Introduction to Chemical Equations and Reactions

A chemical reaction is the
process by which one or

more substances are

changed into one or more

different substances.
(aka chemical change)

Bonds between atoms of
substances are broken, atoms are
rearranged, and new bonds are
formed—forming new substances
with different properties!

A chemical reaction is

represented by a

chemical equation.

Reactant:

✔ substances that combine
✔ substances to the left

In any chemical reaction, the
original substances are known as
the reactants . . .

Two parts of a

chemical equation

Product:

✔ substances formed
✔ substances to the right

Two parts of a

chemical equation

. . . and the resulting substances are
know as the products.

The arrow (➔) separates the reactants from the
products. (Read as “yields”)

Chemical Equations

Represent a reaction in writing

2H2 + O2 ➔ 2H2O

• Energy change (Heat, Light)

• Production of gas (bubbles, fizz, smoke)

• Change in color

• Formation of a precipitate

A solid that is produced
as a result of a chemical
reaction in solution and
that separates from the
solution is known as a
precipitate.

Change of state or phase is a
physical change. (melting,
freezing, condensing, etc…)
The same substance is still
present – just in a different
physical state.

Dissolving something is
a physical change. The
substance is still
present- just dissolved in
a solvent. It can be
separated!

Atoms are neither created nor destroyed in
ordinary chemical reactions. (just
rearranged)

•Same number of atoms of each element
must appear on both sides of the
equation.

• For an equation to correctly
describe a reaction, it must be “balanced.”

According to the law of
conservation of mass, the total
mass of reactants must equal
the total mass of products for
any given chemical reaction.

Before we begin balancing
chemical equations, we must
know how to use coefficients.

A coefficient is a whole
number that appears in front
of a formula in a chemical
equation.

• Coefficients are numbers placed in

front of a symbol or formula and are
used to balance an equation.

• Example:

H2 + O2🡪 H2O
2 H2 + O2🡪 2 H2O

• Never alter a subscript when trying to

balance a chemical equation.

2H2O

Coefficient
(# molecules)

Subscript
(#atoms)

Number of Atoms =

(subscript) x (coefficient)

If there is no coefficient, it
is understood to be one (1)

4 Si2H3

8 Si
12 H

2 H2SO4

4 H
2 S
8 O