Defining the scale
Every metal dipped in a solution of its ions sets up an equilibrium such as Mn+(aq) + ne− ⇌ M(s). The standard electrode potential (E°) measures how strongly a half-cell attracts electrons under standard conditions: 25 °C, 1 mol dm−3 solutions and 1 atm for gases. Because a single electrode potential cannot be measured alone, all values are compared with the standard hydrogen electrode (SHE), which is assigned E° = 0.00 V.
Reading the series
A more negative E° means the metal loses electrons more readily (a stronger reducing agent); a more positive E° means the species gains electrons more readily (a stronger oxidising agent).
| Half-cell | E° / V |
|---|---|
| Zn2+ + 2e− ⇌ Zn | −0.76 |
| 2H+ + 2e− ⇌ H2 | 0.00 |
| Cu2+ + 2e− ⇌ Cu | +0.34 |
Key idea
The cell voltage under standard conditions is E°cell = E°positive terminal − E°negative terminal. The half-cell with the more positive E° becomes the positive terminal (cathode).
Worked example
For a Zn/Cu cell, Cu (+0.34 V) is more positive, so it is the positive terminal. E°cell = (+0.34) − (−0.76) = +1.10 V. A positive value confirms the reaction Zn + Cu2+ → Zn2+ + Cu is feasible, with electrons flowing from Zn to Cu in the external circuit.
The series lets us predict displacement: a metal will displace the ions of any metal that sits below it (more positive E°).
Electrode potentials are equilibrium values, so they predict the direction in which a reaction tends to go but say nothing about its rate. A cell with a positive E°cell may still react very slowly. The standard hydrogen electrode itself is built from a platinum plate coated with platinum black, over which hydrogen gas at 1 atm is bubbled while the plate dips into a 1 mol dm−3 solution of H+ ions.