2P32 – Principles of Inorganic Chemistry
Dr. M. Pilkington
Lecture 21 – Cations, Anions and Hydrolysis in Water:
1. Hydration.energy
2. Hydrolysis of metal cations
3. Categories of acidity and observable behavior
4. Speciation of elements in water
1. Hydration
„
The formation of a solution involves the interaction of solute with solvent
molecules.
„
Many different liquids can be used as solvents for liquid solutions, and water is
the most commonly used solvent.
„
When water is used as the solvent, the dissolving process is called hydration. The
interaction between water molecules and a sodium ion is illustrated in the diagram
below.
„
This is a typical ion-dipole interaction. At the molecular level, the ions interact
with water molecules from all directions in a 3-dimensional space.
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The Enthalpy of hydration, of an ion is the amount of energy released when a
mole of the gaseous ion is dissolved in a large amount of water.
Mx+ (g) + nH2O
„
Mx+(aq)
where Mx+(aq) represents ions surrounded by water molecules and dispersed in
the solution.
„
As the size of the cation decreases then the enthalpy of hydration increases.
„
This is because the interaction between the water and the cation is stronger so
more energy is released on hydration.
„
Remember Coloumb’s Law E is proportional to 1/r
„
As the charge of the cation increases, the enthalpy of hydration increases as
with increasing charge the atomic radii of the ions decreases.
2. Hydrolysis of cations
„
Metal ions in aqueous solution behave as Lewis acids. The positive charge on the
metal ion draws electron density from the O-H bond in the water. This increases
the bond's polarity making it easier to break. When the O-H bond breaks, an
aqueous proton is released producing an acidic solution.
„
The equilibrium constant for this reaction can be measured.
[M(H2O)n]x+ + H2O
[M(H2O)n-1(OH)](x-1)+ + H3O+
2
„
Note the similarity of the equation for the hydrolysis of a hydrated cation with
the equation for the ionization of a weak acid in aqueous solution.
„
The Keq for the hydrolysis of a hydrated cation is analogous to the Ka for the
ionization of a weak acid. Keq is an acid ionization constant. Generally, hydrolysis
constants for cations are tabulated as -log Ka. These tabulated hydrolysis
constants are averages of different experimental measurements that sometimes
differ by more than one pKa unit. For this reason, small differences are not
considered significant when comparing the values of these constants.
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4
Factors Affecting pKa – lower values of pKa and pH correspond to greater extent
of hydrolysis and higher acidity of the solution.
„
Cation
K+
Na+
Li+
Radius, pm
152
116
90
pKa
14.5
14.2
13.6 (too small to
measure directly)
Ba2+
Ca2+
Mg2+
149
116
86
13.5
12.8
11.4
Rate of hydrolysis of metal or acidity of a hydrated cation INCREASES with
INCREASING charge and DECREASING Radii.
„
The smaller the metal the more tightly it holds the lone pairs on the oxygen
waters.
„
In turn the hydrogen’s will become more positively charged and the O-H bond
weakened so it easily breaks to give the H+ ions.
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„
Be2+ has an atomic radii of 59 pm and an electronegativity of 1.57
„
From the table, Be2+ is more electronegative than the Na+, K+, Li+
cations.
„
Be2+ is more acidic than would otherwise be predicted. Why?
1.
The partial covalent character in the BeOH2 bond weakens the O-H
bond.
2.
Plus the small coordination number means the oxygen lone pairs are
held very tightly by the cation weakening O-H bonds more than would
otherwise be expected.
„
To summarize there are important consequences on the acidic
properties and hydrolysis reactions of compounds of elements with
high oxidation numbers or small radii and high electronegativities.
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3.
Categories of Acidity (see handout)
1.
Non Acidic Cations: for example Na+ (in NaCl) are usually not acidic in Group 1
with the exception of Li+. When we dissolve sodium in water it dissolves
smoothly, the solution is definitely NOT detectably acidic.
