2.2.2 A Qualitative View Of Aqueous Features Of Solubilities
Hereafter in this chapter we shall be concerned exclusively with substances that form ionic solutions in water. Since each substance is electrically neutral before it dissolves, it must form ions of positive charge and ions of negative charge. Ions with positive charges are called cations. Ions with negative charges are called anions. A conducting solution is electrically neutral; it contains both anions and cations.
First, let us consider substances with high solubility. Chemists consider a substance to be soluble if it dissolves to a concentration exceeding one-tenth of a mole per liter (0.1 M) at room temperature. Using this meaning of the word soluble, we can say that some cations (positive ions) form soluble compounds with all anions (negative ions). These cations are the hydrogen ion, H+(aq), ammonium ion, NH4+ , and the alkali ions, Li+, Na+, K+, Rb+, Cs+, Fr+. Figure 2.8 shows the placement of these ions in the periodic table.
Figure 2.8 Almost all alkali metals, hydrogen ions, and ammonium ions are soluble in water.
The same sort of remark can be made about two anions (negative ions). Almost all compounds involving nitrate ion, NO3–, and acetate ion, CH3COO–, are soluble in water.*
*There are a few compounds of alkalis, nitrate, and acetate that have low solubilities, but most of them are quite complex in composition. For example, sodium uranyl acetate, NaUO2(CH3COO)3 has low solubility. Silver acetate is an exception but its solubility is moderate.
Other anions (negative ions) form compounds of high solubility in water with some metal cations (positive ions) and compounds of low solubilities with others.
Figure 2.9 indicates, for five anions, the metal ions that form compounds of low solubilities. Figure 2.9A refers to chlorides, bromides, and iodides, Cl–, Br–, and I–; Figure 2.9B refers to sulfates, SO42–; Figure 2.9C refers to sulfides, S2–. Notice the difference between Figure 2.8 and Figure 2.9. The cross-hatching in Figure 2.8 identifies metal ions that form soluble compounds. Figure 2.9 identifies the ones with low solubility.
Figure 2.10 continues this pictorial presentation of solubilities. Figure 2.10A shows the positive ions that form hydroxides of low solubility. Figure 2.10B shows the positive ions that have low solubility when combined with phosphate ion, PO43–, carbonate ion, CO32–, and sulfite ion, SO32–.
These figures furnish a handy summary of solubility behavior. We see from Figure 2.9A that few chlorides have low solubilities. The few that do contain cations of metals clustered toward the right side of the periodic table (silver ion, Ag+, copper (I) ion, Cu+, mercury (II) ion, Hg22+, and lead (II) ion, Pb2+) but they do not fall in a single column.
Figure 2.9 Positive ions (colored in pink) form compounds of low solubilities with various anions (colored in blue).
Figure 2.10 More examples positive ions (colored in pink) forming compounds of low solubilities with various anions (colored in blue).
This irregularity is not unusual in solubility behavior and is seen again in Figures 2.9B and 2.10A. In these two figures, the elements in the second column (the alkaline earth metals) show a trend in behavior. Thus, beryllium and magnesium ions (Be2+ and Mg2+) form soluble sulfates. The others—calcium, strontium, barium, and radium ions (Ca2+, Sr2+, Ba2+, and Ra2+) form sulfates with low solubilities. The opposite behavior is seen in Figure 2.10A for the compounds of these same elements with hydroxide ion, OH–. As for the elements in the middle of the periodic table, we see that they form compounds of low solubilities with the ions: sulfide, S2–, hydroxide, OH–, phosphate, PO43–, carbonate, CO32–, and sulfite, SO32–.
The information contained in Figures 2.8, 2.9, and 2.10 is collected for reference in Table 2.1.
*Except for Li2CO3 which has a low solubility
Summary
The solubility of salts in aqueous solution can be summarized in the following five rules:
- All ammonium (NH4+), alkali metal (Li+, Na+, K+, Rb+, Cs+), nitrate (NO3–) and acetate (CH3COO–) salts are soluble except for Li2CO3 which has a low solubility.
- All hydroxides (OH–) [Ba2+ ; Sr2+], oxides (O2–), phosphates (PO43–), sulfites (SO32–) and carbonates (CO32–) are insoluble [except for thecations of rule 1].
- All sulfates (SO42–) are soluble except Sr2+, Ba2+ and Pb2+, Ca2+.
- All chlorides, bromides and iodides (Cl–, Br– and I–) are soluble except Ag+, Pb2+ and Cu+.
