Themes > Science > Chemistry > Inorganic Chemistry > Chemical Reaction , Chemical Formulas , Chemical Equation > Chemical Equation > Balancing Equations by Counting Atoms
Suppose that we discover by experiment that combination of hydrogen molecules and oxygen molecules produce water molecules. We further discover that the formulas of these substances are H2, O2, and H2O. We can then write the chemical equation,

H2 + O2 => H2O

The chemical equation is a before-after picture of the locations of the atoms. However, in a chemical reaction, atoms can neither be created nor destroyed. But it would appear, in the above equation, that an oxygen atom has been destroyed. The problem is not with the formulas but rather with the coefficients, assumed to be "1" since no other number is written. The process of balancing an equation is simply finding those coefficients so that no atoms are created or destroyed. The formulas, determined by other means, may not be altered. Thus water is H2O and not H2O2 although we can make 2 H2O. To balance the equation we must balance the oxygens, and the only means allowable is to place a "2" before the H2O.

H2 + O2 => 2 H2O.

Now there are 2 oxygen atoms on each side but the hydrogens, once balanced, are no longer. Placing a "2" in front of H2 will balance the equation.

2 H2 + O2 => 2 H2O.

Since no atoms may be created or destroyed in a chemical reaction, all that can happen is rearrangement. The diagram to the right illustrates how this chemical reaction results from bonds breaking and forming.
In more difficult equations, trial and error is not effective because it wastes a lot of time. Some rules to simplify the process follow. Find the elements which appear in the fewest numbers of molecules and balance these first. Continue in sequence until you balance the element which appears in the most molecules last. For example:

NaOH + H2SO4 => Na2SO4 + H2O

Na and S appear in only 2 molecules, H in 3 molecules, and O in four. This is the order in which they should be balanced. The S is already balanced (1 on each side), and the Na may be balanced as follows:

2 NaOH + H2SO4 => Na2SO4 + H2O

The hydrogen is a little more difficult. There are two H atoms in 2 NaOH and another two in H2SO4 so there are four H atoms before. But there are only two H atoms after. The H is then balanced by placing a two in front of the water:

2 NaOH + H2SO4 => Na2SO4 + 2 H2O


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