This chapter covers the IB syllabus content in Sections 8 and 18.

SPREAD 1: What is an acid?

A Bronsted–Lowry acid is a proton/H+ donor and a Bronsted–Lowry base is a proton/H+ acceptor.

Introduction

Acids and bases have been known about since ancient times, although defining precisely what an acid and a base is perhaps a little harder.

The most useful definition of an acid for IB students is the Brønsted–Lowry definition:

An acid is a proton donor. By proton here we mean a hydrogen ion (H+) that consists only of a proton.

A base is a proton acceptor. By this we usually mean a compound that contains hydroxide ions (OH-) that can accept a proton to form water.

In more simple terms an acid is a compound that releases hydrogen ions when dissolved in water. The technical term for this is DISSOCIATION.

It is often easier to see this in terms of equations:

The HCl donates its proton to the OH-ion in the base to form water.

As an ionic equation:

More complicated examples of acid–base behavior still fit the same, simple, logical ionic equation. For example, in the generation of an electrophile for benzene in the organic section (Chapter 11).

Here the sulfuric acid acts as an acid and the nitric acid acts as the base! The donated proton is shown in bold.

Products of neutralization

Reactions involving acids and bases produce compounds called “salts.” Such reactions are known as neutralization reactions. Salts are more properly defined as “compounds where the H+ of an acid has been replaced by another positive ion.”

An alkali is a subgroup of bases that are soluble in water.

Some typical acids

The three common mineral acids are:

Hydrochloric acid (HCl)

Sulfuric acid (H2SO4)

Nitric acid (HNO3)

Knowing the formulas of these three acids is considered absolutely essential for any chemistry course.

There are thousands of weak acids. Their structures and reactions will be looked at in more detail in the organic chemistry course, but it is important that when writing equations with such acids you show exactly which hydrogen is donated.