Question

Which chemical reaction involving water is a decomposition reaction (breaking up) and which is a synthesis (building up) reaction?

Answer 1

A decomposition reaction would be the electrolysis.

Usually it is separated in two equations, one for oxidation and another for reduction, which are called half reactions:

2 H+ + 2e− → H2

2 H2O → O2 + 4 H+ + 4e−

Then you should add them up. In order for the equation to be balanced, the first equation has to be added twice:

2 H+ + 2e− + 2 H+ + 2e−+2 H2O → H2+ H2 + O2 + 4 H+ + 4e−

4 H+ + 4e− +2 H2O → 2H2 + O2 + 4 H+ + 4e−

Then you should cancel the substances which are both in reactives and in products, and also the electrons:

2 H2O → 2H2 + O2

And there you have the overall equation for electrolysis.

Electrolysis does not happen naturally – you have to provide electric energy for it to happen.

A synthesis reaction would be the opposite reaction, which would be called: electrosyntesis.

2H2 + O2 → 2 H2O

Answer 2

Water breaking down into hydrogen and oxygen is a decomposition reaction.
Hydrogen and oxygen combining to form water is a synthesis reaction.