Question

Is the addition of hydrogen to a compound a reduction reaction? Why?

Answer 1

Not necessarily.

Explanation:

If you add hydrogen molecules to a compound, nothing will happen most of the time.
If you add hydrogen ions to a compound, generally it will reduce the compound and create a compound that contains a hydrogen bonded to it.
If you add a hydride anion to a compound, generally it will oxidise the compound and create a compound that contains a hydrogen bonded to it.

Having only one valence electron, hydrogen is very flexible and experiences virtually no shielding effect from its nucleus. Therefore, hydrogen is often caught up in dative bonds as well as other types (like ionic or covalent bonds).

Therefore, the addition of hydrogen to a compound may not always result in a reduction reaction. You may also want to read up on hydrogenation. Hope this helps!