On addition of base we do not get the formation of an insoluble hydroxide
(NaOH is soluble in H2O).
2.
Feebly Acidic Cations: pKa = 11-14 (e.g. Ba2+ in BaCl2)
BaCl3 is very soluble in water, it dissolves smoothly, the hydroxide is somewhat
soluble in water but less soluble than the hydroxides of non acidic cations.
3.
Weakly Acidic Cations: pKa = 6-11.5 (e.g. Fe2+ in FeCl2).
FeCl2 is soluble in water with barely detectable change in the pH towards a
more acidic solution.
Fe(OH)2 precipitates as the pH is increased.
4.
Moderately Acidic Cations: pKa = 1-6 (e.g. Fe3+ in FeCl3)
FeCl3 is soluble in water, produced a distinctly acidic solution.
Fe(OH)2 precipitates at pH = 3 or higher. Insoluble in excess hydroxide.
5.
Strongly Acidic Cations: pKa +4 to +1 (e.g. TiCl4).
TiCl4 reacts VIOLENTLY with water produces a lot of heat and insoluble TiO2.
TiCl4(aq) + 2H2O
6.
TiO2 + 4HCl
Very Strong Acidic Cations: pKa = < -4 (e.g. P5- and PCl5)
Extremely VIOLENT reaction with water produce solutions of the oxo acids
i.e. H3PO4 when PCl5 reacts with H2O.
The solution is highly acidic. When base is added no precipitate appears as the
acid gets neutralized.
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Distribution and predominance diagrams for a phosphate species (see
handout)
„
Indicates the fraction of all phosphorous present in the form of
phosphoric acid H3PO4, the phosphate ion PO43- and the two partially
protonated phosphate ions HPO42- and H2PO4-.
„
Natural waters usually have pH values between 6 and 9, so we can see
from the diagram what form of phosphorous might be present.
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Past Exam Question 2006 – Periodicity
1.(a) (4 marks) What is the “inert-pair” effect? How is it related to the
common oxidation states of Tl and Pb?
(b) (3 marks) What is a diagonal relationship? To which elements does this
term apply in the periodic table?
(c) (4 marks) Explain the difference between hydrolysis and hydration of a
metal ion. Illustrate your answer with an appropriate example.
1 (a) Answer
In group 13-17, in 4th 5th and 6th periods – ns2 electroms are much harder to
remove than expected (i.e. they are “inert”). (these elements are less reactive than
expected on the basis of ENC trend, atomic size and IE). Therefore the highest
oxidation state in the group is less stable than the one that is 2 less than the highest.
e.g.
Tl 5d10 6s2 6p1
+1 is more stable than +3
Pb 5d10 6s2 6p2
+2 is more stable than +4
The 6s2 are the inert pair, not easily removed. Al is +3 is only known state so no inert
pair.
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1(b) Answer
An element in the second row (period) has behavior more like the element in the
next group and next period than to its own congeners.
e.g. BeO and Al2O3 are amphoteric when other group II oxides are basic.
Happens because the second period element ions are small and their charge density
(charge/size of ion) is similar to the diagonal element in the period below.
The three diagonal pairs are similar in atomic radii, electronegativity and charge
density:
Li Be B
Mg Al Si
1 (c) Answer
Hydration involves interaction of chemical species with water. For ions the result is:
O
H
O
+Z
H
H
δ+
H
δ- O
O
H
H
δ+
H
δ+
H O
H
δ-
O
O
H
δ+
H
H
δ-
H
-Y
δ+
H
H
H
O
Hydrolysis involves further chemical reaction of hydrated ions with the solvent
(water). Metal ions in aqueous solution behave as Lewis acids. The positive charge on
the metal ion draws electron density from the O-H bond in the water. This increases
the bond's polarity making it easier to break. When the O-H bond breaks, an
aqueous proton is released producing an acidic solution.
e.g. [M(OH2)6]n+ + H2O
[M(OH2)(OH)](n-1)+ + H3O+
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