2.1 Balancing the equation
In writing an equation for the combustion of methane, the formulas of the reactants and products are now in place.
Can you see a problem with the equation?
On the left, there is one molecule of methane with four hydrogen atoms. Hydrogen appears on the right of the equation in the water molecule but the one water molecule contains just two hydrogen atoms. There are different numbers of oxygen atoms on each side: two on the left and three on the right.
So in going from the left to the right, there is a change in the numbers of atoms.
This can’t happen.
Recall, the Law of Conservation of Mass says that mass is neither lost nor gained in chemical reactions – the total mass of products at the end of a reaction is identical to the total mass of reactants at the beginning. Reactions are all about rearranging atoms on going from reactants to products.
When writing an equation this law must be accommodated– the equation must be balanced.
To write a balanced equation, the relative number ofeach of the formula units that is required is written immediately in front of that formula in the equation:
To represent two water molecules, the number 2 is placed immediately before the formula for water. This number indicates there is two of everything that comes after the number. So above there are two water molecules, i.e. two H2O units. Where a number is not shown (such as in front of CH4), you should assume (as with formulas) that a number 1 is implied.
Now a final check on the numbers of atoms: on the left (before the go to), there are 4 hydrogen atoms, 4 oxygen atoms and 1 carbon atom.
On the right, there are 4 hydrogen atoms, 4 oxygen atoms and 1 carbon atom.
There is a balance in the numbers of atoms of each type on each side – which is as it should be.
Finally, the words ‘go to’ can be replaced with an equals sign to give:
The equals sign indicates that the representation is a balanced chemical equation. This is why the words go to were retained earlier because, at that point, the reaction representation was not balanced. Note, instead of an equals sign you will often find an arrow (à) used.
It’s also important to realise that you cannot balance equations by changing chemical formulas. For example, methane molecules always contain one carbon atom and four hydrogen atoms, CH4. It is only the relative numbers of formula units in the equation that are changed in the balancing process.
An additional piece of information that is often included in chemical equations is an indication of the physical states of the species.
Gas is represented by (g) coming immediately after the species, (l) represents liquid and (s) solid. These are called state symbols. You’ll also frequently come across (aq) short for aqueous – this is used to indicate species that are dissolved in water.
At 25 °C, methane, oxygen and carbon dioxide are gases and water is a liquid.
So following all these steps, you end up with the following balanced chemical equation: