Zinc in dilute acid
To see what goes on inside a cell, start with just one metal in an acid.
Activity 12.1.1
You will need: a small beaker, dilute sulphuric acid and a strip of zinc
Pour dilute sulphuric acid into the beaker and stand the zinc strip in it so that part of it is under the acid, as in figure 12.1.3. Record what you see.
Figure to be added
The observation
Bubbles of gas rise from around the zinc, and the zinc slowly wears away.
Zinc atoms go into solution as zinc ions, leaving their electrons on the metal. Sulphuric acid splits in water into hydrogen ions and sulphate ions. The hydrogen ions are drawn to the zinc to take those electrons, and become hydrogen gas.
Zn (s) → Zn2+ (aq) + 2e
2H+ (aq) + 2e → H2 (g)
(1) zinc gives up electrons; (2) hydrogen ions take them
Half reactions
Equations like (1) and (2), which show one species gaining or losing electrons and turning into another, are half reactions. Adding two suitable half reactions gives the balanced ionic reaction.
Zn (s) + 2H+ (aq) → Zn2+ (aq) + H2 (g)
(1) + (2): the 2e on each side cancel
The sulphate ions came into the solution with the hydrogen ions but take no part: they are the same on both sides. Adding them to each side turns the ionic equation into the full one — zinc and sulphuric acid giving zinc sulphate and hydrogen.
Zn (s) + H2SO4 (aq) → ZnSO4 (aq) + H2 (g)
One reaction, two halves
Zinc in acid is really two things happening at once: zinc losing electrons, and hydrogen ions gaining them. Here both happen on the same piece of zinc — so the electrons never go anywhere useful.