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

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

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Answer 1 · 2016-02-24T06:36:07

Because you are making strong $A l - C l$ $Al-Cl$ and $H - H$ $H-H$ bonds, i.e.

$A l (s) + 3 H C l (a q) \to A l C l_{3} (a q) + \frac{3}{2} H_{2} (g)$ $Al(s) + 3HCl(aq) rarr AlCl_3(aq) + 3/2H_2(g)$

Explanation:

A mantra worth committing to memory for these problems is:

$bond breaking requires energy and bond making releases energy .$ $"bond breaking requires energy and bond making releases energy".$
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 $A l - A l$ $Al-Al$ bonds. We make STRONGER $H - H$ $H-H$ and $A l - C l$ $Al-Cl$ bonds, (Of course the aluminum chloride is solvated in water, bond formation takes places nonetheless.) Capisce?

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

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