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 compounds just concentrate on balancing the equations; if you would like more help on working out formula then simply click on the link.

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 equation and the model equations you will see that they are not balanced. On the reactants side of the equation there is 1 atom of magnesium and 2 atoms of oxygen. On the products side of the arrow there is 1 atom of magnesium and 1 atom of oxygen. We are missing 1 atom 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 and model equations:

This equation can be balanced by inserting "2" as shown below: It is worth mentioning that really the only way to master balancing equations is to have a go yourself. So please click the link for the practice questions below. 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.