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Balancing equations
Equations in chemistry are very important.
• They convey information about the nature of the reactants and the products of the reaction.
• They give us the ratio of the amounts in which the reactants combine and the products are formed. This is important information, because it enables us to mix the reactants in the correct proportion by mass or volume and to predict what mass or volume of products we should expect to obtain.
You can read the tutorial on screen or download from this link a file with the whole of a series of tutorials about formulae and equations.
Stages in writing a balanced equation
To construct equations, there are four stages.
1 You need to be able to write down the word equation and to be certain that you know the names for all the reactants and products.
2 You must convert the names of all the substances into correct formulae.
3 Since atoms can neither be created nor destroyed, the equation needs to be balanced so that there are the same numbers of atoms of each element on both sides of the equation. You can only do this by altering the proportions of the substances reacting together, and also those of the substances created.
4 Next add state symbols to show whether the substances are solids, liquids or gases or dissolved in water. The four commonest state symbols, which are always enclosed in brackets, are:
solid is (s); gas is (g); liquid is (l); a solution in water (aqueous) is (aq)
Example
Consider the reaction of methane, CH4, with oxygen.
1 You should know that carbon dioxide and water are formed
methane + oxygen –> carbon dioxide + water
2 The formulae are: CH4 + O2 –> CO2 + H2O
3 The equation above is not balanced. It accounts for the single carbon atom within the methane molecule which forms part of the carbon dioxide molecule. However, out of the four hydrogen atoms in the methane molecule, only two can be accounted for in the water molecule. So two water molecules must be formed, and we write this as 2H2O.
Now two water molecules must contain two oxygen atoms, and, together with the two oxygen atoms within the carbon dioxide molecule, this makes a total of four oxygen atoms appearing in the products. Yet there are only two oxygen atoms within the oxygen molecule. So there must be two oxygen molecules to start with, and we write this as 2O2. So we can write:
CH4 + 2O2 –> CO2 + 2H2O
4 And as the fourth stage we add the state symbols:
CH4(g) + 2O2(g) –> CO2(g) + 2H2O(l)
Study this equation carefully and compare it with the models shown on the scales in the diagram below. There are four hydrogen atoms, four oxygen atoms and one carbon atom on each side, and the equation is balanced.
The reactants and products of a reaction always have exactly the same mass
When you write balanced equations, you must never change the formula to suit the balancing process. You can only change the proportions of the substances reacting and those of the substances formed, by putting numbers in front of the formulae, until all the atoms are accounted for.
Now you should refer to the Nuffield Chemistry Students’ book. On page 22 you will find some equations to construct and balance.
You will also find a tutorial on this site showing you how to calculate reacting masses from balanced equations.
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updated: 03 November 2003
