The zinc-copper cell
If the electrons zinc gives up could be made to travel through an outside wire to where the hydrogen ions take them, that flow would be an electric current. Activity 12.1.2 tries it.
Activity 12.1.2
You will need: a beaker, zinc and copper strips, dilute sulphuric acid, connecting wires and an ammeter
Connect the zinc and copper strips to the ammeter with wires as in figure 12.1.4, then stand both strips in the beaker of dilute sulphuric acid. Record what you see.
Figure to be added
The observation
- The ammeter needle deflects.
- The zinc wears away.
- Gas bubbles rise from the copper — not from the zinc.
Again zinc atoms leave their electrons on the metal and become Zn2+ ions, so the zinc wears away. But now the electrons have a path: they travel along the outside wire to the copper. That flow of electrons is an electric current, shown by the ammeter. Hydrogen ions in the solution go to the copper and take the electrons there, so hydrogen bubbles come off the copper.
Zn (s) → Zn2+ (aq) + 2e
2H+ (aq) + 2e → H2 (g)
Zn (s) + 2H+ (aq) → Zn2+ (aq) + H2 (g)
at the zinc (1); at the copper (2); the cell reaction (1) + (2)
Electrochemical cell and electrode
An arrangement that uses a chemical reaction to generate electricity is an electrochemical cell. The conducting materials dipped in the electrolyte — here the zinc and copper strips — are the electrodes.
Why the bubbles moved
The reaction is the same as zinc alone in acid: the same two half reactions add to the same cell reaction. What changed is where the second half happens. With a wire to follow, electrons reach the copper, and the hydrogen ions meet them there. Separating the two halves in space is the whole trick of a cell — it forces the electrons through the wire, and anything placed in that wire, a bulb or a meter, is driven by them.
The copper does not react
The bubbles appear on the copper, but it is not the copper that reacts: it only hands over the electrons that came along the wire. The copper strip stays the same size.