Page 137 - Theory and Problems of BEGINNING CHEMISTRY

126 CHEMICAL EQUATIONS [CHAP. 8

is formed. In contrast, in combination with the free element, the higher-charged ion is often formed if sufficient
nonmetal is available.
2Fe + 3Cl 2 −→ 2 FeCl 3
See Table 6-4 for the charges on some common metal ions.

Double-Substitution or Double-Replacement Reactions
Double-substitution or double-replacement reactions, also called double-decomposition reactions or me-
tathesis reactions, involve two ionic compounds, most often in aqueous solution. In this type of reaction, the
cations simply swap anions. The reaction proceeds if a solid or a covalent compound is formed from ions in
solution. All gases at room temperature are covalent. Some reactions of ionic solids plus ions in solution also
occur. Otherwise, no reaction takes place. For example,
AgNO + NaCl −→ AgCl + NaNO 3
3
HCl(aq) + NaOH −→ NaCl + H 2 O
CaCO 3 (s) + 2 HCl −→ CaCl 2 + CO 2 + H 2 O
In the first reaction, two ionic compounds in water are mixed. The AgCl formed by the swapping of anions is
insoluble, causing the reaction to proceed. The solid AgCl formed from solution is an example of a precipitate.
−
In the second reaction, a covalent compound, H 2 O, is formed from its ions in solution, H and OH , causing
+
the reaction to proceed. In the third reaction, a solid reacts with the acid in solution to produce two covalent
compounds.
Since it is useful to know what state each reagent is in, we often designate the state in the equation. The
designation (s) for solid, (1) for liquid, (g) for gas, or (aq) for aqueous solution may be added to the formula.
Thus, a reaction of silver nitrate with sodium chloride in aqueous solution, yielding solid silver chloride and
aqueous sodium nitrate, may be written as
AgNO (aq) + NaCl(aq) −→ AgCl(s) + NaNO 3 (aq)
3
Just as with replacement reactions, double-replacement reactions may or may not proceed. They need a
driving force. In replacement reactions the driving force is reactivity; here it is insolubility or covalence. In order
for you to be able to predict if a double-replacement reaction will proceed, you must know some solubilities of
ionic compounds. A short list of solubilities is given in Table 8-2.

EXAMPLE 8.13. Complete and balance the following equation. If no reaction occurs, indicate that fact by writing “NR.”
NaCl + KNO 3 −→
Ans. NaCl + KNO 3 −→ NR
If a double-substitution reaction had taken place, NaNO 3 and KCl would have been produced. However, both of
these are soluble and ionic; hence, there is no driving force and therefore no reaction.

Table 8-2 Some Solubility Classes

Soluble Insoluble

Chlorates BaSO 4
Acetates Most sulfides
Nitrates Most oxides
Alkali metal salts Most carbonates
Ammonium salts Most phosphates
Chlorides, except for ...... AgCl, PbCl 2 ,Hg 2 Cl 2 , CuCl