Page 135 - Theory and Problems of BEGINNING CHEMISTRY

124 CHEMICAL EQUATIONS [CHAP. 8

Binary compounds may yield two elements or an element and a simpler compound. Ternary (three-element)
compounds may yield an element and a compound or two simpler compounds. These possibilities are shown in
Fig. 8-2.

Two elements
Binary compound
An element and a compound
Ternary compound
Two compounds
Fig. 8-2. Decomposition possibilities

A catalyst is a substance that speeds up a chemical reaction without undergoing a permanent change in its
own composition. Catalysts are often but not always noted above or below the arrow in the chemical equation.
Since a small quantity of catalyst is sufficient to cause a large quantity of reaction, the amount of catalyst need
not be specified; it is not balanced as the reactants and products are. In this manner, the equation for a common
laboratory preparation of oxygen is written as

MnO 2
2 KClO 3 −→ 2 KCl + 3O 2
EXAMPLE 8.9. Write a complete, balanced equation for the reaction that occurs when (a)Ag 2 O is heated, (b)H 2 Ois
electrolyzed, and (c) CaCO 3 is heated.
Ans. (a) With only one reactant, what can happen? No simpler compound of Ag and O is evident, and the compound
decomposes to its elements. Remember that oxygen occurs in diatomic molecules when it is uncombined:
(unbalanced)
2
Ag O −→ Ag + O 2
2Ag O −→ 4Ag + O 2
2
(b) Note that in most of these cases, energy of some type is added to make the compound decompose.
electricity
2H 2 O −−−−→ 2H 2 + O 2
(c) A ternary compound does not yield three elements; this one yields two simpler compounds.
CaCO 3 −→ CaO + CO 2

Substitution or Replacement Reactions
Elements have varying abilities to combine. Among the most reactive metals are the alkali metals and the
alkaline earth metals. On the opposite end of the scale of reactivities, among the least active metals or the most
stable metals are silver and gold, prized for their lack of reactivity. Reactive means the opposite of stable,but
means the same as active.
When a free element reacts with a compound of different elements, the free element will replace one of the
elements in the compound if the free element is more reactive than the element it replaces. In general, a free metal
will replace the metal in the compound, or a free nonmetal will replace the nonmetal in the compound. A new
compound and a new free element are produced. As usual, the formulas of the products are written according
to the rules in Chap. 5. The formula of a product does not depend on the formula of the reacting element or
compound. For example, consider the reactions of sodium with iron(II) chloride and of fluorine with aluminum
oxide:
2Na + FeCl 2 −→ 2 NaCl + Fe
6F 2 + 2Al 2 O 3 −→ 4 AlF 3 + 3O 2
Sodium, a metal, replaces iron, another metal. Fluorine, a nonmetal, replaces oxygen, another nonmetal.
(In some high-temperature reactions, a nonmetal can displace a relatively inactive metal from its compounds.)
The formulas are written on the basis of the rules of chemical bonding (Chap. 5).