Balancing Chemical Equations

When you examine an equation and find more than one atom of an element, you’ll see a small number placed next to that element’s symbol. Let’s continue our example from Writing Chemical Equations:

Calcium + OxygenCalcium oxide

Ca + O2CaO

  • As there are two oxygen atoms, a small 2 is placed next to the “O”.
  • Since there is only one calcium atom, we don’t place a number next to the “Ca” symbol (as writing “1” is unnecessary).

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 and 2 Oxygen atoms
  • Products: 1 Calcium atom and 1 Oxygen atom

Adding Coefficients

Now we know the issue is with balancing the oxygen atoms. However, it’s crucial to remember the following rule:

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

These large numbers are called coefficients.

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 and 2 Oxygen atoms
  • Products: 2 Calcium atoms and 2 Oxygen atoms

As you can see, the equation is still unbalanced. 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 atoms and 2 Oxygen atoms
  • Products: 2 Calcium atoms and 2 Oxygen atoms

The chemical equation is now balanced.

Why Do We Need to Balance Chemical Equations?

Chemical equations are balanced to ensure that the number of atoms of each element on both sides of the equation is the same.

This is important because the law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. Therefore, the total number of atoms of each element before the reaction must be equal to the total number of atoms of that element after the reaction.

Also, the mass of reactants must equal the mass of products. For example:

Reactants (20g)Products (20g)

You’ve used 10 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up