Download acids and bases

CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
ACIDS AND BASES
Classical concept:
Properties of Acids





taste sour (don't taste them!)... the word 'acid' comes from the Latin acere, which
means 'sour'
acids change litmus (a blue vegetable dye) from blue to red
their aqueous (water) solutions conduct electric current (are electrolytes)
react with bases to form salts and water
evolve hydrogen gas (H2) upon reaction with an active metal (such as alkali metals,
alkaline earth metals, zinc, aluminum)
Properties of Bases





taste bitter (don't taste them!)
feel slippery or soapy (don't arbitrarily touch them!)
bases don't change the color of litmus; they can turn red (acidified) litmus back to
blue
their aqueous (water) solutions conduct and electric current (are electrolytes)
react with acids to form salts and water
Examples of Common Acids





citric acid (from certain fruits and veggies, notably citrus fruits)
ascorbic acid (vitamin C, as from certain fruits)
vinegar (5% acetic acid)
carbonic acid (for carbonation of soft drinks)
lactic acid (in buttermilk)
Examples of Common Bases




detergents
soap
lye (NaOH)
household ammonia (aqueous)
BASE VS ALKALI
Bases: Bases are metal oxides, hydroxides or compounds (such as NH3) that give OHions in aqueous solution.
CuO (s) + 2HCl (aq.) → CuCl2 (aq.) + H2O (l)
NaOH (s) + HCl (aq.) → NaCl (aq.) + H2O (l)
H+ + OH- → H2O
NH3 (g) + H2O (l) ↔ NH4+ (aq.) + OH-
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Course-Teacher: Shahrina Alam
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CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
Alkali: A base that dissolves in H2O to give OH- ions is called an alkali. NH3, NaOH are
examples.
‘All alkali are base, but all bases are not alkali.”
Strong and Weak Acids and Bases
Strong Acids
A strong acid is an acid which is fully ionised in solution.
Strong acids are so fully ionised in solution (close to 100%, but not quite) that
the equilibrium constant has very large values much greater than one million.
e.g. HCl(aq) + H2O(l)
H3O+(aq) + Cl-(aq) Greater than 99.99% ionised
There aren't very many strong acids, which makes life very easy for you.
Strong Acids
The Formulae
Sulphuric acid
H2SO4
Hydrochloric acid
HCl
Hybrobromic acid
HBr
Hydroiodic acid
HI
Nitric acid
HNO3
Perchloric acid
HClO4
Weak Acids
A weak acid is an acid which is only partially ionized in solution. This also
means that it has a small value for the equilibrium constant for the ionization
reaction. The value tends to be less than one and quite a lot less than one.
e.g. CH3COOH(aq) + H2O(l)
H3O+(aq) + CH3COO-(aq) Less than 0.4% ionized.
Strong Bases
Similarly, a strong base is one which is fully ionized in solution.
It also has a high value for the equilibrium constant for ionization.
Again there are not many of them, and again you can memorize them all.
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Course-Teacher: Shahrina Alam
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CHEM1101: CHEMISTRY (EEE/COE)
Strong Bases
LECTURE 8
The Formulae
Lithium hydroxide
LiOH
Sodium hydroxide
NaOH
Potassium hydroxide
KOH
Rubidium hydroxide
RbOH
Caesium hydroxide
CsOH
Barium hydroxide
Ba(OH)2
Calcium hydroxide
Ca(OH)2
Strontium hydroxide
Sr(OH)2
Weak Bases
Again, anything else is automatically a weak base.
As before, this means that they have a small value for the equilibrium constant for ionization.
Again, there is no reference to the concentration of the solution, or how corrosive it is.
==================================================================
Three concepts of acids and bases are:
(1) Arrhenius concept
(2) Bronsted-Lowry concept
(3) Lewis concept
Arrhenius concept (1884):
An acid is a hydrogen containing compound that releases H+ ions in H2O.
A base is a hydroxyl containing compound that releases OH- ions in H2O.
HCl + H2O  H3O+ + ClNaOH + H2O  Na+ + OHUsefulness: This concept is useful in the study of chemical reactions.
Limitations:
(a) Free H+ and OH- ions do not exist in water.
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CHEM1101: CHEMISTRY (EEE/COE)
..
O:
H
+
LECTURE 8
+
H
+
..
O
H
H3O+
or
H
H
H
Hydronium ion
(b) Limited to water only.
These definitions are applicable to water only.
(c) Some bases do not contain OH-. Example: NH3, CaO
Bronsted-Lowry concept (1923):
An acid is any molecule or ion that can donate a proton (H+).
A base is any molecule or ion that can accept a proton (H+).
+
H
BASE
ACID
H
H
..
N
H +
H
Cl
H
H
N
+
H
or
Cl-
H
base
CaO
acid
+
H
O
Ca(OH)2
H
base
acid
Bronsted-Lowry concept is superior to Arrhenius concept:
