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PROPERTIES OF LOWRY–BRØNSTED ACIDS AND BASES 241

But a word of caution: species other than metal hydroxides can
We say a proton is
act as bases. Ammonia is such an example, since it can abstract
abstracted when re-
protons in aqueous solution according to
moved selectively.
Similarly, we call a
+ −
NH 3(aq) + H 2 O −−→ NH 4(aq) + OH (aq) (6.11)
selective summary or
pr´ ecis of a piece of
To abstract a proton is to remove only the proton. The substan- prose ‘an abstract’.
tial extent of dissociation in Equation (6.11) helps explain why
‘aqueous ammonia’ is more properly called ‘ammonium hydrox-
−
ide’, NH 4 OH. We generate the solvated hydroxide ion OH (aq) by abstracting a proton
−
from water. The OH (aq) ion in Equation (6.11) is chemically and physically identical
to the solvated hydroxide ion generated by dissolving NaOH or KOH in water.

Why is there no vinegar in crisps of salt
and vinegar flavour?

Conjugate acids and bases

Potato crisps come in many flavours, perhaps the most popular being ‘salt and vine-
gar’. Curiously, a quick glance at the packet’s list of ingredients reveals how the
crisps contain unhealthy amounts of salt, but no vinegar (ethanoic acid) at all. In fact,
the manufacturer dusts the crisps with powdered sodium ethanoate (NaCO 2 CH 3 ),
because ‘real’ vinegar would soon make the crisps limp and soggy. Inside the mouth,
acid from the saliva reacts with the ethanoate anion to form ethanoic acid:

+
CH 3 CO − + H 3 O (aq) −−→ CH 3 CO 2 H (aq) + H 2 O (6.12)
2(aq)
This reaction proceeds inside the mouth, rapidly reaching its position of equilibrium,
and allowing the ethanoic acid to impart its distinctive vinegary flavour.
The solvated proton on the left of Equation (6.12) acts as an
acid, since it donates a proton at the same time as the ethanoate The word ‘conjugate’
ion behaves as a base, because it accepts a proton. To complicate comes from the Latin
the situation, the reaction is one half of a dynamic equilibrium, conjugare, meaning
i.e. it proceeds in both the forward and backward directions. In the ‘to yoke together’
backward direction, we notice how this time the ethanoic acid acts (the prefix con means
as an acid and the water acts as a base. ‘together’ and jugare
is ‘to yoke’). Simi-
The reaction in Equation (6.12) illustrates the coexistence of two
larly, the English word
acids and two bases. We say the ethanoate ion and ethanoic acid
‘conjugal’ relates to
represent a conjugate pair, and the solvated proton and the water marriage and concerns
form a second conjugate pair. Within the ethanoic–ethanoate pair,
the joining of husband
the ethanoic acid is the conjugate acid and the ethanoate anion and wife.
+
is the conjugate base. Similarly, H 3 O is a conjugate acid to the