Higher and foundation tiers

If you think about what happens to the particles during a chemical reaction you would probably realise that all the particles present in the reactants appear in the products. All that really happens during a chemical reaction is that the particles rearrange themselves as they change from reactants into products. This idea is really quite simple but crucial in understanding what happens during chemical reactions. The law of conservation of mass states that the total mass of the reactants must be equal to the total mass of the products; nothing appears or disappears during chemical reactions. All the particles that you start with you end up; they will probably be in a different form but they are all still there!

Working out chemical formulae is another useful skill which is easy to master and will improve your confidence in your chemistry work. However on this page do not worry about any of the formulae for any of the reactants or products just concentrate on balancing the equations; if you would like more help on working out formula then simply click here.

Below are some worked examples on how to balance chemical equations. Go through the worked examples and then try to balance equations yourself by completing the practice questions. The only way to get good at balancing equations is to balance lots of them, it's often trial and error until you get more confident at balancing equations.

Word equation:

Symbolic equation:

Hint: Remember that oxygen gas is a diatomic gas, its goes around in pairs!

Model equation: Now if you look carefully at the symbolic and the model equations you will see that they are not balanced. On the reactants side of the equation there is 1 atom/mole of magnesium and 2 atoms/moles of oxygen. On the products side of the equation there is 1 atom/mole of magnesium and 1 atom/mole of oxygen. We are missing 1 atom/mole of oxygen on the products side of the equation. Now the simple but incorrect way to fix this is simply to change the formula for magnesium oxide as shown below:

To balance equations the first rule is simple - DO NOT change any of the formula for the compounds you are given. You balance equations by putting numbers in front of each of the reactants and products formulae until the number of atoms on each side of the equation balances.

By inserting "2" into the equation above there are now equal numbers of atoms of each element on the reactants and products side of the equation.

Word equation:

Symbolic equation:

Word equation:

Symbolic equation:

This equation can be balanced by:

- First check the carbon atoms on both sides of the equation, these balance as there is only one carbon atom on the reactants and products side of the equation. Next
- Check the number of hydrogen atoms on both sides of the equation. On the reactants side there are 4 hydrogen atoms but only 2 on the product side of the equation, so to balance them place a "2" in front of the water on the products side of the equation. This gives 4 hydrogen atoms on both sides of the equation. This gives:

Finally check the number of atoms of oxygen atoms present on each side of the equation. Here there are 2 atoms of oxygen present on the reactant side of the equation but there are 4 atoms of oxygen on the products side of the equation, so to balance the oxygen atoms insert a "2" in front of the oxygen in the reactants side of the equation:

The equation is now balanced.

It is worth mentioning that really the only way to master balancing equations is to have a go yourself. So please click the link below for the practice questions. You should also be aware that in the examples above we were trying to balance the number of atoms of each element in the equations, however strictly speaking we should be talking about balancing the number of moles of each substance present.- To balance equations do not change the
**formulae**for any of the reactants or products but simply put numbers in front of the formula for the reactant and products where necessary to balance the number of moles of each element on both sides of the equation. - With equations containing lots of elements including hydrogen and oxygen it is often easier to balance these equations if you balance the oxygen atoms last and the hydrogen atoms second last and the other elements present in any order.