(a) Much wider scope.
(b) Not limited to aqueous solutions.
NH3 + HCl
NH4+ + Cl-
(c) Release of OH- not necessary to qualify as a base.
NH3 + H+
NH4+
base
acid
“An acid is a proton donor, while a base is a proton acceptor.”
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CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
Conjugate acid and base:
In acid-base reaction the acid gives up its proton and produces a new base.
Conjugate pair
HA
+
acid
B-
HB
base
Conjugate
acid
+
AConjugate
base
Conjugate pair
The new base (A-) that is related to the original acid (HA) is called a conjugate base.
Similarly, the original base (B-) after a accepting a proton becomes a new acid is called a
conjugate acid.
Conjugate pair: The acid-base pairs associated with the loss or gain of proton is a called
conjugate pairs.
In any acid-base reaction, there are two conjugate acid-base pairs. For example, HA and A-,
HB and B-.
H+
H+
HCl
acid
NH4+
Conj. acid
NH3
base
+
Cl+
Conj. base
H+
H+
A weak acid has a strong conjugate base. A weak base has a strong conjugate acid.
Conjugate pair
CH3COOH
weak acid
+
H3O+
H2O
weak base
stronger
acid
+
CH3COOstronger
base
Conjugate pair
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CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
A strong acid has a weak conjugate base. A strong base has a weak conjugate acid.
Conjugate pair
HCl
H2O
H3O+
strong
base
weak acid
+
strong
acid
Ci-
+
weak base
Conjugate pair
Conjugate acid and base of H2O and HCO3-:
OH-
-H+
conjugate base of H2O
H2O
+H+
H3O+ conjugate acid of H2O
-H+
CO3-2
conjugate base of HCO3-
H2CO3
conjugate acid of HCO3-
HCO3+H+
Classes of Bronsted Acids and Bases:
Bronsted acids: (1) Monoprotic acids- donate one proton
HF → H+ + F-, CH3COOH → CH3COO- + H+
(2) Polyprotic acids-donate two or more protons
H2S → 2H+ + S2-, C2O4H2 → C2O42- + 2H+
Bronsted bases: (1) Monoprotic bases- accept one proton
HS- + H+ → H2S, H2O + H+ → H3O+
(2) Polyprotic bases- accept two or more protons
SO42- + 2H+ → H2SO4, PO43- + 3H+ → H3PO4+
Amphiprotic substances: Molecule or ions that behave both as Bronsted acid and base. For
example, H2O.
HCl
+
acid
NH3
base
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H2O
H3O+
+
Cl-
NH4+
+
OH-
base
+
H2O
acid
Course-Teacher: Shahrina Alam
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CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
Lewis concept (1930):
An acid is an electron-pair acceptor.
A base is an electron-pair donor.
:B
+
A
Lewis
acid
A
Lewis
base
B
Complex
The combination of Lewis acid and Lewis base is called a complex.
All cations or molecules short of an electron pair act as Lewis acids; and all anions or
molecules having a lone pair of electron act as Lewis bases.
Examples:
(a)
Lewis
acid
Lewis
base
H+
..
N
+
H
Complex
H
H
H
N
H
H+
(b)
+
F
(c)
F
B
..
:O
..
F
:N
H
H
..
O
..
H
H
H
+
+
H
F
H
H
F
H
B
N
F
H
H
Factors that are responsible for the strength of acids and bases
(1) Size of metal ions:
LiOH
NaOH
KOH
CsOH
Mg(OH)2
Size of metal ion
(Å)
0.60
0.95
1.33
1.69
0.65
Al(OH)3
0.50
Base
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Charge of ion
+1
+1
+1
+1
+2
+3
Qualitative
strength
Weak base
Strong base
Very strong base
Strongest base
Weaker base
Weaker base
(amphoteric)
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CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
(2) Ionization constant: The smaller the value of ionization constant (K), the less is the
degree of ionization and hence less strong the acids (or bases).
H3PO4  H+ + H2PO41
1
[ H  ][ H 2 PO4 ]
K=
= 0.75  10 2
[ H 3 PO4 ]
(3) Oxidation number: The oxidation number of the central atom determines the
strengths of the acids.
5
6
4
H 2 SO4 > H 3 PO4 > H 2 SO3
7
5
6
4
3
1
HClO4 > H 2 SO4 > HNO3 > H 2 SO3 > H 3 AsO3 > HClO
Note: S with oxidation number +6 in H2SO4 exerts a greater full on all the electron pairs
of O atoms than does S in H2SO3.
(4) Ionic potential: The ratio between the charge and the radius of an ion is called ionic
potential ().
i.e.  =
z
; where z is charge and r is radius.
r
 > 3.2
-acidic
 < 2.3
-basic
2.3 <  < 3.2
-amphoteric (both acidic & basic)
(5) Electro negativity: The decreasing electro-negativity gives rise to decreasing
strengths of acids in the order.
F>O>N
,
HF > H2O > NH3
(6) Size of the halogen atom: As the the atomic size of halogens decreases strengths of
halogens acids also decreases (ii). Increasing bond length, decreasing bond energy, easily
gives up proton. Bond distance of HI (1.7Å) is greater than HF(1.0Å).
HI > HBr > HCl > HF
(7) Number of oxygen and hydrogen atoms: The greater the difference between the
