OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.1.9 Electrode Potentials and Electrochemical Cells Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.1.9 Electrode Potentials and Electrochemical Cells: what you need to know, common mistakes and exam-style questions with answers. Covers half-cells, the standard hydrogen electrode, standard electrode potentials, the electrochemical series, cell notation, EMF calculations, commercial cells and fuel cells.
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About OxfordAQA International A Level Chemistry 3.1.9 Electrode Potentials and Electrochemical Cells
Electrode potentials and electrochemical cells is an A2 topic examined in Unit 3. You need to use IUPAC conventions for half-equations and cell representations, describe the standard hydrogen electrode and the standard conditions used, and use standard electrode potentials in the electrochemical series to calculate the EMF of a cell and predict the direction of redox reactions. The specification states that the Nernst equation is not required. You also need to understand non-rechargeable, rechargeable and fuel cells, including lithium cells and the alkaline hydrogen-oxygen fuel cell. Required practical 6, measuring the EMF of an electrochemical cell, belongs here.
What you need to know
- Half-cells Half-equations are written as reductions, for example Zn2+(aq) + 2e- ⇌ Zn(s). A salt bridge, often filter paper soaked in potassium nitrate, completes the circuit without introducing reacting ions.
- Standard hydrogen electrode Hydrogen gas at 100 kPa bubbles over a platinum electrode in 1.00 mol dm-3 H+ ions at 298 K. Its standard electrode potential is defined as 0.00 V.
- Electrochemical series The more positive the E value, the more readily the species on the left is reduced, so it is a stronger oxidising agent. The more negative, the stronger the reducing agent on the right.
- Cell notation and EMF Cell diagrams use the format Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s). EMF = E right - E left, which equals E(positive electrode) - E(negative electrode).
- Commercial cells Non-rechargeable cells use irreversible reactions; rechargeable cells use reversible reactions. Lithium cells have a lithium negative electrode and a CoO2 positive electrode; you may be asked to deduce electrode reactions from data.
- Fuel cells In an alkaline hydrogen-oxygen fuel cell, H2 + 2OH- → 2H2O + 2e- at the negative electrode and O2 + 2H2O + 4e- → 4OH- at the positive electrode. Fuel cells produce electricity continuously while fuel is supplied, but hydrogen production and storage raise issues.
Common mistakes in 3.1.9 Electrode Potentials and Electrochemical Cells
- Calculating EMF as left minus right. Use E right - E left (positive electrode minus negative electrode).
- Multiplying E values when balancing half-equations. Electrode potentials are not multiplied by the number of electrons.
- Saying a positive EMF guarantees a fast reaction. It shows feasibility only; rate depends on activation energy.
- Forgetting the platinum electrode for half-cells with no solid metal, such as Fe3+/Fe2+.
Try three 3.1.9 Electrode Potentials and Electrochemical Cells exam-style questions
1. E for Cu2+/Cu = +0.34 V and Zn2+/Zn = -0.76 V. Calculate the EMF of the cell and identify the negative electrode. (2 marks)
Show the mark scheme answer
EMF = 0.34 - (-0.76) = 1.10 V; zinc is the negative electrode.
2. Give the conditions of a standard hydrogen electrode. (3 marks)
Show the mark scheme answer
Hydrogen gas at 100 kPa; 1.00 mol dm-3 H+ ions (for example HCl); 298 K, with a platinum electrode.
3. Explain why fuel cells do not need to be recharged. (1 mark)
Show the mark scheme answer
Fuel (hydrogen) and oxygen are supplied continuously, so the reactants are not used up within the cell.
3.1.9 Electrode Potentials and Electrochemical Cells FAQs
What does OxfordAQA International A Level Chemistry 3.1.9 Electrode Potentials and Electrochemical Cells cover?
It covers half-cells, the standard hydrogen electrode, standard electrode potentials, the electrochemical series, cell notation, EMF calculations, commercial cells and fuel cells.
Which unit is 3.1.9 Electrode Potentials and Electrochemical Cells examined in?
It is examined in Unit 3, an International A2 paper covering inorganic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
Do I need the Nernst equation?
No. The specification says the Nernst equation is not required. You need standard electrode potentials, EMF calculations and predictions of whether reactions are feasible.
