AQA A Level Chemistry 3.1.11 Electrode Potentials and Electrochemical Cells Exam Questions

About AQA A Level Chemistry 3.1.11 Electrode Potentials and Electrochemical Cells

Electrode potentials connect redox chemistry with electrical energy. On the AQA A Level course you need to describe half-cells and the standard hydrogen electrode, use conventional cell representation, calculate cell EMF from standard electrode potentials, and use the electrochemical series to predict whether redox reactions are feasible. You also study commercial cells, including non-rechargeable, rechargeable lithium ion and hydrogen-oxygen fuel cells. Required practical 8 involves measuring the EMF of an electrochemical cell. The topic links back to 3.1.7 Redox and forward to transition metal redox chemistry. Examiners often set unfamiliar half equations and ask for EMF values, cell diagrams and feasibility predictions.

What you need to know

• Half-cells and cells A half-cell contains a species in two oxidation states, such as a metal in a solution of its ions. Two half-cells connected by a wire and a salt bridge (often filter paper soaked in KNO3) make a cell, and the voltmeter measures the EMF.
• Standard hydrogen electrode Hydrogen gas at 100 kPa bubbled 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. Platinum is inert and conducts electricity.
• Cell representation Write the oxidation half-cell on the left and the reduction on the right, for example Zn(s)|Zn2+(aq)||Cu2+(aq)|Cu(s). A single line is a phase boundary and a double line is the salt bridge. Platinum electrodes go on the outside.
• EMF calculations Ecell = E(right, positive electrode) - E(left, negative electrode). Electrode potentials are not multiplied when half equations are scaled up.
• Predicting reactions The species with the more positive E value is the stronger oxidising agent and is reduced. A reaction is predicted to be feasible if the overall EMF is positive, but it may be slow or conditions may not be standard.
• Commercial cells Rechargeable cells such as lithium ion cells have reactions that can be reversed by applying a current. In an alkaline hydrogen-oxygen fuel cell, H2 + 2OH- → 2H2O + 2e- at the negative electrode and O2 + 2H2O + 4e- → 4OH- at the positive electrode.

Common mistakes in 3.1.11 Electrode Potentials and Electrochemical Cells

• Multiplying E values when a half equation is doubled. Electrode potentials do not depend on the amount of substance.
• Using a wire as a salt bridge, or not saying what the salt bridge contains. It must allow ions to move, for example filter paper soaked in potassium nitrate solution.
• Writing cell representations in the wrong order or with the platinum electrode in the middle. Platinum goes on the outside.
• Stating that a reaction with a positive EMF will definitely be observed. It may have a high activation energy or non-standard conditions.
• Describing a fuel cell as being recharged. Fuel cells keep working as long as fuel and oxygen are supplied; they are not recharged.

Try three 3.1.11 Electrode Potentials and Electrochemical Cells exam-style questions