Page 272 - Physical chemistry understanding our chemical world

PROPERTIES OF LOWRY–BRØNSTED ACIDS AND BASES 239

Why does cutting an onion make us cry?

Other aqueous acids in the environment

The reason why our eyes weep copiously when peeling an onion is because the onion
contains minute pockets of sulphur trioxide, SO 3(g) . Cutting the onion releases this
gas. A mammalian eye is covered with a thin film of water-based liquid (‘tears’) to
minimize friction with the eyelid. The tears occur in response to SO 3 dissolving in
this layer of water to form sulphuric acid:

(6.8)
SO 3(g) + H 2 O(l) −−→ H 2 SO 4(aq)
2−
The sulphuric acid produced dissociates in the water to form SO 4 and two protons.
The eyes sting as a direct consequence of contact with this acid.

Why does splashing the hands
with sodium hydroxide solution make
them feel ‘soapy’?

Proton abstraction
Care: It is a common
Sodium hydroxide in solution dissociates to yield solvated cations mistake to call the
−
and anions, Na and the hydroxide ion OH respectively: OH − ion a ‘hydroxyl’.
+
It is not: a hydroxyl is
+ −
correctly a covalently
NaOH (s) −−→ Na (aq) + OH (aq) (6.9)
bound –OH group, for
The solvated hydroxide ion in Equation (6.9) is formed in addi-
example in an alcohol.
tion to the hydroxide ions produced during water autoprotolysis,
so there are more hydroxide ions in solution than solvated pro-
tons, yielding excess hydroxide in solution. We say the solution Fullers’ earth is a type
is alkaline. As an alternative name, we say hydroxide is a base of clay named after a
(see p. 241). fuller,whose jobwas to
Oils in the skin react readily with the hydroxide ions via the clean cloth, e.g. strip-
same chemical process occurring when spray-on oven cleaner ‘eats’ ping wool of its grease.
− Fullers’ earth removes
into the grime in an oven, reacting principally by the OH (aq)
ion consuming protons. Let us start, for example, with a molecule oils and grease from
possessing a proton capable of being donated; call it HA, where cloth because of its
‘A’ is merely an anion of some sort. This proton must be labile. alkalinity, just like an
oven cleaner solution.
The hydroxide ion removes this labile proton to generate water,
according to
−
−
HA + OH (aq) −−→ A (aq) + H 2 O (6.10)
The Lowry–Brønsted
theory says a base is a
This proton-removing ability characterizes the reactions of hy-
droxide ions in aqueous solutions, and of bases in general. We proton remover.