CHEMICAL REACTIONS & EQUATIONS
OBJECTIVE
To be able to recognize/distinguish the different types of chemical reactions.
THEORETICAL BACKGROUND
A chemical reaction, also known as a chemical change, occurs when one or more
substances are transformed into one or more new substances. A chemical equation includes
symbols and formulas of all the substances involved in a reaction. It shows the changes in the
composition that the substances undergo. It also indicates the relative amount of each substance
that participates in the change. The substances present at the start of the reaction are called
reactants. The substances formed in the chemical reaction are known as the products.
According to the law of conservation of matter, in an ordinary chemical reaction no mass
is lost or gained. The same amount of matter is present before and after a reaction. The atoms of
the reactants are just rearranged to form the products. Therefore, in a balanced chemical
equation, the same number and kinds of atoms are present on both sides of an equation.
Chemical reactions maybe classified into four general types.
1. Combination Reaction. In a combination reaction, two or more substances combine
to form one new substance. The general form of this type of reaction is element or compound +
element or compound  compound
2 Na + Cl2  2 NaCl
2. Decomposition Reaction. In a decomposition reaction, a single substance breaks
down into two or more simple substances. This type of reaction has the general form compound
 two or more elements or compounds
PCl5  PCl3 + Cl2
3. Single-Replacement Reaction. A single-replacement reaction is one in which an
element replaces another in a compound. Here the general form is element + compound 
element + compound
Mg + Ni(NO3)2  Ni + Mg(NO3)2
4. Double-Replacement Reaction. Positive and negative portions of two compounds
are interchanged in a double-replacement reaction. This type of reaction is recognizable by its
general form compound + compound  compound + compound
AgNO3 + NaCl  AgCl + NaNO3
A double-replacement reaction is expected to take place when an insoluble compound, a gas, a
non-electrolyte or a weak electrolyte is formed as a product.