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When an acid reacts with a base (an alkaline compound), a salt (d) The reactant is a compound, and the products are a simpler
and water are formed compound and an element, following the generalized form of a
decomposition reaction, XY → X + Y. The delta sign (Δ) also
HCl(aq) + NaOH(aq) → NaCl(aq) + H 2 O(l) means that heat was added, which provides another clue that this
is a decomposition reaction. The formula for the chlorate ion is
The reactions of acids and bases are discussed in chapter 11.
in Table 9.3. The balanced equation is

Δ
2 KClO 3 (s) → 2 KCl(s) + 3 O 2 ↑

CONCEPTS Applied
Chemical Reactions
Look around your school and home for signs that a chemical
reaction has taken place. Can you find evidence that a 10.4 INFORMATION FROM
reaction has taken place with oxygen? Can you find new CHEMICAL EQUATIONS
substances being made or decomposition taking place?
A balanced chemical equation describes what happens in a
chemical reaction in a concise, compact way. The balanced
equation also carries information about (1) atoms, (2) mol-
ecules, and (3) atomic weights. The balanced equation for the
EXAMPLE 10.8 combustion of hydrogen, for example, is
Write complete balanced equations for the following, and identify if
2 H 2 ( g) + O 2 ( g) → 2 H 2 O(l)
each reaction is combination, decomposition, replacement, or ion
exchange: An inventory of each kind of atom in the reactants and prod-
ucts shows
(a) silver(s) + sulfur(g) → silver sulfide(s)
(b) aluminum(s) + iron(III) oxide(s) → aluminum oxide(s) + iron(s) Reactants: 4 hydrogen Products: 4 hydrogen
(c) sodium chloride(aq) + silver nitrate(aq) → ? 2 oxygen 2 oxygen

(d) potassium chlorate(s) Δ potassium chloride(s) + oxygen(g) Total: 6 atoms Total: 6 atoms
→
There are six atoms before the reaction and there are six atoms
SOLUTION
after the reaction, which is in accord with the law of conserva-
tion of mass.
(a) The reactants are two elements, and the product is a compound,
following the general form X + Y → XY of a combination reac- In terms of molecules, the equation says that two diatomic
1+
tion. Table 9.2 gives the charge on silver as Ag , and sulfur (as the molecules of hydrogen react with one (understood) diatomic
2–
other nonmetals in family VIA) is S . The balanced equation is molecule of oxygen to yield two molecules of water. The num-
ber of coefficients in the equation is the number of molecules
2 Ag(s) + S(g) → Ag 2 S(s)
involved in the reaction. If you are concerned how two mol-
Silver sulfide is the tarnish that appears on silverware. ecules plus one molecule could yield two molecules, remember
(b) The reactants are an element and a compound that react to form that atoms are conserved in a chemical reaction, not molecules.
a new compound and an element. The general form is Since atoms are conserved in a chemical reaction, their
XY + Z → XZ + Y, which describes a replacement reaction. The atomic weights should be conserved, too. One hydrogen atom
balanced equation is
has an atomic weight of 1.0 u, so the formula weight of a
diatomic hydrogen molecule must be 2 × 1.0 u, or 2.0 u. The
2 Al(s) + Fe 2 O 3 (s) → Al 2 O 3 (s) + 2 Fe(s)
formula weight of O 2 is 2 × 16.0 u, or 32 u. If you consider the
This is known as a thermite reaction, and in the reaction,
aluminum reduces the iron oxide to metallic iron with the release equation in terms of atomic weights, then
of sufficient energy to melt the iron. The thermite reaction is Equation
sometimes used to weld large steel pieces, such as railroad rails.
(c) The reactants are water solutions of two compounds with the 2 H 2 + O 2 →2 H 2 O
general form of AX + BY →, so this must be the reactant part of
Formula weights
an ion exchange reaction. Completing the products part of the
equation by exchanging parts as shown in the general form and 2(1.0 u + 1.0 u) + (16.0 u + 16.0 u) → 2(2 × 1.0 u + 16.0 u)
balancing,
4 u + 32 u → 36 u
NaCl(aq) + AgNO 3 (aq) → NaNO 3 (?) + AgCl(?)
36 u → 36 u
Now consult the solubility chart in appendix B to find out if
either of the products is insoluble. NaNO 3 is soluble and AgCl is The formula weight for H 2 O is (1.0 u × 2) + 16 u, or 18 u. The
insoluble. Since at least one of the products is insoluble, the reac-
coefficient of 2 in front of H 2 O means there are two molecules
tion did take place, and the equation is rewritten as
of H 2 O, so the mass of the products is 2 × 18 u, or 36 u. Thus,
NaCl(aq) + AgNO 3 (aq) → NaNO 3 (aq) + AgCl↓ the reactants had a total mass of 4 u + 32 u, or 36 u, and the

10-13 CHAPTER 10 Chemical Reactions 263