Page 267 - Physical chemistry understanding our chemical world

234 ACIDS AND BASES

The ethanoic acid molecule is essentially covalent, explaining
The German chemist why it is liquid when pure at room temperature. Nevertheless, the
Liebig, in 1838, was molecule is charged, with the O–H bond characterized by a high
the first to suggest
mobile, replaceable, percentage of ionic character. Because water is so polar a sol-
hydrogen atoms being vent, it strongly solvates any solute dissolved within it. In aqueous
responsible for acidic solutions, water molecules strongly solvate the oxygen- and proton-
properties. Arrhenius containing ends of the O–H bond, causing the bond to break in a
extended the idea in significant proportion of the ethanoic acid molecules, according to
1887, when he said the the following simplistic reaction:
hydrogen existed as a
proton. − +
CH 3 COOH (aq) −−→ CH 3 COO (aq) + H (aq) (6.1)
We say the acid dissociates. The bare proton is very small, and has
The O–H bond in an a large charge density, causing it to attract the negative end of the
acid is sometimes said water dipole. The proton produced by Equation (6.1) is, therefore,
to be ‘labile’, since it is hydrated in aqueous solutions, and is more accurately represented
so easily broken. The by saying H (aq) .
+
word derives from the We see how solvated protons impart the subjective impression
Latin labi,to lapse (i.e. of a sour, bitter flavour to the ethanoic acid in vinegar. In fact, not
to change).
only the sour flavour, but also the majority of the properties we
typically associate with an acid (see Table 6.1) can be attributed to
+
an acidic material forming one or more solvated protons H (aq) in
solution.
This classification of an acid is called the Lowry–Brønsted theory
The ‘Lowry–Brønsted after the two scientists who (independently) proposed this definition
theory’ says an acid is of an acid in 1923. More succinctly, their theory says an acid is
a proton donor.

Table 6.1 Typical properties of Lowry–Brønsted acids
Acid property Example from everyday life
Acids dissolve a metal to form a salt plus Metallic sodium reacts with water, and
hydrogen ‘fizzes’ as hydrogen gas evolves
Acids dissolve a metal oxide to form a salt The ability of vinegar to clean tarnished
and water silver by dissolving away the coloured
coating of Ag O
2
Acids react with metal carbonates to form a The fizzing sensation in the mouth when
salt and carbon dioxide eating sherbet (saliva is acidic, with a pH
of 6.5); sherbet generally contains an
organic acid, such as malic or ascorbic
acids
Acids are corrosive Teeth decay after eating sugar, and one of the
first metabolites from sugar is lactic acid
Acids react with a base to form a salt and Rubbing a dock leaf (which contains an
water (‘neutralization’) organic base) on the site of a nettle sting
(which contains acid) will neutralize the
acid and relieve the pain