Page 86 - Chemistry_Form_2

Chemistry
for Secondary Schools

A full-headed arrow is used to separate reactants and products for an irreversible
reaction.
For example:
Reactants Products
FOR ONLINE READING ONLY
Double half-headed arrows pointing in the opposite directions are used to separate
reactants and products for a reversible reaction. For example:

Reactants forward reaction Products
backward reaction

The arrows show the direction of a reaction; thus, it means ‘produce’ or ‘yield’.
Each individual substance is separated from the other by a plus sign (+). Note that
the number of reactants and products are not necessarily the same. A chemical
equation has the following key characteristics:
(a) Reactants and products: It lists the substances involved in the reaction, with
reactants on the left side and products on the right side.
(b) Chemical formulas: Each substance is represented using its chemical formula
(for example H O for water).
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(c) Direction of reaction: An arrow points from the reactants to the products,
indicating the direction of the reaction.

(d) Balanced equation: A chemical equation follows the law of conservation of
mass, ensuring that the number of atoms for each element is the same on
both sides of the equation.

(e) States of matter: The physical state of each substance is often indicated.
(f) Energy changes: If applicable, energy changes such as heat or light may be
noted.
(g) Reaction conditions: Temperature, pressure or catalysts may be noted above
or below the arrow.

Molecular equations
A molecular equation is an equation representing a reaction showing the

reactants and products in undissociated form. In molecular equations, reactants
and products are considered neutral regardless of their exact physical states. The
following are examples of molecular equations:

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