Page 133 - Theory and Problems of BEGINNING CHEMISTRY

122 CHEMICAL EQUATIONS [CHAP. 8

5. Eliminate any coefficients equal to 1.
6. Always check to see that you have the same number of atoms of each element on both sides of the equation
after you finish.

EXAMPLE 8.4. Sulfuric acid, H 2 SO 4 , reacts with excess sodium hydroxide to produce sodium sulfate and water. Write a
balanced chemical equation for the reaction.
Ans. Step 1: Write down the formulas for reactants and products. Assume a coefficient of 1 for a complicated reactant
or product. Add question marks:

1H 2 SO 4 + ? NaOH −→ ?Na 2 SO 4 + ?H 2 O
Step 2: Balance the S 1 H 2 SO 4 + ? NaOH −→ 1Na 2 SO 4 + ?H 2 O
Balance the Na 1 H 2 SO 4 + 2 NaOH −→ 1Na 2 SO 4 + ?H 2 O
Balance the H 1 H 2 SO 4 + 2 NaOH −→ 1Na 2 SO 4 + 2H 2 O
Since the coefficient of H 2 SO 4 is 1, there is one sulfur atom on the left of the equation. Sulfur appears in
only one product, and so that product must have a coefficient of 1. The one Na 2 SO 4 has two Na atoms in
it, and so there must be two Na atoms on the left; the NaOH gets a coefficient of 2. There are two H atoms
in H 2 SO 4 and two more in two NaOH, and so two water molecules are produced. The oxygen atoms are
balanced, with six on each side.
Step 4 is not necessary.
Step 5: We drop the coefficients of 1 to finish our equation.
Eliminate the 1s H 2 SO 4 + 2 NaOH −→ Na 2 SO 4 + 2H 2 O
Step 6: Check: We find four H atoms, one S atom, two Na atoms, and six O atoms on each side. Alternatively
(step 3), we count four H atoms, one SO 4 group, two Na atoms, and two other O atoms on each side of the
equation.

EXAMPLE 8.5. Magnesium metal reacts with HCl to produce MgCl 2 and hydrogen gas. Write a balanced equation for
the process.
Ans. Step 1: ? Mg + ? HCl −→ 1 MgCl + ?H 2
2
We note that hydrogen is one of the seven elements that form diatomic molecules when in the elemental
state.
Step 2: 1Mg + 2 HCl −→ 1 MgCl + ?H 2
2
1Mg + 2 HCl −→ 1 MgCl + 1H 2
2
Step 5: Mg + 2 HCl −→ MgCl + H 2
2
Step 6: There are one Mg atom, two H atoms, and two Cl atoms on each side of the equation.
EXAMPLE 8.6. Balance the following equation:

CoF 3 + NaI −→ NaF + CoI 2 + I 2
Ans. Step 1: 1 CoF 3 + ? NaI −→ ? NaF + ? CoI 2 + ?I 2
I
Step 2: 1 CoF 3 + 3 NaI −→ 3 NaF + 1 CoI 2 + 1 2 2
3rd 2nd 2nd 4th
Steps 4 and 5: 2 CoF 3 + 6 NaI −→ 6 NaF + 2 CoI 2 + I 2

8.3. PREDICTING THE PRODUCTS OF A REACTION
Before you can balance a chemical equation, you have to know the formulas for all the reactants and products.
If the names are given for these substances, you have to know how to write formulas from the names (Chap. 6).
If only reactants are given, you have to know how to predict the products from the reactants. This latter topic is
the subject of this section.