Chemistry Reference
In-Depth Information
B. What difference, if any, is there between the following equations?
R
Q £ T
Z
Q
R £ Z
T
8.2
Balancing Equations
Our first major task of this chapter is to learn to balance equations for chemi-
cal reactions. Balancing simple equations will be covered in this chapter; equa-
tions for more complicated oxidation-reduction reactions will be considered in
Chapter 16.
The first step in writing a complete and balanced equation for a chemical
reaction is to write correct formulas for the reactants and products. To help us
as we learn, we might write the equation in words and later write the formulas.
Correct formulas cannot be changed to make an equation balance! Only after
the correct formulas have been written can we go on to the next step. Then, we
use coefficients to change the numbers of formula units to get the same num-
ber of atoms of each element on the two sides of the equation.
For example, the unbalanced equation for the reaction of sulfur dioxide with
oxygen to give sulfur trioxide is
To complete an equation for any
type of reaction, write the proper
formulas for the reactants and
products before starting to
balance the equation.
SO
2
O
2
£ SO
3
(Not balanced)
Don't forget that elemental oxygen occurs as With one molecule of each
substance, the numbers of oxygen atoms on the two sides of the equation are
not equal, so the equation is not balanced. We can balance the equation by
inserting proper coefficients in front of the formulas:
O
2
.
2 SO
2
O
2
£ 2 SO
3
(Balanced)
The number 2 before the indicates that there are two sulfur dioxide mole-
cules, containing two sulfur atoms and four oxygen atoms. There are two more
oxygen atoms in the molecule. Because there are two sulfur atoms and six
oxygen atoms in the two molecules, the equation is now balanced. We must
always check an equation after we balance it to make sure that the numbers of
atoms of each element on each side of the arrow are equal.
One of the problems encountered by students just learning to balance
equations is that the absence of a coefficient in a balanced equation means a
coefficient of 1, but the absence of a coefficient before the equation is fully bal-
anced might mean that this substance has not yet been considered. To avoid any
confusion, we can place a question mark before each formula when we start to
balance an equation. (After we have had a lot of practice, we will not need to
use the question marks.)
SO
2
O
2
SO
3
EXAMPLE 8.1
Balance the equation for the reaction of barium hydroxide and hydrobromic acid
to give barium bromide and water.
