Question

When aluminum is placed in concentrated hydrochloric acid, hydrogen gas is produced.

Answer 1

2 Al(s) + 6 H3O+(aq) -> 2 Al+3(aq) + 3 H2(g) + 6 H2O(l)

This is a redox reaction:

reduction: 2 H3O+(aq) + 2 e -> H2(g) + 2 H2O(l)
oxidation: Al(s) -> Al+3(aq) + 3 e

Aluminum is a very active metal, which means it can very easily lose its outer 3 valence electrons, which makes it a good reducer. In this case, it reduces the H+ in the H3O+ to H2, and gets itself oxidized.

Most metals will i principle reduce H+, which is why most metals will in priciple react with acid. There are some exceptions, like copper, gold, silver, lead, platinum. Also, some metals are prevented by reaction because they have a passivating oxide coating on their surface.