📘 IB Understanding
The standard electrode potential (E°) measures the tendency of a half-cell to gain electrons under standard conditions (298 K, 1 mol dm-3, 100 kPa). All values are measured relative to the Standard Hydrogen Electrode (SHE), which is assigned E° = 0.00 V.
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
Formula
E°cell = E°cathode - E°anode
Worked Example
Q: Calculate E° for a Zn/Cu cell.
E°(Cu2+/Cu) = +0.34 V (cathode, more positive)
E°(Zn2+/Zn) = -0.76 V (anode, more negative)
E°cell = +0.34 - (-0.76)
E°cell = +1.10 V (spontaneous, as E° > 0)
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
The data booklet lists half-reactions as reductions. The half-cell with the more negative E° will be reversed (oxidised) in the overall reaction.