Question

Why is the oxidation of aluminum metal by hydrochloric acid exothermic?

Answer 1

Because you are making strong `Al-Cl` and `H-H` bonds, i.e.

`Al(s) + 3HCl(aq) rarr AlCl_3(aq) + 3/2H_2(g)`

Explanation:

A mantra worth committing to memory for these problems is:

`"bond breaking requires energy and bond making releases energy".`
The energy released or consumed by a chemical reaction is always the balance between bonds broken and bonds made.

When aluminum metal is oxidized by hydrochloric acid we break reasonably strong `Al-Al` bonds. We make STRONGER `H-H` and `Al-Cl` bonds, (Of course the aluminum chloride is solvated in water, bond formation takes places nonetheless.) Capisce?