Therefor, the Mg is oxidized and the H are reduced. The Cu loses 2 electrons and the S gains 2. Therefor, the Cu is oxidized and the S is reduced.
Ammonia is a weak acid and hydrobromic acid is a strong acid.
There is no oxidation or reduction taking place. S 2- on both sides of the reaction, so no electrons shift around. The clue here is the reaction of a strong acid with a pure metal, with the evolution of hydrogen gas. This reaction is characterized by two types of reactions.