A balanced equation has equal numbers of each type of atom on each side of the equation.

 

The Law of Conservation of Mass is the rationale for balancing a chemical equation. The law was discovered by Antoine Laurent Lavoisier (1743-94) and this is his formulation of it, translated into English in 1790 from the Traité élémentaire de Chimie (which was published in 1789):

 

"We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements."

 

A less wordy way to say it might be:

 

"Matter is neither created nor destroyed."

 

Therefore, we must finish our chemical reaction with as many atoms of each element as when we started.

 

 

Example: Balance the following equation: H2 + O2 ---> H2O

It is an unbalanced equation (sometimes also called a skeleton equation). This means that there are UNEQUAL numbers at least one atom on each side of the arrow. By the way, a skeleton equation is not wrong, it just hasn't been balanced yet. Presenting it as being balanced would be wrong.

In the example equation, there are two atoms of hydrogen on each side, BUT there are two atoms of oxygen on the left side and only one on the right side.

Remember this: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow.

An equation is balanced by changing coefficients in a somewhat trial-and-error fashion. It is important to note that only the coefficients can be changed, NEVER a subscript.

Important point: the coefficient times the subscript gives the total number of atoms.

Three quick examples before balancing the equation.

(a) 2H2 - there are 2 x 2 atoms of hydrogen (a total of 4).

(b) 2H2O - there are 2 x 2 atoms of hydrogen (a total of 4) and 2 x 1 atoms of oxygen (a total of 2).

(c) 2(NH4)2S - there are 2 x 1 x 2 atoms of nitrogen (a total of 4), there are 2 x 4 x 2 atoms of hydrogen (a total of 16), and 2 x 1 atoms of sulfur (a total of 2).

And, one more: how many oxygens are indicated: 3Ca(NO3)2

Answer: 18. The 3 on the nitrate times 2 outside the parenthesis equals 6 oxygen in one formula unit. The coefficient of three times the 6 gives the final answer of 18.

 

 

 

Two things you CANNOT do when balancing an equation.

1) You cannot change a subscript.

You cannot change the oxygen's subscript in water from one to two, as in:

H2 + O2 ---> H2O2

True, this is a balanced equation, but you have changed the substances in it. H2O2 is a completely different substance from H2O. So, it's not the answer to the question that was asked.

2) You cannot place a coefficient in the middle of a formula. <--- I actually had a student do this. I was gentle in my correction of his mistake.

The coefficient goes at the beginning of a formula, not in the middle, as in this example:

H2 + O2 ---> H22O

Water only comes as H2O and you can only use whole formula units of it.