### KS3 Chemistry

Atoms, Elements and Compounds
Pure and Impure Substances
Chemical Reactions
Energetics
The Periodic Table
Materials
Earth and the Atmosphere

# Balancing Chemical Reactions

When you look at equations, if there is more than one atom of an element, there is a small number placed next to the element’s symbol. In our example from ‘Writing Chemical Equations’:

Calcium + OxygenCalcium oxide

Ca + O2CaO

• As there are two oxygen atoms, there is a small 2 placed next to the O
• There is only one calcium atom so we do not place a number next to the Ca symbol (As we don’t need to write the 1)

On the left side of the equation (the reactants), we have one calcium atom and two oxygen atoms. However, on the right side of the equation (the products), we have one calcium atom and one oxygen atom. So, the equation is unbalanced.

When balancing an equation you first separate the reactants from the products and list how many of each element you have.

Reactants:

• 1 Calcium atom
• 2 Oxygen atoms

Products:

• 1 Calcium atom
• 1 Oxygen atom

Now we know the issue is with balancing the oxygen atoms. However, it is important to remember an important rule!

When balancing chemical equations, you can only place numbers in front of each chemical. This means that we can’t add any small (subscript) numbers to chemicals.

As there are two oxygen atoms on the left of the equation and only one on the right, we place a 2 in front of CaO.

Ca + O22CaO

We now count the reactants and products again.

Reactants:

• 1 Calcium atom
• 2 Oxygen atoms

Products:

• 2 Calcium atom
• 2 Oxygen atom

As you can see, there is still an imbalance. There is only one calcium atom on the left side of the equation and two calcium atoms on the right side of the equation. So we place a 2 in front of Ca.

2Ca + O2 2CaO

Now count the reactants and products again.

Reactants:

• 2 Calcium atom
• 2 Oxygen atoms

Products:

• 2 Calcium atom
• 2 Oxygen atom

The chemical equation is now balanced.

## Why do we balance chemical equations?

The mass of reactants must equal the mass of products.

Reactants (20g)Products (20g)

As no atoms are made or lost in a chemical reaction.

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