What is the standard hydrogen electrode? With a labelled diagram, explain how it is used to measure the standard electrode potentials of the Cu²⁺/Cu and Zn²⁺/Zn electrodes.

1. 1 · Define the standard electrode potential. The reduction potential of a half-cell when every species is at unit concentration (1 M, or 1 bar for a gas).
2. 2 · Describe the SHE. A platinum foil coated with platinum black, dipped in 1 M H⁺, with pure H₂ at 1 bar bubbled over it. By convention, E° = 0 V at all temperatures.
3. 3 · Draw the SHE. Glass tube with an H₂ inlet, Pt black foil at the bottom, beaker of 1 M HCl. Label every part.
4. 4 · Measure Cu²⁺/Cu. Pt | H₂(1 bar) | H⁺(1 M) || Cu²⁺(1 M) | Cu gives 0.34 V. Copper is the cathode, so E°(Cu²⁺/Cu) = +0.34 V.
5. 5 · Measure Zn²⁺/Zn. With zinc the value is −0.76 V: zinc is really the anode and reduces H⁺, so E°(Zn²⁺/Zn) = −0.76 V.
6. 6 · Close in one line. So E° values rank reducing power: zinc is a stronger reducing agent than hydrogen, and copper is weaker.

Scoring tip: Write 'by convention, E° of the SHE is zero at all temperatures', and use 1 bar for H₂, as your textbook does.