(3) Single replacement reactions. The general equation for a single
replacement reaction is A + BC → AC + B. An example is:
Zn + CuSO4 → ZnSO4 + Cu
This equation tells us that one atom of zinc and one molecule of cupric sulfate yield one
molecule of zinc sulfate and one atom of copper.
(4) Double replacement reactions. The most commonly occurring reaction
is the double replacement reaction. The general equation for this reaction is AB + CO
→ AD + CB. Double replacement reactions can be further subdivided into several
classes. The most common of these classes are the precipitation reaction, the acid-
base reaction, and the oxidation-reduction reaction. An example of the precipitation
BaCl2 + Na2SO4 → 2 NaCl + BaSO4
This equation tells us that one molecule of barium chloride reacts with one molecule of
sodium sulfate to yield two molecules of sodium chloride and one molecule of barium
sulfate as a precipitate. Acid-base and oxidation-reduction reactions will be covered
WRITING CHEMICAL EQUATIONS
At this point, you have seen several examples of chemical equations and should
be familiar with the symbols used in an equation. We will now examine the process of
writing an equation when we are given a verbal description of the reaction. One general
rule that must be kept in mind is that there will always be the same number and kinds of
atoms in the products of a reaction as in the reactants. This is because matter can
neither be created nor destroyed in a chemical reaction and atoms always combine in
certain proportions. When given a written verbal description of a chemical reaction, the
following steps are used to write the equation for the reaction.
a. Write the symbols for all elements involved.
b. Write the correct formulas for any compounds and check for diatomic
molecules. (Some elements never exist as single atoms but only as diatomic
molecules. These elements can be identified from their names, which end in -gen or
-ine. The common diatomic molecules are hydrogen (H2), nitrogen (N2), oxygen (O2),
chlorine (Cl2), fluorine (F2), and bromine (Br2).)
c. Balance the equation by placing coefficients where appropriate.
Remember that there must be equal numbers of atoms of each kind on both sides of the
equation. In this step, the subscripts that were used in writing the correct formulas
cannot be changed.