This sometimes seems hard, but it isn’t really.
Reactants go on the left, products on the right. The number of reactant and product particles has to be the same on both sides, which means that we can’t actually lose anything. The see-saw balances, or mass is conserved in other words – the particles are like money and we have to account for every single atom. Here’s an example.
When heated, aluminium reacts with solid black copper oxide to produce copper metal and aluminium oxide:
Writing the reaction down in symbols, making sure you KNOW them:
Al + CuO→ Al2O3 + Cu
The equation doesn’t balance, until we do this:
2Al + 3CuO→ Al2O3 + 3Cu
To solve this, I started by saying ‘I need 2 Al’s on the left and 3 more oxygens, so 3CuO’s’
A few tricks:
You can only put numbers in front of molecules, never change the formula of the compound itself.
H4O5 No! No! This isn’t water!
Don’t worry if when you start adding up, the numbers turn out to be fractions – you can always double or treble all the numbers at a later stage.
Balance complicated molecules with lots of different atoms first. Putting numbers in front of these may mess up other molecules, so use the simpler molecules to adjust these major changes.
If you recognise the atoms making up a standard group such as sulphate, nitrate, phosphate, ammonium etc.that survive unscathed throughout the chemical reaction, treat them as an indivisible item to be balanced as a whole. This makes life easier and helps understanding of the chemistry.
Leave molecules representing elements until last. This means that any numbers you put in front of those molecules won’t unbalance any other molecule.
Click here for all the details. Work through all of this, there’s an exercise at the end.