Page 131 - Theory and Problems of BEGINNING CHEMISTRY

CHAPTER 8

Chemical

Equations

8.1. INTRODUCTION
A chemical reaction is described by means of a shorthand notation called a chemical equation. One or
more substances, called reactants or reagents, are allowed to react to form one or more other substances, called
products. Instead of using words, equations are written using the formulas for the substances involved. For
example, a reaction used to prepare oxygen may be described in words as follows:
Mercury(II) oxide, when heated, yields oxygen gas plus mercury.

Using the formulas for the substances involved, the process could be written
heat
HgO −→ O 2 + Hg (unfinished)
A chemical equation describes a chemical reaction in many ways as an empirical formula describes a chemical
compound. The equation describes not only which substances react, but also the relative number of moles of each
reactant and product. Note especially that it is the mole ratios in which the substances react, not how much is
present, that the equation describes. To show the quantitative relationships, the equation must be balanced. That
is, it must have the same number of atoms of each element used up and produced, except for special equations
that describe nuclear reactions (Chap. 19). The law of conservation of mass is thus obeyed as well as the “law of
conservation of atoms.” Coefficients are written in front of the formulas for elements and compounds to tell how
many formula units of that substance are involved in the reaction. A coefficient does not imply any chemical
bonding between units of the substance it is placed before. The number of atoms involved in each formula unit
is multiplied by the coefficient to get the total number of atoms of each element involved. Later, when equations
with individual ions are written (Chap. 9), the net charge on each side of the equation, as well as the numbers of
atoms of each element, must be the same to have a balanced equation. The absence of a coefficient in a balanced
equation implies a coefficient of 1.
The balanced equation for the decomposition of HgO is
heat
2 HgO −→ O 2 + 2Hg
| ↑
——–coefficients—
EXAMPLE 8.1. Draw a ball-and-stick diagram depicting the reaction represented by the following equation:
2H 2 + O 2 −→ 2H 2 O
Ans. The diagram is shown in Fig. 8-1.
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