Balancing Equations

Chemical equations express how much and what type of reactants must be used to obtain a given
quantity of product. From the law of conservation of mass, the mass of the reactants in a reaction
must be equal to the mass of the products. More specifically, chemical equations must be balanced
so that there are the same number of atoms of each element in the products as there are in the
reactants. Stoichiometry is essentially the study of how the quantities of reactants and products are
related in a chemical reaction. Stoichiometric coefficients are numbers used to indicate the number
of moles of a given species involved in the reaction. For example, the reaction for the formation of
water is:

2H 2    (g) +   O 2     (g) →   2H 2 O  (g)

The coefficients indicate that two moles of H 2 gas must be reacted with one mole of O 2 gas to
produce two moles of water. In general, stoichiometric coefficients are given as whole numbers.

Given the identities of the compounds participating in a reaction (the reactants and products), you
need to balance the equation for the reaction before you can deduce any stoichiometric
information from it. When balancing an equation, the important thing to realize is that you can only
change the number in front of a compound, the one that tells you how many molecules (or moles)
of that compound are needed for the reaction to occur. You may not change the subscripts—that
would change the nature or the identity of the compound, and hence the reaction itself.

For example, in the reaction on the previous page, if we are just given the information H 2 and O 2
react to form H 2 O and are told to balance the equation to determine the molar relationships, we
may not write H 2 + O 2 → H 2 O 2 . Yes, all the elements are balanced, but the reaction has changed: We
have written the formation of hydrogen peroxide instead of water! To correctly balance the
equation, we must ensure that there are the same number of atoms of each type of element. Since
we cannot change subscripts, we place coefficients in front of the molecules.