The Foundations of Chemistry

 

CHEMICAL EQUATIONS

Chemical reactions always involve changing one or more substances into one or more dif-

ferent substances. In other words, chemical reactions rearrange atoms or ions to form

other substances.

Chemical equationsare used to describe chemical reactions, and they show (1) the

substances that react,called reactants;(2) the substances formed,called products;and (3) the

relative amounts of the substances involved.We write the reactants to the leftof an arrow and

the products to the rightof the arrow. As a typical example, let’s consider the combustion

(burning) of natural gas, a reaction used to heat buildings and cook food. Natural gas is

a mixture of several substances, but the principal component is methane, CH 4. The equa-

tion that describes the reaction of methane with excess oxygen is

CH 4 
2O 2 88nCO 2 
2H 2 O

reactants products

What does this equation tell us? In the simplest terms, it tells us that methane reacts with

oxygen to produce carbon dioxide, CO 2 , and water. More specifically, it says that for every

CH 4 molecule that reacts, two molecules of O 2 also react, and that one CO 2 molecule

and two H 2 O molecules are formed. That is,

heat

CH 4 
 2O 2 8888n CO 2 
 2H 2 O

1 molecule 2 molecules 1 molecule 2 molecules

This description of the reaction of CH 4 with O 2 is based on experimental observations.By

this we mean experiments have shown that when one CH 4 molecule reacts with two O 2

molecules, one CO 2 molecule and two H 2 O molecules are formed. Chemical equations

are based on experimental observations.Special conditions required for some reactions are

indicated by notation over the arrow. Figure 3-1 is a pictorial representation of the

rearrangement of atoms described by this equation.

As we pointed out in Section 1-1, there is no detectable change in the quantity of matter

during an ordinary chemical reaction.This guiding principle, the Law of Conservation of

Matter,provides the basis for “balancing” chemical equations and for calculations based

on those equations. Because matter is neither created nor destroyed during a chemical re-

action,

a balanced chemical equation must always include the same number of each kind of

atom on both sides of the equation.

3-1

Sometimes it is not possible to
represent a chemical change with a
single chemical equation. For example,
when too little O 2 is present, both
CO 2 and CO are found as products,
and a second chemical equation must
be used to describe the process. In the
present case (excess oxygen), only one
equation is required.

The arrow may be read “yields.”
The capital Greek letter delta () is
sometimes used in place of the word
“heat.”

90 CHAPTER 3: Chemical Equations and Reaction Stoichiometry

See the Saunders Interactive
General Chemistry CD-ROM,
Screens 4.1–4.4, Equations.

Figure 3-1 Two representations of
the reaction of methane with oxygen
to form carbon dioxide and water.
Chemical bonds are broken and new
ones are formed in each
representation. Part (a) illustrates the
reaction using models, and (b) uses
chemical formulas.

H

H

H

H H

H

H

H

C

OO

O

O

C

O O

O O

CH 4 

2O 2 CO 2 2H 2 O

(a)

(b)