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220 CHEMISTRY FOR THE UTTERLY CONFUSED
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Our goal in this chapter is to help you understand the equilibrium systems
involving acids and bases. If you don’t recall the Arrhenius acid-base theory,
refer to Chapter 4 on Aqueous Solutions. You will learn a couple of other acidbase
theories, the concept of pH, and will apply those basic equilibrium techniques
we covered in Chapter 14 to acid-base systems. In addition, you will need
to be familiar with the log and 10 x functions of your calculator. And, as usual, in
order to do well you must Practice, Practice, Practice.
15-1 Brønsted-Lowery Acids and Bases
An acid in the Brønsted-Lowery theory is an H + donor and a base is an H +
acceptor. In the Brønsted-Lowery acid-base theory, there is a competition for
an H + . Consider the acid-base reaction between acetic acid, a weak acid, and
ammonia, a weak base:
CH 3COOH(aq) NH 3(aq) K CH 3COO (aq) NH 4 (aq)
Acetic acid donates a proton to ammonia in the forward (left to right) reaction of
the equilibrium to form the acetate and ammonium ions. However, in the reverse
(right to left) reaction, the ammonium ion donates a proton to the acetate ion to
form ammonia and acetic acid. The ammonium ion is acting as an acid and the
acetate ion a base. Under the Brønsted-Lowery system, acetic acid (CH 3COOH)
and the acetate ion (CH 3COO ) are conjugate acid-base pairs. Conjugate acidbase
pairs differ by only a single H . Ammonia (NH 3) and the ammonium ion
(NH 4 ) are also conjugate acid-base pairs. In this reaction, there is a competition
for the H between acetate ion and ammonia. In order to predict on which side
the equilibrium will lie, this general rule applies: the equilibrium will favor (shift
towards) the side in which the weakest acid and base are present.
15-2 Strength of Acids and Bases
In Chapter 4, we introduced the concept of acids and bases. Acids and bases
may be strong or weak. Strong acids completely dissociate in water and weak
acids only partially dissociate. For example, consider two acids HCl (strong)
and CH 3COOH (weak). If we add each to water to form aqueous solutions, the
following reactions take place:
HCl(aq) H 2O(l) l H 3O (aq) Cl (aq)
CH 3COOH(aq) H 2O(l) K H 3O (aq) CH 3COO (aq)