Page 132 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 8] CHEMICAL EQUATIONS 121

H H
H O H
O
+ +
O
H O H
H H
Fig. 8-1. The reaction of hydrogen and oxygen

EXAMPLE 8.2. Interpret the following equation:

2Na + Cl 2 −→ 2 NaCl
Ans. The equation states that elemental sodium reacts with elemental chlorine to produce sodium chloride, table salt.
(The fact that chlorine is one of the seven elements that occur in diatomic molecules when not combined with other
elements is indicated.) The numbers before Na and NaCl are coefficients, stating how many formula units of these
substances are involved. If there is no coefficient in a balanced equation, a coefficient of 1 is implied, and so the
absence of a coefficient before the Cl 2 implies one Cl 2 molecule. The equation thus states that when the two reagents
react, they do so in a ratio of two atoms of sodium to one molecule of chlorine, to form two formula units of sodium
chloride. In addition, it states that when these two reagents react, they do so in a ratio of 2 mol of sodium atoms to
1 mol of chlorine molecules, to form 2 mol of sodium chloride. The ratios of moles of each substance involved to
every other substance are implied:

2 mol Na 2 mol Na 1 mol Cl 2
1 mol Cl 2 2 mol NaCl 2 mol NaCl
2 mol NaCl 2 mol NaCl
1 mol Cl 2
2 mol Na 2 mol Na 1 mol Cl 2

EXAMPLE 8.3. How many ratios are implied for a reaction in which four substances are involved?
Ans. 12. (Each of the four coefficients has a ratio with the other three, and three 4s equal 12.)

8.2. BALANCING SIMPLE EQUATIONS
If you know the reactants and products of a chemical reaction, you should be able to write an equation for
the reaction and balance it. In writing the equation, first write the correct formulas for all reactants and products.
After they are written, only then start to balance the equation. Do not balance the equation by changing the
formulas of the substances involved. For simple equations, you should balance the equation “by inspection.”
(Balancing of oxidation-reduction equation will be presented in Chap. 14.) The following rules will help you to
balance simple equations.
1. Before the equation is balanced, there are no coefficients for any reactant or product; after the equation
is balanced, the absence of a coefficient implies a coefficient of 1. Assume a coefficient of 1 for the most
complicated substance in the equation. (Since you are getting ratios, you can assume any value for one of
the substances.) Then work from this substance to figure out the coefficients of the others, one at a time. To
prevent ambiguity while you are balancing the equation, place a question mark before every other substance.
2. Replace each question mark as you figure out each real coefficient. After you get some practice, you will not
need to use the question marks. If an element appears in more than one reactant or more than one product,
balance that element last.
3. Optionally, if a polyatomic ion that does not change during the reaction is involved, you may treat the whole
thing as one unit, instead of considering the atoms that make it up.
4. After you have provided coefficients for all the substances, if any fractions are present, multiply every
coefficient by the same small integer to clear the fractions.