2P32 – Principles of Inorganic Chemistry
Dr. M. Pilkington
Lecture 22 – The Acid-Base Character of Oxides and
Hydroxides in Aqueous Solution
• Oxides; acidic, basic, amphoteric
• Classification of oxides - oxide acidity and basicity
• Hydrolysis of oxo anions
• Periodicity of oxide acid-base character
• Reactions of acid and basic oxides
1. Oxides

Oxygen forms compounds with all elements except He, Ne, Ar, and Kr. It reacts directly with all
elements except the halogens, a few noble metals e.g. Ag and Au and the noble gases.

As encountered for the hydrides, there is variability in the types of bonding encountered.

Metal oxides are ionic solids, nonmetal oxides are discrete molecular covalent gases and liquids. The
oxides of heavier nonmetals and the semimetals tend to be covalent polymeric solids. Transition
metal oxides are occasionally nonstiochiometric.
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
The variation of acid-base properties of the oxides in aqueous solution, is
strongly correlated with the position of the metal-nonmetal line.
Oxides in aqueous solution (Acidic and Basic Anhydrides)

One of the most important aspects of the properties of oxides is their acid-base
properties.
ti

Many oxides are basic or acidic anhydrides; that is they are compounds that are
formed by the removal of water from a corresponding base or acid.

Ionic oxides are usually basic anhydrides, whereas covalent oxides are usually
acidic anhydrides.

Oxides of the semimetals are amphoteric anhydrides,
anhydrides capable of acting as either
an acid or base, depending on the circumstances.

If we list the oxides of a given period, e.g. 3rd, we find an orderly progression of
their acid-base character.

The acid strength increases with the acidity of the cation involved
Na+
Cl+7
Na2O,
MgO,
Al2O3,
Na(OH) Mg(OH)2 Al(OH)3
(sol)
(insol)
amphoteric
SiO2,
P4O10, SO3,
Cl2O7
Si(OH)4 H3PO4 H2SO4 HClO4
weak acid
very strong acid.
ACIDIC OXIDE
BASIC OXIDE
AMPHOTERIC
OXIDE
strong base
Amphoteric species
Strong Acid
2

The ionic oxides are characterized by the presence of the oxide ion, O2-, that like
the H+ ion, cannot exist alone in aqueous solution.

The reaction between the oxide and a water molecule is shown below:
O2-


+ H O
2O
H-
H 

The 2- charged oxide attacks and forms a bond with a partially positive hydrogen
atom of the water molecule. The subsequent breaking of the O-H bond produced
two hydroxide
h d
d ions.

The equilibrium constant for this reaction is greater than 1022, so this reaction
lies far to the right.

For example sodium oxide:
Na2O (s) + H2O(l)

2NaOH (aq)
2Na+ + 2OH- (aq)
The process is a sequence from the metal oxide, to the metal hydroxide that
dissociates into the aqueous hydroxide and metal ions.

Sodium oxide is therefore a basic anhydride; it produces the base sodium
hydroxide in aqueous solution.

Note the greater degree of ionic character of the oxide, the more basic it is.
Nonmetal oxides react with water to produce what are known as oxo-acids or
oxyacids, i.e. acids containing NM-O-H unit, where NM = nonmetal.

A non metal oxide is usually characterized by polar covalent bonds rather than
the ionic bonds of a metal oxide.
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For example,
H
+
 O
 NM
O
O H +
NM

H
2H2O
O
NMO22- + 2H3O+
H
oxoacid
id

The partially negative oxygen atom of a water molecule will attack the partially
positive nonmetal atom at the same time that the oxygen of the oxide is
attracted to one of the hydrogen atoms of the water.

The breaking of the O-H bond of the water produces an oxoacid that, splits to
produce the corresponding aqueous anion and hydronium ions.

For example, sulfur trioxide (non metal) produces sulfuric acid as the oxoacid
that in turn dissaciates into sulfate and hydronium ions.

Sulfur trioxide is the acid anhydride of sulfuric acid.

