AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
The conventional representation of cells.
Full specification wording
Electrode potentials and cells (A-level only): The conventional representation of cells.
A cell diagram is a shorthand for a whole cell.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 2 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A cell diagram is a shorthand for a whole cell. A single vertical line marks a boundary between two phases, a double vertical line marks the salt bridge, and a comma separates two species that share the same solution. The half-cell with the more negative electrode potential is written on the left, and within each half-cell the species are ordered so that the most reduced form sits on the outside: Zn(s) | Zn²⁺(aq) ‖ Cu²⁺(aq) | Cu(s). Where neither species is a solid metal that can conduct, an inert platinum electrode is written in, as in Cu(s) | Cu²⁺(aq) ‖ Fe³⁺(aq), Fe²⁺(aq) | Pt. Read this way, EMF = E⦵(right) − E⦵(left), and oxidation is always happening at the left-hand electrode.
What earns the marks
- Placing the half-cell with the more negative electrode potential on the left.
- A single vertical line at each phase boundary and a double line for the salt bridge.
- Including a platinum electrode where neither species is a solid metal conductor.
- Ordering the species so that the most reduced form is furthest from the salt bridge.
Common mistakes and how to put them right
Writing the more positive half-cell on the left and then reporting a negative EMF.
Put it right: The more negative half-cell goes on the left.
Using a single vertical line where the salt bridge belongs.
Put it right: Use a double vertical line.
Omitting Pt from the Fe³⁺/Fe²⁺ or H⁺/H₂ half-cell, so the diagram has no conductor.
Put it right: Include an inert electrode if no solid metal is present.
Separating two species in the same solution with a vertical line instead of a comma.
Put it right: Use a comma for species in the same phase.
Exam tips
- Build the diagram outwards from the salt bridge: oxidised species next to the double line on both sides.
- Check your diagram by calculating E⦵(right) − E⦵(left); a positive answer means you have the halves the right way round.
Try the lesson
The start of the Electrode potentials and cells (A-level only): The conventional representation of cells. lesson, made from checked teaching notes for this specification point.
Have a go first
Which symbol is used in the conventional cell representation to show a phase boundary between two different states of the same substance?
Listen
A cell diagram is a shorthand for a whole cell. A single vertical line marks a boundary between two phases, a double vertical line marks the salt bridge, and a comma separates two species that share the same solution. The half-cell with the more negative electrode potential is written on the left, and within each half-cell the species are ordered so that the most reduced form sits on the outside: Zn(s) | Zn²⁺(aq) ‖ Cu²⁺(aq) | Cu(s). Where neither species is a solid metal that can conduct, an inert platinum electrode is written in, as in Cu(s) | Cu²⁺(aq) ‖ Fe³⁺(aq), Fe²⁺(aq) | Pt. Read this way, EMF = E⦵(right) − E⦵(left), and oxidation is always happening at the left-hand electrode. A strong answer earns marks for these points. placing the half-cell with the more negative electrode potential on the left. a single vertical line at each phase boundary and a double line for the salt bridge. including a platinum electrode where neither species is a solid metal conductor. ordering the species so that the most reduced form is furthest from the salt bridge. Exam tip: Build the diagram outwards from the salt bridge: oxidised species next to the double line on both sides.
See it
Key words
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cell diagrams
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phase boundaries
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salt bridge notation
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inert electrodes
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See every definition
- cell diagrams
- A shorthand for a whole cell, in which a single vertical line marks a boundary between two phases and a double vertical line marks the salt bridge.
- phase boundaries
- Marked by a single vertical line, these separate two phases within a cell diagram.
- salt bridge notation
- Shown as a double vertical line in a cell diagram, marking the salt bridge.
- inert electrodes
- A platinum electrode is written in where neither species is a solid metal that can conduct.
Label the diagram
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1Box 1
2Box 2
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