Chemistry › Electrochemistry
Electrochemistry
Unit C10 · 3 lessons.
Electrode potentials measure how strongly a species gains electrons. Each is measured against the standard hydrogen electrode, ordered into the electrochemical series, and used to predict whether a redox reaction will go and what voltage a cell will give. The same numbers explain the cells in phones and cars and the hydrogen fuel cell.
Two answers lose marks. A cell's EMF is E°right minus E°left, with the more positive electrode on the right, and writing the subtraction the other way round gives a negative value and the wrong conclusion. And a positive EMF says only that a reaction is feasible: it says nothing about rate, so a reaction with a large activation energy may not be seen at all.
The papers that set it are AQA Papers 1 and 3, OCR A Papers 1 and 3, Edexcel Papers 1 and 3 and Cambridge Paper 4.
Specification sections: AQA 3.1.11.1, 3.1.11.2 · OCR A 5.2.3 · Edexcel 14.3 to 14.17, 13.22 · CAIE 24.2, 24.1
Notes
Illustrated notes for each topic in this unit, with worked examples and a checklist of objectives.
- Electrode potentials and the standard hydrogen electrode
Half-cells and how a potential difference arises at an electrode, the standard hydrogen electrode and standard conditions, the standard electrode potential, platinum electrodes for ion and gas half-cells, the salt bridge and why it holds potassium nitrate, the high-resistance voltmeter, conventional cell representation, E°cell = E°right − E°left, what a positive E°cell does and does not predict, required practical 8, measuring the EMF of a cell, and the conventions other boards use.
- The electrochemical series and predicting feasibility
The electrochemical series as an ordered table of E° values, the relative strengths of oxidising and reducing agents, predicting whether a redox reaction is feasible from the sign of E°cell and writing its equation, the anticlockwise rule, disproportionation, why a feasible reaction may not be seen, how concentration changes an electrode potential and the EMF of a cell, and, for other boards, the Nernst equation, ΔG° = −nFE°cell and the Faraday constant, and the link between E°cell, total entropy change and ln K.
- Commercial cells, rechargeable cells and fuel cells
Non-rechargeable, rechargeable and fuel cells, the electrode reactions and EMF of the lithium-ion cell during discharge and charging, the lead–acid cell in outline, the hydrogen–oxygen fuel cell with alkaline and acidic electrolytes, deducing the reactions in an unfamiliar cell from electrode potentials, the benefits and risks of each kind of cell, and, for other boards, the direct methanol fuel cell and electrolysis: the products at inert electrodes from molten and aqueous electrolytes, Q = It, F = Le and the moles liberated.
Key definitions for this unit
Practical work
The practical capabilities this unit's work belongs to, each with its techniques, its calculations and the errors that cost marks.
Worked in full, from the apparatus to the evaluation:
More for this topic