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Atomic Structure

Balancing Chemical Equations - Example 2.

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 2 is the reaction between lithium and oxygen.

(1)   Li  +    O₂              Li₂O
On the left side of the arrow, there is 1Li and 2O.
On the right side of the arrow, there are 2Li and 1O.
The reaction is not balanced
because atoms are not gained or lost in a chemical reaction.

The two oxygens in O₂ cannot be changed
since O₂ is the formula of oxygen.
The two lithiums in Li₂O cannot be changed
since Li₂O is the formula of lithium oxide. 

A big number in front of the symbol
changes the number of elements in the formula that follows it.
2Li means two lithium ions.
2Li₂O means four lithium ions with two oxide ions.

(1)   Li  +    O₂              Li₂O

First balance for the metal (lithium).
There is one Li on the left, but two Li on the right.
So, put a 2 before Li on the left.

The equation becomes
(2)   2Li  +    O₂              Li₂O
Now the equation is balanced for lithium, 2 on each side.

But the equation is still unbalanced for oxygen.
There are 2 on the left and 1 on the right.
To get an extra oxygen on the right, we can put a 2 before Li₂O.
(3)   2Li  +    O₂              2Li₂O
The equation is now balanced for oxygen, 2 on each side.

Re-check for lithium.
There are two Li on the left
but now there are four Li on the right.
So, put a 4 before Li on the left.
(4)   4Li  +    O₂              2Li₂O
The equation is now balanced.

There are four Li with two O on the left.
There are four Li with two O on the right.

Showing the four steps together, we have

(1)  Li  +  O₂      Li₂O unbalanced for lithium and oxygen.
(2)   2Li  +  O₂      Li₂O        unbalanced for oxygen.       
(3)   2Li  +  O₂     2Li₂O      unbalanced for lithium again.
(4)   4Li  +  O₂     2Li₂O                  Balanced!                 

See example 1.   The reaction of potassium with chlorine.

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