Siyapath
Sign up

Lesson: Chapter 12 — Electrochemistry

More simple cells 3 of 3

Predicting any cell

With the activity series you can predict every part of a simple cell before building it.

The steps

  1. Find which metal is higher in the activity series.
  2. That metal is the anode and the negative terminal; the other is the cathode and positive terminal.
  3. Anode: the metal is oxidised, M (s) → Mn+ (aq) + ne.
  4. Cathode, in an acid: hydrogen ions are reduced, 2H+ (aq) + 2e → H2 (g).
  5. Balance the electrons and add the half reactions for the cell reaction.

Worked example — a zinc-lead cell in dilute sulphuric acid

Zinc is above lead in the activity series.

Anode, negative terminal: zinc. Cathode, positive terminal: lead.

Anode (oxidation): Zn (s) → Zn2+ (aq) + 2e

Cathode (reduction): 2H+ (aq) + 2e → H2 (g)

Cell: Zn (s) + 2H+ (aq) → Zn2+ (aq) + H2 (g)

You would see the zinc dissolve and bubbles on the lead.

Why the series predicts it

A metal's place in the activity series is a measure of how readily it gives up electrons. Put two metals in the same acid and join them, and the one more eager to lose electrons does so; its electrons are pushed through the wire to the other metal, where hydrogen ions take them. The further apart the two metals are in the series, the harder the electrons are pushed — which is why a zinc-copper cell gives a larger voltage than a zinc-iron one.

Higher metal: anode, negative, dissolves

In a simple cell the metal higher in the activity series is the anode and negative terminal and dissolves; the lower one is the cathode and positive terminal, where hydrogen is given off.