Acids and Bases: An Introduction

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Table of Contents

Introduction

Molecular Structure

Ionization Constants

Salts

Buffers

Lewis Theory

In addition to describing acids and bases by their observable properties, their chemical properties can also be described. What is happening on a molecular level when acids and bases react?

A
R
R
H
E
N
I
U
S

In 1884, Svante Arrhenius came up with a theory to answer this question. By his definition, acids increase the concentration of H⁺ and bases increase the concentration of OH^- in an aqueous solution. When acids and bases react, neutralization occurs:

The Arrhenius theory of acids and bases does not describe acid-base behavior in non-aqueous solutions.

B
R
Ø
N
S
T
E
D

In 1923, two chemists, Johannes N. Brønsted and Thomas M. Lowry, independently came up with a theory that would include non-aqueous acid-base chemistry. In this theory, an acid donates H⁺ ions (or protons) and a base accepts H⁺ ions (or protons). An acid base reaction can be described as a proton transfer:

Since Brønsted developed these ideas to a greater extent than Lowry, this is known as the Brønsted theory, although some chemists refer to it as the Brønsted-Lowry Theory.

NOTE: Since a hydrogen atom is made up of one electron and one proton, when it loses an electron to become H⁺, it is just a proton. Hence, H⁺ and proton are interchangeable when describing acid-base reactions.

L
E
W
I
S

There is one other common theory of acids and bases, the Lewis Theory. This theory describes acids as electron pair acceptors and bases as electron pair donors. This theory is more thoroughly described in a later module.

The acid base theories are summarized in the table below:

	A C I D S	B A S E S
Arrhenius	produce H⁺ in aqueous solutions	produce OH^- in aqueous solutions
Brønsted	donate H⁺	accept H⁺
Lewis	accept an electron pair	donate an electron pair

NOTE: While the Lewis theory is the most broad, the Brønsted theory is widely used and simpler. It is easier to track protons than electron pairs. For the remainder of this tutorial (with the exception of the module on Lewis Theory), the Brønsted theory will be used.

An Introduction

Acids and Bases: An Introduction

A C I D S