Chemical equations

The changes in chemical reactions can be modelled using equations. In general, you write:

curriculum-key-fact
reactants → products

The reactants are shown on the left of the arrow, and the products are shown on the right of the arrow. Do not write an equals sign instead of an arrow. If there is more than one reactant or product, they are separated by a plus sign.

Word equations

A word equation shows the names of each substance involved in a reaction, and must not include any chemical symbols or formulae. For example:

copper + oxygen → copper oxide

In this reaction, copper and oxygen are the reactants, and copper oxide is the product.

Balanced equations

A balanced equation gives more information about a chemical reaction because it includes the symbols and formulae of the substances involved. There are two steps in writing a balanced equation:

  1. replace the name of each substance with its symbol or formula
  2. use numbers to balance the equation, if it is not already balanced

In the example above (the reaction between copper and oxygen to make copper oxide), we get this in the first step:

Cu + O2 → CuO

This is unbalanced because there is one copper atom on each side of the arrow, but two oxygen atoms on the left and only one on the right. To balance the equation, you need to adjust the number of units of some of the substances until we get equal numbers of each type of atom on both sides. You should never change the formula of a substance to do this.

Here is the balanced symbol equation:

2Cu + O2 → 2CuO

You can see that we now have two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction:

Two copper atoms react with one oxygen molecule to produce two units of copper oxide.Two copper atoms react with one oxygen molecule to produce two units of copper oxide

Here are some other examples of balanced equations. Check that you understand why they are balanced:

  • C + O2 → CO2
  • 2H2 + O2 → 2H2O
  • 2Mg + O2 → 2MgO
  • CuCO3 → CuO + CO2
  • Mg + 2HCl → MgCl2 + H2