2.2 Aqueous Solutions
Expression (4), K = [I2], is applicable to the solubility equilibria of some substances in water, but not of all. Contrast, for example, water solutions of sugar, salt, and hydrochloric acid. Glucose (a sugar) forms a molecular solid and, as it dissolves in water, the sugar molecules remain intact. These molecules leave the crystal and become a part of the liquid:
C6H12O6(s) → C6H12O6(aq)
This is exactly the situation we described for iodine in alcohol, and is applicable to aqueous sugar solutions. But sodium chloride, NaCl, behaves quite differently. As salt dissolves, positively charged sodium ions and negatively charged chloride ions enter the solution and these ions behave quite independently:
NaCl(s) → Na+(aq) + Cl–(aq) (15)
Hydrochloric acid, HCl, is similar. This substance is a gas at normal conditions. At very low temperatures it condenses to a molecular solid. When HCl dissolves in water, positively charged hydrogen ions and negatively charged chloride ions are found in the solution. As with sodium chloride, a conducting solution containing ions is formed:
HCl(g) → H+(aq) + Cl–(aq) (16)
Substances like NaCl(s) and HCl(g) that dissolve in water to form conducting solutions are called electrolytes. The conduction involves movement of the ions through the solution, positive ions moving in one direction and negative ions in the other. This shows that the positive and negative ions behave independently. In view of this independence of the ions, solubility behavior in an electrolyte solution is more complicated than that given by expression (4). We shall find that equilibrium principles are correspondingly more important.
Match the correct answer.
Glucose is
NaCl is
HCl gas is
Glucose is
= molecular.
NaCl is
= ionic.
HCl gas is
= molecular. |
| Response: That's the correct answer 2.2.1 Types Of Compounds That Are Electrolytes
The ions in an electrolyte solution can arise in two major ways.
They may already be present in the pure compound, as in ionic solids.
When such a solid is placed in water, the ions separate and move throughout the solution. 1
However, some compounds that form ions in water are not considered to contain ions when pure,
whether in the solid, liquid, or gas phase. Hydrochloric acid, HCl, and sulfuric acid, H2SO4,
are good examples of the second type of compound.
They form molecular liquids (or solids, if cold enough).
But in water they form ions: HCl gives hydrogen ion, H+(aq), and chloride ion, Cl–(aq);
H2SO4 gives hydrogen ion, H+(aq), hydrogen sulfate or bisulphate ion, HSO4– (aq),
and sulfate ion, SO42– (aq):
HCl(g) + water → H+(aq) + Cl–(aq) (16)
H2SO4(l) + water → H+(aq) + HSO4– (aq) (17)
HSO4– (aq) ⇌ H+(aq) + SO42– (aq) (18)
In whatever way they are formed, and from whatever source,
aqueous ions are individual species with properties not possessed by the materials from
which they came.
Furthermore, the properties of a particular kind of ion are independent of the source.
Chloride ions from sodium chloride, NaCl(s),
have the same properties as chloride ions in an aqueous solution of hydrochloric acid, HCl.
In a mixture of the two, all of the chloride ions act alike; none “remembers” whether it entered
the solution from an ionic NaCl lattice or from a gaseous HCl molecule.
Since the properties of an ionic solution (that is, a solution containing ions) differ in important ways
from those of nonconducting solutions, it is important to be able to predict which substances are
likely to form ionic solutions in water. The periodic table guides us.
In Chapter 7 of last year’s book we saw that the chemistry
of sodium can be understood in terms of the special stability
of the inert gas electron population of neon. An electron can be
pulled away from a sodium atom relatively easily to form a
sodium ion, Na+. Chlorine, on the other hand, readily accepts
an electron to form chloride ion, Cl–, achieving the inert gas
population of argon. When sodium and chlorine react,
the product, sodium chloride, is an ionic solid, made up of Na+ ions and Cl– ions packed in
a regular lattice. Sodium chloride dissolves in water to give Na+(aq) and Cl–(aq) ions.
Sodium chloride is an electrolyte; it forms a conducting solution in water.
This example illustrates the guiding principles. Sodium is a metal—electrons can be pulled away from
sodium relatively easily to form positive ions. Chlorine is a nonmetal—it tends to accept electrons
readily to form negative ions. When a metallic element reacts with a nonmetallic element, the resulting
compound usually forms a conducting solution when dissolved in water.
1 Liquids that form conducting solutions are called ionizing solvents. A few other compounds
(ammonia, NH3, sulfur dioxide, SO2, sulfuric acid, H2SO4 etc.) are “ionizing solvents” but water is
by far the most important. We will discuss water exclusively but the same ideas apply to the other solvents
in which ions form.
The metals are found toward the left side of the periodic table and the nonmetals are at the right side.
A compound containing elements from the opposite sides of the periodic table can be expected to
form a conducting solution when dissolved in water. Notice from our examples that hydrogen reacts
with nonmetals to form compounds that give conducting solutions in water. In this sense, hydrogen
acts like a metallic element.
Using the periodic table as a guide, choose the compounds which form ionic solutions in water.
magnesium bromide, MgBr2
carbon tetrabromide, CBr4
chromium(III) chloride, CrCl3
silicon carbide, SiC
Match to determine which of the following substances can be expected to dissolve in the indicated solvent to form, primarily, ions and which would form molecules.
Sucrose in water
RbBr in water
CHCl3 in water
CsNO3 in water
HNO3 in water
S8 in carbon disulfide, CS2
ICl in ethyl alcohol
Which of the solutions listed below is an electrolyte?
ICl in ethyl alcohol
CsNO3 in water
Sucrose in water
RbBr in water
CHCl3 in water
HNO3 in water
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