For application of these steps, consider this description of a reaction. Calcium
metal and water react to yield calcium hydroxide and hydrogen gas.
a. Write the symbols for all elements involved.
Ca, O, H
b. Write the correct formulas for any compounds and check for diatomic
Ca + H2O → Ca(OH)2 + H2
c. Balance the equation by placing coefficients where appropriate. Look at the
number of atoms of each element in the products and reactants.
It is apparent here that there are twice as many oxygen and hydrogen atoms in the
products as reactants. How can this equation be balanced to give equal numbers of
atoms on both sides? Fill in the coefficients of the molecules in the equation below.
H2O → _____
Ca(OH)2 + ________ H2 ↑
Since there are twice as many hydrogen and oxygen atoms on the right as on the left, if
we could double the numbers of these atoms on the left, we would have a balanced
equation. This can be done by placing a two in front of H2O. All the other coefficients
would be one (if there is no coefficient, we assume it is one, so there is no need to write
it in front of each molecule).
We have implied that all reactions only go in the direction of the products, but this
is not always the case. Sometimes as products are formed, they react with one another
or decompose to form the reactants. Thus, the reaction is going in both directions at the
same time, and if allowed to continue indefinitely, would result in a constant amount of
products and reactants. Reactions that go in both directions are called equilibrium
reactions, and when the rate of formation of product is the same as the rate of formation