written by Dr Richard Clarkson : © Saturday, 1 November 1997
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The process of writing a formula equation involves three stages.
The first stage is to know the names of the reactants and products and possibly write a word equation. As your ability to write equations increases the need to write a word equation decreases; however, you still have to know what happens when particular chemicals react together.
e.g. hydrogen + oxygen → water
Then the formulae for the various chemicals are written out (under the respective chemicals if you have written out the word equation).
e.g. hydrogen + oxygen → water
H2 + O2 → H2O
The final stage is to count up the number of each individual type of atom on the left hand side and right hand side of the arrow. For a proper formula equation these numbers must be balanced, i.e. they must be equal to each other.
This balancing is achieved by putting numbers in front of the chemicals as required, to multiply the number of them present in the equation.
2H2 + O2 → 2H2O
Before balancing there are 2 hydrogen atoms and 2 oxygen atoms on the left and 2 hydrogen atoms and 1 oxygen atom on the right.
So, to balance the equation the number of hydrogen molecules is doubled to give 4 hydrogen atoms in total on the left and the number of water molecules is doubled to give 4 hydrogen atoms and 2 oxygen atoms on the right.
This ability to write a balanced formula equation is one of the most important aspects of chemistry and requires a lot of practice. Here is another example drawn from the reaction of a metal with water (see alkali metals below),
sodium + water → sodium hydroxide + hydrogen gas
2Na + 2H2O → 2NaOH + H2
| number of atoms on left | number of atoms on right | |||||
| Na | O | H | Na | O | H | |
| before balancing | 1 | 1 | 2 | 1 | 1 | 3 |
| after balancing | 2 | 2 | 4 | 2 | 2 | 4 |
The finishing touch for a balanced formula equation is to put state symbols next to each chemical to denote what kind of matter it is, i.e. solid, liquid, gas or in an aqueous solution.
| state symbol | represents |
| (s) | solid |
| (l) | liquid |
| (g) | gas |
| (aq) | aqueous solution |
e.g. 2H2(g) + O2(g) → 2H2O(g)
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
back to top written by Dr Richard Clarkson : © Saturday, 1 November 1997
Updated : Friday, 16th March, 2012
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