Balancing Equations

If you want to get a good grade you’re going to have to become a ninja at balancing equations. We’ve included a few examples at the bottom of this page so you can get practicing your skills.

Everything On One Side Equals Everything On The Other

During chemical reactions nothing is lost or made…it’s just rearranged. At the end of a reaction you still have the same number of atoms you had at the start.

Balanced symbol equations show the atoms at the start (the reactant atoms) and the atoms at the end (the product atoms).

Let’s take an example:

Word Equation: Copper + Oxygen →    Copper Oxide

 Balanced symbol equation: 2Cu + O2    →    2CuO

The diagram below shows what is going on. As you have two copper and two oxygen atoms at the start that’s what you need to have at the end.

copper and oxygen reaction

Note: The mass of both sides of the equation is equal too.

So How The Hell Do You Balance An Equation?

So examiners like to ask you to balance an equation. What you basically need to do is pout numbers in front of the formulas until both sides of the equation balance out. You can’t change numbers like H2O – you can only change the numbers in front!

So let’s have a look at an example – say in an exam you were given this formula:

H2SO4 + NaOH arrow Na2SO4 + H2O

This is the equation for the reaction between sulphuric acid and sodium hydroxide which gives you sodium sulfate and water.

Let’s add up what we have on each side of the equation:

Left Side: 3 hydrogen, 5 oxygen, 1 sulphur, 1 sodium       Right Side: 2 hydrogen, 5 oxygen, 1 sulphur, 2 sodium

You now have to have a little play around with the numbers. Use a pencil because you will be using trial and error.

Here we can see we need more hydrogen on the right hand side. So let’s add in a two infron of the water molecule on the right hand side:

H2SO4 + NaOH arrow Na2SO4 + 2H2O

Left Side: 3 hydrogen, 5 oxygen, 1 sulphur, 1 sodium       Right Side: 4 hydrogen, 6 oxygen, 1 sulphur,  2 sodium

Now we have too many hydrogens and oxygens on the right hand side. So let’s try adding a two to the left hand side:

H2SO4 + 2NaOH arrow Na2SO4 + 2H2O

Left Side: 4 hydrogen, 6 oxygen, 1 sulphur, 2 sodium       Right Side: 4 hydrogen, 6 oxygen, 1 sulphur,  2 sodium

So it all balances! We did it quite quickly – but it will probably take a lot more fiddling around for you.

Let’s take an example:

H2 + O2 → H2O

[item title=”Can you balance this out?”]2H2 + O2 → 2H2O[/item][/accordion]

What about this one: Al + H2SO4 → Al2(SO4)3 + H2

Remember the number outside the brackets applies to everything inside it. So on the left side you have 3 x 4 oxygen atoms…i.e. 12 oxygen atoms

[item title=”Click for answer”]OK this is a really tricky one!

2Al + 3H2SO4 Al2(SO4)3 + 3H2