient but hydrogen storage and production raise practical and environmental issues.
• Manganate(VII) titrations MnO4- + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O. Acidify with dilute sulfuric acid; the end point is the first permanent pale pink colour.
• Iodine-thiosulfate titrations An oxidising agent releases I2 from excess KI, then I2 + 2S2O3 2- → 2I- + S4O6 2-. Starch is added near the end point, and the blue-black colour disappears.

Common mistakes in Topic 14: Redox II

• Using hydrochloric acid to acidify manganate(VII) titrations; chloride is oxidised, so dilute sulfuric acid is used.
• Adding starch at the start of an iodine titration rather than near the end point.
• Multiplying electrode potentials by the coefficients in the balanced equation.
• Saying a positive E cell guarantees a reaction will be observed; it may be too slow.
• Forgetting that 2 mol of thiosulfate react with 1 mol of iodine.

Try three Topic 14: Redox II exam-style questions