HL Extension
Definition of Standard Electrode Potential
The standard electrode potential (\(E^\circ\)) of a half-cell is the potential difference measured relative to the Standard Hydrogen Electrode (SHE) under standard conditions (\(298\text{ K}, 100\text{ kPa}, 1.00\text{ mol dm}^{-3}\)).
Standard Conditions
- Temperature: 298 K (25°C)
- Concentration: 1.00 mol dm-3
- Pressure: 100 kPa
Common E° Values
| Half-reaction (reduction) | E° / V |
|---|---|
| Li+ + e- ⇌ Li | -3.04 |
| Zn2+ + 2e- ⇌ Zn | -0.76 |
| 2H+ + 2e- ⇌ H2 | 0.00 |
| Cu2+ + 2e- ⇌ Cu | +0.34 |
| Ag+ + e- ⇌ Ag | +0.80 |
| F2 + 2e- ⇌ 2F- | +2.87 |
Calculating Cell Potential
HL Extension
Cell Potential Calculation Formula
\[\mathbf{E^\circ_{\text{cell}} = E^\circ_{\text{reduction (cathode)}} - E^\circ_{\text{oxidation (anode)}}}\]
All \(E^\circ\) values are tabulated as reduction potentials in the IB Data Booklet.
Worked Example
Calculating Standard Cell Potential
Given \(E^\circ(\text{Zn}^{2+}/\text{Zn}) = -0.76\text{ V}\) and \(E^\circ(\text{Cu}^{2+}/\text{Cu}) = +0.34\text{ V}\):
\[E^\circ_{\text{cell}} = +0.34 - (-0.76) = \mathbf{+1.10\text{ V}}\]Key Rules
- If E°cell > 0, the reaction is spontaneous
- More negative E° = stronger reducing agent (loses electrons easily)
- More positive E° = stronger oxidising agent (gains electrons easily)
- E° values are intensive; they don't change if you multiply the half-equation
Exam Tip
Predicting Cathode vs Anode from E°
The half-cell with the more positive \(E^\circ\) undergoes reduction (cathode). The half-cell with the more negative \(E^\circ\) undergoes oxidation (anode).
AQA GCSE & IB Chemistry
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