Page 258 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 17] ACID-BASE THEORY 247

molecule, and is the species that we have been abbreviating H thus far in this book. (H is not stable; it does
+
+
not have the configuration of a noble gas; see Sec. 5.4.)
The acid on the left of this equation is related to the base on the right; they are said to be conjugates of each
−
other. The HC 2 H 3 O 2 is the conjugate acid of the base C 2 H 3 O 2 . Similarly, H 2 O is the conjugate base of H 3 O .
+
Conjugate differ in each case by H .
+
EXAMPLE 17.1. Write an equilibrium equation for the reaction of NH 3 and H 2 O, and label each of the conjugate acids
and bases.
Ans. Conjugates
———————–
| |
−→ + + OH −
NH 3 + H 2 O ←− NH 4
base acid acid base
| ———————— ↑
Conjugates
The NH 3 is a base because it accepts a proton from water, which is therefore an acid. The NH 4 is an acid because
+
−
it can donate a proton to OH , a base.
We have now labeled water as both an acid and a base. It is useful to think of water as either, because it
really has no more properties of one than the other. Water is sometimes referred to as amphiprotic. It reacts as
an acid in the presence of bases, and it reacts as a base in the presence of acids.
Various acids have different strengths. Some acids are strong; that is, they react with water completely
to form their conjugate bases. Other acids are weak, and they form conjugate bases that are stronger than the
conjugate bases of strong acids. In fact, the stronger the acid, the weaker its conjugate base. Some acids are so
extremely weak that they do not donate protons at all at ordinary temperatures. In this text we refer to these
conjugates as feeble. We can classify the acids and the related conjugate bases as follows:
Conjugate Acid Conjugate Base

Strong Feeble
Weak Weak
Feeble Strong
The same reasoning applies to bases that are molecules and their conjugate acids (that are ions).
Note especially that weak acids do not have strong conjugate bases, as stated in some texts. For example,
acetic acid is weak, and its conjugate base, the acetate ion, is certainly not strong. It is even weaker as a base
than acetic acid is as an acid.

EXAMPLE 17.2. Classify the following acids and bases according to their strength: HNO 3 ,HC 2 H 3 O 2 , KOH, and NH 3 .
Ans. HNO 3 is a strong acid; HC 2 H 3 O 2 is a weak acid; KOH is a strong base; NH 3 is a weak base.

EXAMPLE 17.3. Classify the conjugates of the species in the prior example according to their strength.

+
+
−
−
Ans. NO 3 is a feeble base; C 2 H 3 O 2 is a weak base; K is a feeble acid; NH 4 is a weak acid.
+
This classification indicates that NO 3 and K have no tendency to react with water to form their conjugates.
−
The C 2 H 3 O 2 does react with water to some extent to form HC 2 H 3 O 2 and OH . The NH 4 reacts with water to
+
−
−
a small extent to form H 3 O and NH 3 . Consider the following equation:
+
HNO 3 + H 2 O ←− NO 3 + H 3 O +
−
−→
acid base base acid
Since HNO 3 reacts with water 100%, NO 3 does not react with H 3 O at all. If the nitrate ion cannot take a
−
+
proton from the hydronium ion, it certainly cannot take the proton from water, which is a much weaker acid than
the hydronium ion is.