AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Students should be able to: Ɵ use E values to predict the direction of…
Full specification wording
Electrode potentials and cells (A-level only): Students should be able to: Ɵ use E values to predict the direction of simple redox reactions calculate the EMF of a cell write and apply the conventional representation of a cell.
Three skills sit together here.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 6 of 7
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Three skills sit together here. To predict direction, compare E⦵ values: the more positive half-equation runs forwards as a reduction and the more negative one is forced backwards as an oxidation. To calculate EMF, subtract: EMF = E⦵(more positive) − E⦵(more negative). For zinc and copper, EMF = +0.34 − (−0.76) = +1.10 V. To build the overall equation, scale the two half-equations so the electrons cancel and then add them. E⦵ itself is never multiplied by that scaling factor. For conventional cell representation, place the more negative half-cell on the left (oxidation), use a single vertical line for phase boundaries, a double line for the salt bridge, and include a Pt electrode if no solid metal is present: Zn(s) | Zn²⁺(aq) ‖ Cu²⁺(aq) | Cu(s).
What earns the marks
- Identifying which half-equation runs in the reverse, oxidation, direction.
- Subtracting the less positive E⦵ from the more positive E⦵.
- Scaling the half-equations so the electrons cancel before adding them.
- Correctly applying conventional cell representation with phase boundaries and salt bridge.
Common mistakes and how to put them right
Adding the two E⦵ values instead of subtracting them to find the EMF.
Multiplying E⦵ by the factor used to balance the electrons.
Put it right: E⦵ is independent of stoichiometry.
Omitting the Pt electrode in a cell diagram when neither species is a solid metal.
Put it right: A conductor is always required.
Exam tips
- Write both half-equations as reductions first, then decide which one to reverse; do not reverse anything on the page until you have compared the values.
- When writing cell diagrams, build outwards from the salt bridge: oxidised species next to the double line.
Try the lesson
The start of the Electrode potentials and cells (A-level only): Students should be able to: Ɵ use E values to predict the direction of simple redox reactions calculate the EMF of a cell write and apply the conventional representation of a cell. lesson, made from checked teaching notes for this specification point.
Have a go first
A cell is made from Zn²⁺/Zn (EƟ = -0.76 V) and Cu²⁺/Cu (EƟ = +0.34 V). What is the standard cell EMF?
Listen
Three skills sit together here. To predict direction, compare E⦵ values: the more positive half-equation runs forwards as a reduction and the more negative one is forced backwards as an oxidation. To calculate EMF, subtract: EMF = E⦵(more positive) − E⦵(more negative). For zinc and copper, EMF = +0.34 − (−0.76) = +1.10 V. To build the overall equation, scale the two half-equations so the electrons cancel and then add them. E⦵ itself is never multiplied by that scaling factor. For conventional cell representation, place the more negative half-cell on the left (oxidation), use a single vertical line for phase boundaries, a double line for the salt bridge, and include a Pt electrode if no solid metal is present: Zn(s) | Zn²⁺(aq) ‖ Cu²⁺(aq) | Cu(s). A strong answer earns marks for these points. identifying which half-equation runs in the reverse, oxidation, direction. subtracting the less positive E⦵ from the more positive E⦵. scaling the half-equations so the electrons cancel before adding them. correctly applying conventional cell representation with phase boundaries and salt bridge. Exam tip: Write both half-equations as reductions first, then decide which one to reverse; do not reverse anything on the page until you have compared the values.
Key words
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EMF calculation
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feasibility prediction
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combining half-equations
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See every definition
- EMF calculation
- Subtract the less positive E⦵ from the more positive E⦵, since EMF = E⦵(more positive) − E⦵(more negative).
- feasibility prediction
- Compare E⦵ values so that the more positive half-equation runs forwards as a reduction and the more negative one is forced backwards as an oxidation.
- combining half-equations
- Scale the two half-equations so the electrons cancel and then add them, never multiplying E⦵ by that scaling factor.
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Test your understanding of Electrode potentials and cells (A-level only): Students should be able to: Ɵ use E values to predict the direction of simple redox reactions calculate the EMF of a cell write and apply the conventional representation of a cell.
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