The button batteries in small devices like watches are commonly composed of silver-zinc or mercury-zinc batteries. The reaction in a mercury button battery is: HgO(s) + Zn(s) --> ZnO(s) + Hg(l) What is the line notation for a mercury button battery?

The mercury button battery line notation is:

        Zn⁰| Zn²|| , HgO| Hg⁰||

Explanation:

Mercury batteries either use

pure mercury(II) oxide (HgO)—which is called mercuric oxide

or a mixture of HgO with manganese dioxide (MnO2) which is as the cathode.

Anode has two half reactions:

The first step is an electrochemical reaction step:

Zn + 4OH− → Zn(OH)4−2 + 2e−[3]

cathode has a half reaction :

HgO + H2O + 2e− → Hg + 2OH−[3]

+0.0977 V  standard potential is of this reaction.

which is followed by a chemical reaction step:

Anode consists of oxidation:

Zn+2OH>ZnO+H2O+2e

which gives an overall anode half-reaction of:

Zn + 2OH− → ZnO + H2O + 2e−[3]

Therefore,

The overall reaction for the battery is:

Zn + HgO → ZnO + Hg

the volume increases