Just Ask Antoine!
Atoms & ions
The quantum theory
Electrons in atoms
The periodic table
Acids & bases
History of chemistry
Why does mixing a strong acid with water release so much heat?
Breaking a chemical bond requires energy (just as
stretching a spring until it breaks requires energy).
Forming a chemical bond will release energy. So in a reaction that releases
heat (called an exothermic reaction), there must be net bond formation.
If you look at the equation
for dissolving a strong acid like HCl in water,
you might think at first that this would have to be a heat-absorbing (endothermic)
process, because it looks like the bond between H and Cl is broken. But
there is another reaction hiding here.
The hydrogen ion reacts with water to form a complex of the form
(where the n can vary a bit).
we write H+(aq), we actually mean
(It's much easier to write H+(aq)!) Because the hydrogen ion is so
tiny, a large amount of charge is concentrated in a very small area, and
the polar water molecules are strongly attracted to it. This 'hydration'
of the hydrogen ion involves the formation of a covalent bond to one of the
waters and a large number of strong hydrogen bonds,
so it's a strongly exothermic process. This causes the mixing of a strong acid
with water to be strongly exothermic overall.
(aq) + Cl-(aq)|
Author: Fred Senese email@example.com