Define standard electrode and cell potentials
Interpret electrode potentials as reduction potentials relative to a reference and distinguish them from a complete cell potential.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 24. Treat all tabulated electrode potentials as reduction potentials; state conditions, phases, signs and units. Written practice is self-checked
Before you begin
- Use AS redox half-equations, oxidation states, mole ratios, current and charge.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Relative to which reference are standard electrode potentials defined?
What you will learn
- Define standard electrode potential as a relative reduction potential
- Define standard cell potential for a complete cell
- State standard conditions and the role of the reference electrode
Electrode potentials are measured relative to a reference
A standard electrode potential E° is the potential of a half-cell relative to the standard hydrogen electrode, measured under standard conditions. Tables write each half-equation as a reduction; the value indicates its tendency to gain electrons compared with the reference.
An isolated half-cell potential cannot be measured on its own. The measured voltage comes from connecting it to a reference or another half-cell, so keep the comparison and the reduction direction clear.
A cell potential compares two half-cells
The standard cell potential E°cell is the potential difference for a complete electrochemical cell under standard conditions. When using tabulated reduction potentials, E°cell = E°(cathode) − E°(anode).
Standard aqueous concentrations and gas pressures, temperature and physical states must match the convention in the question. E° is measured in volts and is not an amount of energy by itself.
Pause and explain
How is E°cell calculated from tabulated reduction potentials?
Worked example
What does a more positive standard reduction potential indicate about a half-equation?
Show the worked solution
- All tabulated half-equations are written as reductions.
- A more positive value means reduction is more favourable relative to the reference.
- The oxidised species therefore has a stronger tendency to accept electrons.
Answer It indicates a greater tendency for the oxidised species to gain electrons (be reduced) relative to the standard hydrogen electrode.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Define standard electrode potential in words, including the reference and reaction direction.
Give me a hint
Mention relative measurement against SHE and reduction form.
Compare my reasoning
- A half-cell is compared with the standard hydrogen electrode.
- Use the standard conditions stated for the potential table.
- Write the half-equation as a reduction.
E° is the potential of a half-cell relative to the standard hydrogen electrode under standard conditions, with the half-equation written as a reduction.
Look for these in your work
- Reference named
- Conditions included
- Reduction direction stated
Explain why E°cell is not the same as the standard electrode potential of either half-cell.
Give me a hint
A complete cell contains two electrode processes and measures their potential difference.
Compare my reasoning
- Each E° describes one half-cell relative to SHE.
- E°cell compares the two half-cells in one complete cell.
- It is calculated from the difference of their reduction potentials.
E°cell is the potential difference of a complete cell; a standard electrode potential is a relative potential for one half-cell. E°cell = E°cathode − E°anode using reduction values.
Look for these in your work
- Half-cell and full-cell distinguished
- Potential difference identified
- Calculation relation stated
A student calls E° = +0.80 V an absolute potential that can be measured for an isolated half-cell. Correct the statement.
Give me a hint
A voltage is a comparison, so identify the reference half-cell.
Compare my reasoning
- An isolated half-cell has no measurable absolute potential in this setup.
- Connect it to a reference, conventionally SHE.
- The positive value is relative to that reference and reduction direction.
The value is a relative standard reduction potential measured against the standard hydrogen electrode, not an absolute potential of an isolated half-cell.
Look for these in your work
- Absolute claim corrected
- Reference requirement stated
- Sign interpreted relatively
Common mistakes
- Calling a tabulated reduction potential an oxidation potential
- Treating E° as an absolute potential of an isolated half-cell
- Confusing E°cell with one half-cell value
- Ignoring the stated standard conditions
What can you explain now?
Relative to which reference are standard electrode potentials defined?
Compare with the explanation
The standard hydrogen electrode
It is assigned E° = 0.00 V as the reference.
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Make it stick
What does E° compare, how is a half-reaction written, and how does E°cell combine two comparisons?
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