Page 134 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 8] CHEMICAL EQUATIONS 123

To simplify the discussion, we will classify simple chemical reactions into five types:

Type 1: combination reactions Type 4: double-substitution reactions
Type 2: decomposition reactions Type 5: combustion reactions
Type 3: substitution reactions
More complex oxidation-reduction reactions will be discussed in Chap. 14.

Combination Reactions
A combination reaction is a reaction of two reactants to produce one product. The simplest combination
reactions are the reactions of two elements to form a compound. After all, if two elements are treated with each
other, they can either react or not. There generally is no other possibility, since neither can decompose. In most
reactions like this, there will be a reaction. The main problem is to write the formula of the product correctly and
then to balance the equation. In this process, first determine the formula of the product from the rules of chemical
combination (Chap. 5). Only after the formulas of the reactants and products have all been written down, balance
the equation by adjusting the coefficients.

EXAMPLE 8.7. Complete and balance the following equations:
(a)Na + F 2 −→ (b)Mg + O 2 −→ (c)K + S −→
Ans. The products are determined first (from electron dot structures, if necessary) to be NaF, MgO, and K 2 S, respectively.
These are placed to the right of the respective arrows, and the equations are then balanced.

(a)2 Na + F 2 −→ 2 NaF (b)2 Mg + O 2 −→ 2 MgO (c)2 K + S −→ K 2 S

EXAMPLE 8.8. Write a complete, balanced equation for the reaction of each of the following pairs of elements:
(a) aluminum and sulfur (b) aluminum and iodine (c) aluminum and oxygen
Ans. (a) The reactants are Al and S. The Al can lose three electrons [it is in periodic group IIIA (3)], and each sulfur
atom can gain two electrons [it is in periodic group VIA (16)]. The ratio of aluminum to sulfur atoms is thus
2 : 3, and the compound which will be formed is Al 2 S 3 :
Al + S −→ Al 2 S 3 (unbalanced)
2Al + 3S −→ Al 2 S 3
(b) The reactants are Al and I 2 . (In its elemental form, iodine is stable as diatomic molecules.) The combination of
a group IIIA (13) metal and a group VIIA (17) nonmetal produces a salt with a 1 : 3 ratio of atoms: AlI 3 .
Al + I 2 −→ AlI 3 (unbalanced)
2Al + 3I 2 −→ 2 AlI 3
(c) Al + O 2 −→ Al 2 O 3 (unbalanced)
4Al + 3O 2 −→ 2Al 2 O 3

It is possible for an element and a compound of that element or for two compounds containing a common
element to react by combination. For example,
MgO + SO 3 −→ MgSO
4
PtF 2 + F 2 −→ PtF 4

Decomposition Reactions
The second type of simple reaction is decomposition. This reaction is also easy to recognize. Typically,
only one reactant is given. A type of energy, such as heat or electricity, may also be indicated. The reactant
usually decomposes to its elements, to an element and a simpler compound, or to two simpler compounds.