Page 136 - Theory and Problems of BEGINNING CHEMISTRY

CHAP. 8] CHEMICAL EQUATIONS 125

You can easily recognize the possibility of a substitution reaction because you are given a free element and
a compound of different elements.

EXAMPLE 8.10. Look only at the reactants in the following equations. Tell which of the reactions represent substitution
reactions.
(c)
(a)3 Mg + N 2 −→ Mg N 2 2 KClO 3 −→ 2 KCl + 3O 2
3
(b)2 Li + MgO −→ Li 2 O + Mg (d ) 2 CrCl 2 + Cl 2 −→ 2 CrCl 3
Ans. Reaction (b) only. (a) is a combination, (c) is a decomposition, and (d ) is a combination. Note that in (d ) elemental
chlorine is added to a compound of chlorine.

If the free element is less active than the corresponding element in the compound, no reaction will take place.
A short list of metals and an even shorter list of nonmetals in order of their reactivities are presented in Table 8-1.
The metals in the list range from very active to very stable; the nonmetals listed range from very active to fairly
active. A more comprehensive list, a table of standard reduction potentials, is presented in general chemistry
textbooks.
Table 8-1 Relative Reactivities of Some Metals and Nonmetals

Metals Nonmetals

Most active metals Alkali and alkaline F Most active nonmetal
earth metals O
Al Cl
Zn Br
Fe I Less active nonmetals
Pb
H
Cu
Less active metals Ag
Au

EXAMPLE 8.11. Complete and balance the following equations. If no reaction occurs, indicate that fact by writing “NR”.
(a) KCl + Fe −→ (b)KF + Cl 2 −→

Ans. (a) KCl + Fe −→ NR (b)KF + Cl 2 −→ NR
In each of these cases, the free element is less active than the corresponding element in the compound, and cannot
replace that element from its compound.

In substitution reactions, hydrogen in its compounds with nonmetals often acts as a metal; hence, it is listed
among the metals in Table 8-1.

EXAMPLE 8.12. Complete and balance the following equation. If no reaction occurs, indicate that fact by writing “NR”.
Al + HCl −→
Ans. 2Al + 6 HCl −→ 2 AlCl 3 + 3H 2
Aluminum is more reactive than hydrogen (Table 8-1) and replaces it from its compounds. Note that free hydrogen
is in the form H 2 (Sec. 5.2).

In substitution reactions with acids, metals that can form two different ions in their compounds generally
form the one with the lower charge. For example, iron can form Fe 2+ and Fe . In its reaction with HCl, FeCl 2
3+