E2 Dissolution And Precipitation Solubility Rules Worksheets
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3E2 DISSOLUTION AND PRECIPITATION
Solubility rules
Ionic Compounds
When elements combine by transferring electrons from atoms of one element to those of another, an ionic compound is
+
-
2+
-
formed; for example, NaCl (Na
ions and Cl
ions) and BaI
(Ba
ions and I
ions). An ionic compound, or salt, is
2
composed of oppositely charged ions packed into a crystal lattice and may include polyatomic ions in which a number
2+
2-
+
of atoms covalently bond together to form an ion; for example SrSO
(Sr
ions and SO
ions) and NH
Cl (NH
ions
4
4
4
4
-
and Cl
ions).
Our description of ionic compounds represents an extreme and in practice there may be some covalent character
(electron sharing) in the bonds between the oppositely charged ions, for example Pb(CH
CO
)
and HgCl
. When
3
2
2
2
writing the formula of a compound it is conventional not to indicate whether the compound is ionic or covalent. Thus,
the essentially ionic compound barium chloride is written BaCl
and the essentially covalent compound water is written
2
H
O.
2
In addition, ionic compounds formed from aggregation of ions in a water solution may contain molecules of water
trapped in the crystal lattice, or in other words, water-of-crystallisation; for example CoSO
.H
O, CoSO
.6H
O and
4
2
4
2
CoSO
.7H
O. These exist in addition to the anhydrous compound, CoSO
. In most situations, any water-of-
4
2
4
crystallisation present is unimportant to the chemistry and any of the compounds may be used as a reagent: CoSO
,
4
CoSO
.H
O, CoSO
.6H
O or CoSO
.7H
O.
4
2
4
2
4
2
2+
This is because cobalt(II) sulfate is water soluble and all compounds give rise to the same species in solution, Co
(aq)
2-
and SO
(aq) where (aq) indicates the species is hydrated, ie surrounded by some water molecules.
4
The ions you will most commonly come across in the First Year Chemistry course are listed in Table E2. Note that
(i) ions are colourless in solution unless otherwise stated,
(ii) the table gives no indication of the relative stability of the ions given,
+
(iii) whereas the copper(I) ion is correctly represented as Cu
, the mercury(I) ion contains two mercury atoms
2+
sharing a double charge and is correctly represented as Hg
.
2
Solubility
-5
Salts have widely differing water solubilities. At room temperature these range from very low values such as 10
M for
-17
BaSO
and 10
M for Ag
S to values around 10 M for some fluorides. It is convenient to use the following arbitrary
4
2
definitions:
soluble = solubility at least 0.1 M
slightly soluble = solubility between 0.01 and 0.1 M
insoluble = solubility less than 0.01 M
All salts are strong electrolytes and when the ions dissolve in water they do so with complete dissociation. This can be
represented by an equation, for example
+
-
KCl(s)
K
(aq) + Cl
(aq)
+
2-
Na
SO
(s)
2Na
(aq) + SO
(aq)
2
4
4
Note, that it would not be correct to represent these two solutions as KCl(aq) or as Na
SO
(aq), because any of the
2
4
compound that dissolves is present in solution as separated ions.
The process can be reversed if a solution containing the ions of a salt is evaporated to dryness, thus forming a solid; for
example
+
-
K
(aq) + Cl
(aq)
KCl(s)
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