•Assessment Statement:
•Deduce whether salts
form acidic, alkaline or
neutral aqueous
solutions.
Let us reflect upon what we have
learned. We learned of the Bronsted
Lowry Theory and Lewis theories
that enabled us to distinguish between
acids and bases, we learned of pH and
pOH, and so forth. We also learned
that even ions have a particular Ka or
Kb value which makes a solution
acidic or basic.
Assuming that all salts undergo complete
dissociation in aqueous solution, we determine
that all salts are strong ELECTROLYTES. (The
word was on the scavenger hunt!)
As a result of the dissociation, it is observed that
the ions involved can cause water molecules to
dissociate to form H+ and OH- ions. This
process, of the dissociation of water molecules,
is called HYDROLYSIS.
* (Hydro- water; lysis- split!)
The factor that enables the
determination of the pH of an
aqueous salt solution is the capability
of the constituent ions to bring about
the hydrolysis of water, and further
react with the ions produced. This
can be deduced using the table that
helps us differentiate between strong
and weak acids and bases, and the
relative strengths of their conjugates.
In a salt solution, remember the anion (negative
ion) is the CONJUGATE BASE of an acid.
Therefore, in adding a proton we can deduce
the acid and its strength.
This is generally of the order:
X- + H+  HX
Using this, recollect if the acid is a strong or weak
one, and then deduce the strength of its
conjugate base!
(STRONG ACID  Weak Conjugate Base
Weak Acid  STRONG CONJUGATE BASE)
A weak conjugate base will have negligible
tendency to cause the hydrolysis of water, thus
limiting the production of OH- ions in solution.
There is consequently NO EFFECT on the pH
of the solution.
However, a stronger conjugate base will have
comparatively larger tendencies of evoking the
hydrolysis of water. The H+ ions will react with
the anion in solution, leaving behind OH- ions.
This causes the pH to rise, making the solution
ALKALINE.
The theory discussed previously applies only to
anions that have completely dissociated and
cannot release any more H+ ions in solution.
However another kind of salt exists. These are
the kind whose anion is capable of releasing
more H+ ions in solution. These kind of anions
are called AMPHIPROTIC.
The values of Ka and Kb will determine their
behavior in water. If Ka > Kb, the solution will
be ACIDIC, however if Ka < Kb the solution
will be ALKALINE.
As observed with
anions, cations
(positive ions) also
determine the pH of a
salt solution. Can you
tell how?
(Yes here’s the answer: STRONGER
CONJUGATE ACIDS donate a proton to water
to form a HYDRONIUM ion, making it
ACIDIC!
Weaker conjugate acids have a lower tendency to
do so. Thus, not affecting the pH to a large
extent.)
Now consider Metal Ions…
Metal ions, being positive, attract the unshared
electron pairs on water molecules. The
interaction, called HYDRATION, causes salts
to dissolve in water.
If we think about this in terms of the Lewis
theory, we realize that as the electrons on the
Oxygen atom in water get attracted to the
positive metal ion, the bond between the
hydrogen and oxygen gets more polarized.
Thus, water molecules bonded to the metal ion
are more acidic than the rest of the molecules
in solution.
EXAMPLES:
1.) sodium ethanoate solution
sodium ethanoate is 100% dissociated into ions:CH3COONa CH3COO- + Na+
Sodium ions are from a strong base (sodium hydroxide) and do not
interact with the water ions.
However, the ethanoate ions do interact with the hydrogen ions from the
water equilibrium (H2O H+ + OH-)
CH3COO- + H+ CH3COOH
We know that this last equilibrium lies to the side of the ethanoic acid
(to the right), removing the hydrogen ions from the solution. As [H+]
decreases the pH rises.
Hence a solution of sodium ethanoate has a pH greater than 7. We say
that it is basic by hydrolysis.
2.) Ammonium chloride solution
Ammonium chloride dissociates 100% into ions in solution
NH4Cl NH4+ + ClThe ammonium ions interact with the hydroxide ions from the
water removing them from the solution (equilibrium lies to
the right)
NH4+ + OH- NH3 + H2O
This increases the concentration of hydrogen ions (as [H+] x
[OH-] is constant) increasing the acidity of the solution
(decrease pH)
We say that a solution of ammonium chloride is acidic by
hydrolysis.
General rules
• When the negative ion is from a weak acid then
the salt is basic by hydrolysis
• When the positive ion is from a weak base then
the salt is acidic by hydrolysis
• If the salt is formed from a strong acid and
strong base then it is neutral
• If the salt is formed from a weak acid and weak
base then its hydrolysis is determined by the
relative Ka and Kb values
Charge density
This means the charge to size ratio of the ion.
charge density = ionic charge/ionic size
When the ion has a charge of 3+ or when it is very
small this charge to size ratio is enough to polarise
the water molecules surrounding the ion in solution.
This results in a weakening of the O-H bonds within
the water molecules allowing hydrogen ions to be
released into the solution. Hence the solutions are
acidic.
This effect is typified in aluminum salts (the
aluminum ion has a charge of 3+) which are very
acidic in solution
EXAMPLE:
The aluminum hexa-aqua ion
Aluminum ions are surrounded by six water molecules in
an octahedral arrangement. This is called the aluminum
hexa-aqua ion. The high charge density of the aluminum
ion polarises the water molecules and hydrogen ions are
released into solution. The solution is so acidic that it
releases carbon dioxide from sodium carbonate (this
reaction is used in some fire extinguishers to produce
foam in conjunction with detergent)
[Al(H2O)6]3+ [Al(OH)(H2O)5]2+ + H+
[Al(OH)(H2O)5]2+ [Al(OH)2(H2O)4]+ + H+
Transition metals
As the transition metals have variable
oxidation states the ions that are formed
with high charges (high oxidation state)
also produce acidic solutions. A good
example of this is the Iron III ion. Salts
such as iron III sulphate are acidic in
solution.
[Fe(H2O)6]3+ [Fe(OH)(H2O)5]2+ + H+