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

1 Answer
anor277
Feb 24, 2016

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?

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