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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