Chemistry, Third edition

WRITING AND BALANCING EQUATIONS

Chemical equations are best written in three stages:

1.Write the names of the reactants and products on each side of the equation.

2.Replace the names with formulae.

3.Balance the equation.

25

Example 2.4

Hydrogen gas (dihydrogen) reacts with oxygen gas (dioxygen) to

form water. Write a balanced equation for the reaction.

Answer

1.hydrogen gas oxygen gas water

2.H 2 O 2 H 2 O

3.Count the number of atoms of each element on each side of the equation:

left-hand side – two atoms of H and two atoms of O

right-hand side – two atoms of H and one atom of O

We need to increase the number of atoms of O on the right-hand side.Numbersin

frontof the formulae must be added, thus 2H 2 O means two molecules of water

(containing four atoms of H and two atoms of O), so we can increase the number of O

atoms on the right-hand side:

H 2 O 2 2H 2 O

Now there are more hydrogens on the right-hand side (four) than on the left-hand side

(two), but writing 2 in front of H 2 remedies the situation:

2H 2 O 2 2H 2 O

The equation is now balanced – there are the same number of atoms of hydrogen

and oxygen on each side. This equation tells us that two molecules of H 2 react with

one molecule of O 2 to form two molecules of H 2 O.

Comment

Note that although chemists often use formulae instead of names (a bottle of water

might be labelled H 2 O) the formula H 2 O, strictly speaking, refers to one moleculeof

water. The balanced equation is represented by Fig. 2.5.

Additional comment

Before you try to balance an equation it is very important that you write correct

formulae for the products and reactants. For example, you will come across a

chemical with the formula H 2 O 2. This compound is commonly called hydrogen

peroxide and it is very differentfrom water in its chemical behaviour. The extra

oxygen in the formula makes a big difference to the nature of the substance!

Fig. 2.5Balanced equation for the reaction of hydrogen with oxygen.

Balancing equations

Balance the following

equations:

(i) N 2 H 2 NH 3

(ii) NaCl 2 NaCl

(iii) AlO 2 Al 2 O 3

(iv) AlH 3 PO 4 

AlPO 4 H 2

(v) NaO 2 Na 2 O

(vi) Mg 3 N 2 H 2 O

Mg(OH) 2 NH 3

(vii) Cr 2 O 3 SiCrSiO 2

(viii)CS 2 O 2 CO 2 SO 2

(ix) Fe 2 O 3 COFeCO 2

(x) FeS 2 O 2 

Fe 2 O 3 SO 2.

Exercise 2J