Page 257 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 17

Acid-Base

Theory

17.1. INTRODUCTION
Thus far, we have used the Arrhenius theory of acids and bases (Secs. 6.4 and 8.3), in which acids are defined
−
as hydrogen-containing compounds that react with bases. Bases are compounds containing OH ions or that
−
form OH ions when they react with water. Bases react with acids to form salts and water. Metallic hydroxides
and ammonia are the most familiar bases to us.
The Brønsted-Lowry theory expands the definition of acids and bases to allow us to explain much more of
solution chemistry. For example, the Brønsted-Lowry theory allows us to explain why a solution of ammonium
nitrate tests acidic and a solution of potassium acetate tests basic. Most of the substances that we consider acids in
the Arrhenius theory are also acids in the Brønsted-Lowry theory, and the same is true of bases. In both theories,
strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can
now explain this partial ionization as an equilibrium reaction of the weak acid, the ions, and the water. A similar
statement can be made about weak bases:
HC 2 H 3 O 2 + H 2 O −→ C 2 H 3 O 2 + H 3 O +
−
←−
+
NH 3 + H 2 O −→ NH 4 + OH −
←−
17.2. THE BRØNSTED-LOWRY THEORY
In the Brønsted-Lowry theory, (often called the Brønsted theory for short), an acid is defined as a substance
that donates a proton to another substance. In this sense, a proton is a hydrogen atom that has lost its electron;
2
it has nothing to do with the protons in the nuclei of other atoms. (The nuclei of H are also considered protons
for this purpose; they are hydrogen ions.) A base is a substance that accepts a proton from another substance.
The reaction of an acid and a base produces another acid and base. The following reaction is thus an acid-base
reaction according to Brønsted:
HC 2 H 3 O 2 + H 2 O −→ C 2 H 3 O 2 + H 3 O +
−
←−
acid base base acid
−
+
The HC 2 H 3 O 2 is an acid because it denotes its proton to the H 2 O to form C 2 H 3 O 2 and H 3 O . The H 2 Oisa
+
−
base because it accepts that proton. But this is an equilibrium reaction, and C 2 H 3 O 2 reacts with H 3 O to form
HC 2 H 3 O 2 and H 2 O. The C 2 H 3 O 2 is a base because it accepts the proton from H 3 O ; the H 3 O is an acid
−
+
+
because it donates a proton. H 3 O is called the hydronium ion. It is the combination of a proton and a water
+
246
Copyright © 2005, 1999, 1991 by The McGraw-Hill Companies, Inc. Click here for terms of use.