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242 ACIDS AND BASES

conjugate base of H 2 O. Other examples of conjugate acid–base pairs include nitric
acid and nitrate ion, and ammonium ion and ammonia (the acid being cited first in
each case).
We must treat with caution one further aspect of the Brønsted theory: multiple
−
proton-donation reactions. Consider the example of the bicarbonate ion HCO 3 in
water. When titrating bicarbonate with a base such as hydroxide, the ion behaves as
an acid to form the carbonate anion and water:
− − 2− + H 2 O (6.13)
A substance like bicar- HCO 3 + OH −−→ CO 3
bonate, which can react But, conversely, when titrating ions with an acid, the bicarbonate
as either an acid or as behaves as a base, losing its proton to form carbonic acid:
a base, is said to be
amphoteric.The word HCO 3 + H 3 O −−→ H 2 CO 3 + H 2 O (6.14)
−
+
comes from the Greek
amphoteros, meaning We see how the same ion acts as an acid or as a base, depending
‘both’. on the other reagents in solution. We say the bicarbonate ion is
amphoteric, since it reacts either as an acid or as a base.
SAQ 6.1 Consider the following pairs, and for each decide which is
the conjugate acid and which the base: (a) carbonate and bicarbonate;
−
(b) H 2 EDTA 2− and H 3 EDTA ;(c)HNO 2 and NO 2 .
−

Aside
Related models of acids and bases

The concept of acid and base can be generalized in several ways. In liquid ammonia, for
+ −
example, the ammonium and amide ions (NH 4 and NH 2 respectively) coexist. The
+
−
roles of these ions are directly comparable with H 3 O and OH in water. In ammonia,
the species NH 4 Cl and NaNH 2 can be considered to be the respective acid and base
conjugates, just as HCl and NaOH are an acid–base pair in water. This solvent-based
classification of acids and bases derived from Franklin, in 1905. His ideas are worth
careful thought, although we no longer use his terminology.
Brønsted’s definition of acids and bases (see p. 234 and 240) emphasizes the com-
plementary nature of acids and bases, but it is broader than Franklin’s model because
it does not require a solvent, and can even be applied to gas-phase reactions, e.g.
HCl (g) + NH 3(g) → NH 4 Cl (s) .

How did soldiers avoid chlorine gas poisoning
at the Second Battle of Ypres?
Neutralization reactions with acids and bases

The bloody Second Battle of Ypres was fought in France on 22 April 1915, and was
the first time in modern warfare when poison gases were employed. At a crucial