Amphoteric oxides – are often oxides of the semimetals. Although the oxides
themselves are not too soluble in water, they can react with either acids or
bases. ZnO is also amphoteric.
+ 6H+
2Al3+(aq) + 3H2O
Al2O3(s)
+ OH- + 3H2O
2[Al(OH)4]-(aq)
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Hydrolysis of Oxo anions

Non metals exist in aqueous solution as oxo acids that ionize to form oxoanions in
solution.

Oxo anions (combinations of oxygen and a NM) are hydrated in aqueous solution.

In this case the electrostatic attraction is between the electron pairs on the oxo
anion oxygen atoms and the partially positive H-atoms of the water molecules.

The hydration is an exothermic process. Hydration energies increase with
increasing charge and decreasing size.

If the interaction between the anion and the H-atom of the water is
sufficiently strong, the H can be removed from water generating a
hydroxide ion resulting in a basic solution.
MOxy- + H2O

[MO(x-1)OH](y-1) + OH-
When determining the basicity of an oxo anion we have to take into consideration,
charge, number of oxo groups and electronegativity.

Oxoanions can be placed into categories that describe the extent to which they
hydrolyze.
Effect of charge on basicity

Increasing charge on an anion increases its tendency to hydrolyze and form basic
solutions.

The pKb values of an oxoanion decrease by 4-5 units for each additional negative
charge on the anion.
Effect of number of oxo groups

Since most nonmetals exhibit more than one oxidation state, they can form oxo
anions that differ in the number of oxo groups (oxygen’s bound directly to the
metal). For example
p chlorine forms four different oxo anions: ClO-, ClO2-, ClO3-,
ClO4-. Adding additional oxygen’s decreases the bascity of the oxoanion.
Effect of electronegativity

As the electronegativity of the nonmetal atom decreases the basicity of the oxo
anion increases.
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The classification of oxo anions is summarized below
Classification
Resulting Formula
Nonbasic
contains oxo groups & no charge
Feebly basic
no charge & no oxo groups
Moderatel basic
Moderately
no oxo
o o groups
gro ps & a charge of -1/2
1/2 or -1
1
Very strongly basic
no oxo groups & a charge more
negative an -1
Example
1. Consider an oxo anion of formula MO4-. For this formula, the effect of one unit of negative charge will cancel
out the effect of two oxo groups.
If we remove the negative charge and two oxo groups the resulting formula is MO2. This anion falls into the
category of nonbasic.
nonbasic
2. Consider an oxo anion of formula MO34-. The three oxo groups cancel 1.5 unit of negative charge. The resulting
formula becomes M-2.5. This oxo anion will be very strongly basic.
A table on the next slide lists calculated pKb’s for the important simple oxo anions of the elements.
6
Summary of Trends in Numbers of Oxo Groups in Oxo Anions

The oxo anions having the smallest central atoms are those of the Period 2pblock. These can have a maximum coordination of 3 and will accommodate either
3 oxo groups or two oxo groups plus one unshared p electrons.
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
Central atoms of the p-block of Periods 3 and 4 and those of the 3 block 4 and
those of the d-block of Periods 4 and 5 have larger radii and can have a
maximum coordination number of 4. If the valence orbitals are 3p, 4p or 4d, the
central atom can accommodate 4 oxo groups and unshared p electrons.

Central atoms of the p-block and d-block of Period 6, have a maximum
coordination number of 6. If the valence orbitals on the central atom are 5p, 5d,
5f, 6p or 6d, the central atom can accommodate 4-6 oxo groups and unshared porbital pairs.
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2. To Summarize - classification of oxides

Acidic Oxide – produces an acid when it reacts with H2O
e.g.

H2SO4
SO2 + H2O
H2SO3
CO2 + H2O
H2CO3
Basic Oxide – produces a base when reacts with H2O
e.g.

SO3 + H2O
Na2O + H2O
2NaOH
CaO + H2O
Ca(OH)2
Amphoteric Oxides – react with H2O to produce a hydroxide that reacts
further with either acids or bases.
For Example ZnO, BeO-, Al2O3-, Ga2O3-, Sn2O2-, PbO2-
The “E-OH Unit in Aqueous solution; (where E = an element)

We know both metal and monmetal oxides react with water to produce
compounds with an E-O-H unit.

If E is a metal,
metal the unit acts as a base releasing hydroxide ions in solution
solution.

If E is a nonmetal, hydronium ions are released.

