412
Reactions: Prediction and Synthesis
A reaction will "go" for any one of the following reasons.
1. For electron-transfer reactions, if the electrode potential (E) for the
reducing half-reaction is more negative than that for the oxidizing halfreaction.
2. For non-electron-transfer reactions, if
a. an insoluble product is formed (AT e > 1 because A" sp < 1), or
b. a weak (or non-) electrolyte is formed (K e > 1 because K, < 1 or
tf.nst < 1), or
c. a gas is formed in an open vessel (loss of gas product continually
shifts equilibrium to right).
Guidelines
Some simple, general guidelines for using these principles are listed here.
1. Electrode potentials for half-reactions are most easily obtained from a
table of standard electrode potentials (Table 17-1, p 275); they may be
modified for concentrations appreciably different from unit activity (p
276). A list of features characteristic of electron-transfer reactions is
given on p 300.
2. General solubility rules for water as a solvent must be stated in two
parts, one emphasizing the negative ions of the salts, and the other
emphasizing the positive ions, as follows.
a. All nitrates, acetates, perchlorates, halides (Cl~, Br~, and I~), and
sulfates are soluble, with the following important exceptions:
(1) the halides of Ag
+ , Hgl
+ , and Pb
2+ are insoluble;
(2) the sulfates of Ba
2+ and Pb
2+ are insoluble.
b. All alkali metal (Li
+ , Na
+ , K
+ , Rb
+ , and Cs
+ ) and ammonium (NH 4 +)
salts and bases are soluble. Ba(OH) 2 , a fairly important base, also is
soluble.
c. In general, all other common types of inorganic compounds are
insoluble.
3. Strong electrolytes (those that ionize in water) include with minor exceptions, the following compounds:
a. all soluble salts (this does not include acids and bases);
b. all strong acids (there are eight common ones: HC1, HBr, HI,
HNO 3 , HC1O 4 , H 2 SO 4 , H(NH 2 )SO 3 , and H 3 PO 4 —the latter is included only because it loses its first H
+ readily);
c. all strong bases (these are the alkali metal and Ba
2+ hydroxides).
4. Weak electrolytes include all those substances that are not classed as
strong electrolytes. A particularly important weak electrolyte is water.
Reactions: Prediction and Synthesis
A reaction will "go" for any one of the following reasons.
1. For electron-transfer reactions, if the electrode potential (E) for the
reducing half-reaction is more negative than that for the oxidizing halfreaction.
2. For non-electron-transfer reactions, if
a. an insoluble product is formed (AT e > 1 because A" sp < 1), or
b. a weak (or non-) electrolyte is formed (K e > 1 because K, < 1 or
tf.nst < 1), or
c. a gas is formed in an open vessel (loss of gas product continually
shifts equilibrium to right).
Guidelines
Some simple, general guidelines for using these principles are listed here.
1. Electrode potentials for half-reactions are most easily obtained from a
table of standard electrode potentials (Table 17-1, p 275); they may be
modified for concentrations appreciably different from unit activity (p
276). A list of features characteristic of electron-transfer reactions is
given on p 300.
2. General solubility rules for water as a solvent must be stated in two
parts, one emphasizing the negative ions of the salts, and the other
emphasizing the positive ions, as follows.
a. All nitrates, acetates, perchlorates, halides (Cl~, Br~, and I~), and
sulfates are soluble, with the following important exceptions:
(1) the halides of Ag
+ , Hgl
+ , and Pb
2+ are insoluble;
(2) the sulfates of Ba
2+ and Pb
2+ are insoluble.
b. All alkali metal (Li
+ , Na
+ , K
+ , Rb
+ , and Cs
+ ) and ammonium (NH 4 +)
salts and bases are soluble. Ba(OH) 2 , a fairly important base, also is
soluble.
c. In general, all other common types of inorganic compounds are
insoluble.
3. Strong electrolytes (those that ionize in water) include with minor exceptions, the following compounds:
a. all soluble salts (this does not include acids and bases);
b. all strong acids (there are eight common ones: HC1, HBr, HI,
HNO 3 , HC1O 4 , H 2 SO 4 , H(NH 2 )SO 3 , and H 3 PO 4 —the latter is included only because it loses its first H
+ readily);
c. all strong bases (these are the alkali metal and Ba
2+ hydroxides).
4. Weak electrolytes include all those substances that are not classed as
strong electrolytes. A particularly important weak electrolyte is water.