414
Reactions: Prediction and Synthesis
Remember that a weak electrolyte need not be a molecule; it also may be a
weakly dissociated ion. For example, Ca 3 (PO 4 ) 2 dissolves in an excess of strong
acid, because the weak acid ion HPO
2
," (as well as some H 2 PO;") is formed by
the reaction
Ca 3 (P0 4 ) 2 + 2H+ -> 3Ca
2+ + 2HPOr
In general, you could expect the insoluble salt of a weak acid to dissolve in an
excess of strong acid for the reasons just given. (Extremely insoluble substances, such as some of the metal sulfides, are exceptions because H 2 S is not a
weak enough acid to take the S
2 ~ away from the metal ion.)
The last two examples illustrate how precipitation or solution of an insoluble
compound may be controlled by controlling the pH (see also pp 378-383).
Summary of Prediction and Equation-Balancing Procedures
1. First, decide whether an electron-transfer reaction is possible, using
approximate half-reaction potentials (p 301) and/or the characteristics
of electron-transfer reactions (p 300). If it is possible, follow the
equation-balancing procedure outlined on pp 295-299.
2. If the reaction is non-electron-transfer, then treat it as a metathesis
reaction.
a. Write a preliminary balanced molecular equation in which the reaction partners have been interchanged:
A 2 B + 2CD -> 2AD + BC 2
b. Study each compound in the molecular equation and decide
whether it is an insoluble solid, a weak (or non-) electrolyte, or a
gas. Rewrite the equation as appropriate. If A 2 B is a strong electrolyte, CD a weak electrolyte, AD a gas, and BC 2 an insoluble solid,
then
2A+ + B
2 - + 2CD ?± 2AD| + BC 2 |
Eliminate all entities that are identical in form on both sides to get
the final balanced ionic equation.
c. The reaction will go if any one of the products is a gas, a solid, or a
weak electrolyte. If there is competition (insoluble solids or weak
electrolytes onboth sides), you must consider (1) whether one of the
reactants is used in excess or high concentration to favor a shift to
the right, and (2) the relative insolubilities of solids or weaknesses
of electrolytes.
Reactions: Prediction and Synthesis
Remember that a weak electrolyte need not be a molecule; it also may be a
weakly dissociated ion. For example, Ca 3 (PO 4 ) 2 dissolves in an excess of strong
acid, because the weak acid ion HPO
2
," (as well as some H 2 PO;") is formed by
the reaction
Ca 3 (P0 4 ) 2 + 2H+ -> 3Ca
2+ + 2HPOr
In general, you could expect the insoluble salt of a weak acid to dissolve in an
excess of strong acid for the reasons just given. (Extremely insoluble substances, such as some of the metal sulfides, are exceptions because H 2 S is not a
weak enough acid to take the S
2 ~ away from the metal ion.)
The last two examples illustrate how precipitation or solution of an insoluble
compound may be controlled by controlling the pH (see also pp 378-383).
Summary of Prediction and Equation-Balancing Procedures
1. First, decide whether an electron-transfer reaction is possible, using
approximate half-reaction potentials (p 301) and/or the characteristics
of electron-transfer reactions (p 300). If it is possible, follow the
equation-balancing procedure outlined on pp 295-299.
2. If the reaction is non-electron-transfer, then treat it as a metathesis
reaction.
a. Write a preliminary balanced molecular equation in which the reaction partners have been interchanged:
A 2 B + 2CD -> 2AD + BC 2
b. Study each compound in the molecular equation and decide
whether it is an insoluble solid, a weak (or non-) electrolyte, or a
gas. Rewrite the equation as appropriate. If A 2 B is a strong electrolyte, CD a weak electrolyte, AD a gas, and BC 2 an insoluble solid,
then
2A+ + B
2 - + 2CD ?± 2AD| + BC 2 |
Eliminate all entities that are identical in form on both sides to get
the final balanced ionic equation.
c. The reaction will go if any one of the products is a gas, a solid, or a
weak electrolyte. If there is competition (insoluble solids or weak
electrolytes onboth sides), you must consider (1) whether one of the
reactants is used in excess or high concentration to favor a shift to
the right, and (2) the relative insolubilities of solids or weaknesses
of electrolytes.