GCSE CHEMISTRY - How can I Balance a Chemical Equation? - Rules for Balancing Chemical Reactions

How to Balance Chemical Equations - Example 1.

When we are balancing chemical equations we first
need to know what the symbols and numbers mean.
Click here if you are not sure.

Example 1 is the reaction between potassium and chlorine.

(1) K + Cl₂ KCl
On the left side of the arrow, there is 1K and 2Cl.
The two chlorines in Cl₂ cannot be changed,
since Cl₂ is the formula of chlorine.
On the right side of the arrow, there is 1K and 1Cl.
The reaction is not balanced
because atoms are not gained or lost in a chemical reaction.
The right side needs an extra chlorine.

A big number in front of the symbol
changes the number of elements in the formula that follows it.
2K means two potassium ions. 2Cl₂ means four chlorine atoms.
2KCl means two potassium ions with two chlorine ions.

To get an extra chlorine on the right side, we can put a 2 before KCl.
The equation becomes
(2) K + Cl₂ 2KCl
Now the equation is balanced for chlorine, 2 on each side.

But the equation has now become unbalanced for potassium.
There is 1 on the left and 2 on the right.
To get an extra potassium on the left, we can put a 2 before K.
(3) 2K + Cl₂ 2KCl
The equation is now balanced.
On the left there are 2 potassiums and 2 chlorines.
On the right there are 2 potassiums and 2 chlorines.

Showing the three steps together, we have

(1)    K + Cl₂   KCl      unbalanced for chlorine.
(2)    K + Cl₂   2KCl     unbalanced for potassium.
(3)    2K + Cl₂   2KCl                   Balanced!

See example 2. The reaction of lithium with oxygen.

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