Why the difference? How does the nature of E determine whether the unit will
be an acid or a base or amphoteric?

To answer this we look closely
y at the relative electronegativities
g
of the atoms
within the unit.
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hydroxide = OH- = base
X
O
H
hydronium = H3O+ = acid
where does the bond break?
For a metal (ENs 0.7-1.5)
X
O
H
X+ + OH-
Producing a base in solution.

The greater difference in electronegativity is between the metal and the
oxygen, making the M-O bond the more polar. The more polar M-O bond is
susceptible to attack by polar water molecules.

This results in breaking the M-O bond to afford the aqueous metal cation and
the hydroxide anion in solution.
For a Non Metal (ENs 2.3-3.5)

In this case the O-H bond is the more polar and susceptible to attack by water
molecules, and this results in the oxoanion and hydronium ions in solution.
X
O
H
XO- + H+
Producing an acidic solution.
For a semimetal

The two bonds of the E-O-H
E O H unit are approximately of the same polarity,
polarity and
either can be broken, depending on the circumstances. In this case the unit is
amphoteric.
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3.

Periodicity of oxide acid-base character
We have just seen that for a non metal, how the X-O-H unit is split between
the oxygen and the hydrogen atoms by the attack of polar water.

Two additional factors are also important in determining the relative acid
strengths:
1.
Th electronegativity
The
l t
ti it of
f the
th central
t l atom
t
2.
The number of nonhydroxyl oxygens bound to the central atom.

As the electronegativity of the central atom increases, so does its ability to
withdraw electron density from its neighboring atoms.

The surrounding oxygen atoms have high electronegativity, so the central
atom cannot withdraw electron density from these.

The electron density comes from the O-H bonds. The H atom has low
electronegativity.

Electron density is withdrawn from the O-H bond, the H becomes more
partially positive and therefore the O-H bond is more polar and susceptible to
attack by water molecules.

The acid strength of an oxoacid increases as the electronegativity of the central
atom increases. For example, sulfuric acid is a stronger acid than selenic acid,
phosphoric acid is stronger than arsenic acid and perchloric acid is stronger than
perbromic acid.

As the number of nonhydroxyl oxygens increases
increases, they withdraw more electron
density from the central atom, making it partially positive. In turn the central
atom now withdraws more electron density from the only source available, i.e the
O-H bond. Again the H atom becomes partially positive, and the O-H bond
becomes more polar and therefore more susceptible to attack by water. The
result is that as the number of nonhydroxyl oxygens increases so does the
strength of the oxoacid.
oxoacid

For example, acid strength increases from nitrous acid to nitric acid and from
sulfurous to sulfuric acid.
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What about Transition Metals?
CrIIO is basic and dissolves in acids.
CrIII2O3 is amphoteric it dissolves in acids (Cr3+) and bases [Cr(OH)3]CrVIO3 is an acidic oxide it dissolves in water to produce H3CrO4 (chromic acid).

A idit of
Acidity
f the
th oxide
id increases
i
s s ass the
th charge
h
on the
th “cation”
“ ti ” increases
i
s s (so
(s with
ith
increasing oxidation number of the central atom).

Acidity of the oxide also increases as the size decreases ( as the ionic radius
decreases the X-OH bond strength increases and the H+ ion dissociates).

CrO3 is analogous to SO3
H2CrO4
H2SO4
Chromic acid
Sulfuric acid
4.
Reactions of acidic and basic oxides
1.
With H2O to produce acid or bases
CaO + H2O
Ca(OH)2
basic oxide
N2O5 + H2O
2HNO3
acidic oxide
2.
React with each other to afford salts.
Na2O(s) + SO3(g)
basic
acidic
oxide
oxide
Na2SO4 (s)
salt
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3
3.
CaO + Cl2O
Ca(OCl)2
basic
acid
salt
oxide
oxide
A id oxide
Acid
id + base
b
salt
lt or salt
lt + H2O
CO2 + NaOH
NaHCO3
CO2 + 2NaOH
Na2CO3 + H2O
(CO2 is the anhydride of H2CO3)
3.
Basic oxide + acid
MgO + 2HCl
salt + H2O
MgCl2 + H2O
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