Page 87 - Basic physical chemistry for the atmospheric sciences

Solution chemistry and aqueous equilibria 7 1

common in the atmosphere, plants, and animals (e.g. , the human body
consists primarily of aqueous solutions).
Solids can dissolve in water in two ways: ( 1 ) with their molecules
intact (e.g. , when sugar dissolves in water individual sugar molecules
pass from the solid to the liquid phase but the sugar molecule does not
break up), and (2) by their molecules breaking up into positively and
e
negatively charged ions. For exampl , common salt (NaCl) dissolves
in the latter way, which can be represented by
NaCl(s) � N a + ( aq) + Cl- (aq) (4.8)
where the plus sign indicates that the sodium ion carries one unit of
positive charge, and the negative sign indicates that the chloride ion
carries one unit of negative charge. (If an ion carries two units of
positive charge, it is indicated by the superscript 2 + , etc. )
Aqueous solutions containing charged ions are electrically conduct­
ing and are called electrolytes. Aqueous ions are individual species,
the properties of which are independent of their source. For example,
chloride ions from NaCl are just the same as chloride ions from y dro­
h
chloric acid (HCI) or any other electrolyte containing chlorine.
A compound will dissolve in water to form ions if the attractive
forces between the water molecule and the ions are stronger than the
attractive force between the ions. For example, NaCl dissolves in
water because the attractive forces between the water molecule and
the Na+(aq) and Cl - (aq) ions are stronger than the attractive force
between Na+ (aq) and c1 - (aq). In a sodium chloride solution, water
molecules surround both the Na+ (aq) and c - (aq) ions, with the
1
negatively charged end of the water dipole (i. . , the oxygen atom)
e
pointed toward the Na ( aq) ions and the positively charged end of the
+
-
e
water dipole (i. . , the hydrogen atoms) pointed toward the Cl ( aq)
ions. This type of arrangement between solution and solvent molecules
is known as salvation, and when the solvent is water as hydration.
4
Table . 1 shows the solubilities of various compounds in water. A
substance is considered to be soluble if it dissolves to produce a
solution with a concentration of at least one-tenth of a mole per liter
(0. 1 M) at room temperature.

4.6 Aqueous equilibria
In Chapter I we discussed the basic principles of chemical equilib­
rium. We will now apply these principles to ionic equilibria in aque­
ous solutions.