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 is the particles are rearranged as they go from reactants to 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, probably in a different form but they are still there!
Balancing equations
When we balance symbolic equations all we are doing really is simply counting atoms, to make sure that all the atoms are accounted for
and none are lost or have gone missing! The only way to master this skill is to practice it. Luckily there a few simple
rules which make it easy for us.
Working out chemical formula is another useful skill which is easy to do and will improve your confidence in your
chemistry work. However on this page do not worry about any of the formula just concentrate on balancing the equations, if
you would like more help on working out formula then simply click on the link.
Worked examples
Below are some worked examples on how to balance chemical equations, there is also a short video opposite which contains more examples on
balancing equations. Go theough the worked examples and watch the video. When you have done this click on the link below to try the "questions to check your understanding".
The only way to get good at balancing equations is to balance lots of them!
example 1.
magnesium burns in air with a brillant white flash to form magnesium oxide. We can represent this reaction with word and symbolic equations:
Word equation:
magnesium + oxygen → magnesium oxide
symbolic equation:
Mg + O2 → MgO
Now if you look carefully at this symbolic equation you will see that it is 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:
Mg + O2 → MgO2
However MgO2 is not the correct formula for magnesium oxide!
To balance equations the first rule is simple - DO NOT change any of the formula for the compounds. You balance
equations by putting numbers in front of each of the reactants and products till the number of atoms on each side of
the equations balance.
2Mg + O2 → 2MgO
By inserting "2" into the equation above there ae now equal numbers of atoms of each element on the reactants and products side of the equation.
example 2.
Hydrogen burns in air to form hydrogen oxide or water. We can represent this reaction with word and symbolic equations:
Word equation:
hydrogen + oxygen → hydrogen oxide
symbolic equation:
H2 + O2 → H2O
As in example 1 this equation is not balanced. There are 2 atoms of oxygen present on the reactants side of the equation and only 1
atom on the product side. So remember to balance the equation you can only put numbers in the front of each substance,
you cannot change any of the formula given.
2H2 + O2 → 2H2O
by inserting "2" into the equation above we now have 4 hydrogen atoms on the reactants side and 4 on the products side of the equation. There are 2 oxygen atoms on each
side of the equation so it is balanced.
example 3.
Methane is the gas used in Bunsen burners, it burns to form carbon dioxide and water vapour.. We can represent this reaction with word and symbolic equations:
Word equation:
This equation is not balanced, there are not enough hydrogen atoms on the products side of the equation and too many oxygen atoms
on the products side of the equation. To make balancing equations which contain hydrogen and oxygen as well as other elements as a genral rule
balance the oxygen atoms last, the hydrogen atoms second last and any other elements first.
This equation can be balanced by inserting "2" as shown:
CH4 + 2O2 → CO2 + 2H2O
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.
Key points
To balance equations do not change the formulae for any of the reactants or products but simply put numbers
in front of 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.
