Everything Science Grade 11

8.1 CHAPTER 8. TYPES OFREACTIONS

Tip H^2 O� H^3 O++ OH−

For more information
on dissociation, refer to
Grade 10.

Arrhenius described anacid as a compound that increases the concentration of H 3 O+ions in solution,

and a base as a compound that increases theconcentration of OH−ions in a solution. Look at

the following examplesshowing the dissociation of hydrochloric acid and sodium hydroxide (abase)

respectively:

1. HCl + H 2 O→ H 3 O++ Cl−
   Hydrochloric acid in water increases the concentration of H 3 O+ions and is therefore an acid.


2. NaOH + H 2 O→ Na++ OH−
   Sodium hydroxide in water increases the concentration of OH−ions and is therefore a base.

However, this definitioncould only be used foracids and bases in water. Since there are manyre-

actions which do not occur in water it was important to come up with amuch broader definitionfor

acids and bases.

In 1923, Lowry and Bronsted took the work ofArrhenius further to develop a broader definition for

acids and bases. The Bronsted-Lowry model defines acids and basesin terms of their ability to donate

or accept protons.

DEFINITION: Acids and bases

According to the Bronsted-Lowry theory of acidsand bases, an acid is a substance that

gives away protons (H+), and is therefore calleda proton donor. A base is a substance

that takes up protons, and is therefore called a proton acceptor.

Below are some examples:

1. HCl(g) + NH 3 (g)→ NH+ 4 + Cl−

In order to decide which substance is a protondonor and which is a proton acceptor, we need

to look at what happensto each reactant. The reaction can be broken down as follows:

HCl→ Cl−+ H+and

NH 3 + H+→ NH+ 4

From these reactions, itis clear that HCl is a proton donor and is therefore an acid, and that NH 3

is a proton acceptor and is therefore a base.

2. CH 3 COOH + H 2 O→ H 3 O++ CH 3 COO−

The reaction can be broken down as follows:

CH 3 COOH→ CH 3 COO−+ H+and

H 2 O + H+→ H 3 O+

In this reaction, CH 3 COOH (acetic acid) is aproton donor and is therefore the acid. In this case,

water acts as a base because it accepts a proton to form H 3 O+.

3. NH 3 + H 2 O→ NH+ 4 + OH−

The reaction can be broken down as follows:

H 2 O→ OH−+ H+and