- All sulfides (S2–) are insoluble or slightly soluble [except for rule 1 and alkaline earth salts Be2+, Mg2+, Ca2+, Sr2+, Ba2+].
Decide on the solubility of each of the compounds listed below.
Match "sol" if the compound is soluble and "low" if it has low solubility.
Silver carbonate
Aluminum chloride
Aluminum hydroxide
Copper(I) chloride
Copper(II) chloride
Ammonium bromide
Which compounds containing the anions and cations below are soluble?
Anion: hydroxide OH– Cations: cations of the fourth period (row) of the periodic table
Anion: chloride Cl– Cations: cations of the fifth period of the periodic table except Ag+
Anion: sulfide S2– Cations: alkaline earth ions: Be2+, Mg2+, Ca2+, Sr2+, Ba2+
Anion: carbonate CO32– Cations: alkali ions: Li+, Na+, K+, Rb+, Cs+
Anion: carbonate CO32– Cations: alkaline earth ions: Be2+, Mg2+, Ca2+, Sr2+, Ba2+
2.2.3 Using Qualitative Rules To Predict The
Formation Of A Precipitate
Our solubility rules help us to predict when a solid may form if two solutions are mixed.
Example 1
What will happen if solutions of BaCl2 and Na2SO4 are mixed? What ions will remain in the solution?
Solution
In water, BaCl2 gives two types of ions, Ba2+(aq) and Cl–(aq), Na2SO4 gives two types of ions,
Na+(aq) and SO42–(aq).
These four ions can combine in only four possible ways, since positive ions can only combine with
negative ions. These four ways are:
Step 1: Ba2+(aq) and Cl–(aq): These will not precipitate because of rule 4.
Step 2: Na+(aq) and SO42–(aq): These will not precipitate because of rule 1.
Step 3: Na+(aq) and Cl–(aq): These will not precipitate because of rule 1.
Step 4: Ba2+(aq) and SO42–(aq): These will precipitate because of rule 3.
Step 5: The only reaction that takes place is as follows:
Ba2+(aq) + SO42–(aq) → BaSO4(s)
The ions that remain in the solution are Na+(aq) and Cl–(aq).
Remark 1: When answering this question, there is no need to write steps 1 to 4. It is enough
to write step 5.
Remark 2: If initially the number of ions of Ba2+(aq) and SO42–(aq) are not exactly equal,
the one in excess will remain in solution. For example, if the Ba2+(aq) are in excess, the ions that will
be left in the solution will be Ba2+(aq), Na+(aq) and Cl–(aq).
Example 2
What will happen if solutions of BaBr2 and NaNO3 are mixed? What ions will remain in the solution?
Solution
In water, BaBr2 gives two types of ions, Ba2+(aq) and Br–(aq), NaNO3 gives two types of ions, Na+(aq)
and NO3–(aq)
These four ions can combine in only four possible ways, since positive ions can only combine with
negative ions: These four ways are
Step 1: Ba2+(aq) and Br–(aq): These will not precipitate because of rule 5.
Step 2: Na+(aq) and NO3–(aq): These will not precipitate because of rule 1.
Step 3: Na+(aq) and Br–(aq): These will not precipitate because of rule 1.
Step 4: Ba2+(aq) and NO3–(aq): These will not precipitate because of rule 1.
Step 5: There will be no precipitate. The ions that remain in the solution are Na+(aq) and Br–(aq),
Ba2+(aq) and NO3–(aq).
Remark 1: Again, when answering this question, there is no need to write steps 1 to 4. It is enough
to write step 5.
Example 3
What will happen if solutions of KCl and Pb(NO3)2 are mixed? What ions will remain in the solution?
Solution
In water, KCl gives two types of ions, K+(aq) and Cl–(aq), Pb(NO3)2 gives two types of ions, Pb2+(aq) and
NO3–(aq)
A precipitate forms as follows:
Pb2+(aq) + 2Cl–(aq) → PbCl2(s)
T
he ions that remain in the solution are K+(aq) and NO3–(aq) provided that the Pb2+(aq) ions are half
he ions that remain in the solution are K+(aq) and NO3–(aq) provided that the Pb2+(aq) ions are half
the number of Cl–(aq), otherwise, the one in excess will remain.
Fill in the blank.
The name of Mg(OH)2 is magnesium
.Fill in the blank.
Name of CuSO3 is Copper (II)
Fill in the blank.
Name of PbI2 is Lead (II)