number of oxygen and number of hydrogen atoms, the stronger the acid.
H2SO4 > H2SO3, HNO3 > HNO2
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CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
(8) Number of non-hydrogenated oxygen molecule: The greater the number of nonhydrogenated O atoms per molecule, the greater is the strength of acid.
Hard and Soft Acids and Bases (HSAB). HSAB is an extremely useful qualitative theory
that enables predictions of what adducts will form in a complex mixture of potential Lewis
acids and bases.
Hard acids (HA) are characterized by (s,f blocks, left side of d block in higher OS's)
Low electronegativity values in the range 0.7-1.6 of the metal atom;
Relatively small size;
Relatively high charge (> 3+).
High charge often results in small size, because the remaining electrons are contracted toward
the nucleus by the substantial excess positive charge. Specific examples of hard acids are the
metal cations from the s and f blocks, and the higher-charged ions from the left side of the d
block. Na+, Mg2+, Fe3+, and Al3+ are examples of hard acids. Exceptions- H+, B+3, C+4 are
hard acids.
Hard bases (HB) are characterized by
Very high electronegativity values in the range 3.4-4 of the donor atom;
Relatively small size.
The combination of high electronegativity and small size results in a nonpolarizable electron
cloud surrounding the donor atom. The only 2 donor atoms with electronegativities in the
specified range are oxygen and fluorine. So the hard bases are those in which the donor atom
is either O or F. Specific examples are O2-, F-, SO42-, CO32-, and PO43-.
Soft acids (SA) are characterized by an acceptor atom of
intermediate to high electronegativity (1.9-2.5);
large size;
low charge (1+, 2+)
Species of large size generally have many electrons, some of which can be quite far from the
nucleus. The low charge of the species results in a polarizable (distortable) electron cloud.
Specific examples of soft acids include Cu+, Hg2+, Au+, Ag+, and Pb2+. Note that these metals
are all clustered in the same region of the periodic table.
Soft bases (SB) are characterized by donor atom of
Intermediate to high electronegativity (2.1-3.0)
large size, leading to polarizability
Specific examples of soft bases are S2-, PEt3, RSe-, I-, and Br-. Note that these fall in groups
15-17 in periods with n > 3.
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CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
Applications of the HSAB Principle.
1) Predicting favorable equilibria.
2) Geochemistry of the elements.
3) Toxicology, Medicinal Chemistry. Ions of many so-called heavy metals, such as Hg2+ and
Pb2+, are highly toxic. Why? Heavy metal ions are soft acids, and therefore have high affinity
for S2-, a soft base. S occurs in the side chains of two amino acids, methionine and cystine,
and is important in maintaining tertiary structure of proteins and enzymes upon which life
depends. Ingested heavy metal ion seeks out and coordinates with amino acid sulfur,
disrupting protein structure and deactivating the protein. Eventual death is the usual result of
prolonged exposure to heavy metal ions.
Exercise: Why are HCN, CO, H2S, H2Se, and PH3 poisons?
4) Ligand selections in metalloproteins and enzymes. Ions of many of the 3d transition metals
are essential for life in trace amounts. A number of proteins and enzymes incorporate these
metal ions specifically into their structures, forming adducts with the metal ion using donor
atoms on the side chains of their amino acids. Side chains containing oxygen, nitrogen, and
sulfur donors are usually involved in adduct formation.
Exercise: What donor atoms might be appropriate for binding Cu+ in an enzyme or protein?
Na+? K+? Ca2+?
5) Reduction Potentials. The electron has been termed the "ultimate soft base". Viewing the
electron in these terms, standard reduction potential can be understood in terms of HSAB
theory. Several standard reduction potentials are given below.
Fe3+(aq) + 3eCu+(aq) + eNa+(aq) + eLa3+(aq) + 3e-
→ Fe(s),
→ Cu(s),
→ Na(s),
→ La(s),
o = -.036
o = .522
o = -2.711
o = -2.37
Recall that the more positive the value of o, the more spontaneous the reduction. The data
reveal that the o values parallel the hardness/softness of the metal cation (acid). Thus the soft
acid, Cu+, interacts quite spontaneously with the soft electron. The hard acids, Fe3+, Na+, and
La3+ are very difficult to reduce because their interaction with the soft electron is not
favorable.
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CHEM1101: CHEMISTRY (EEE/COE)
LECTURE 8
Basic Principles of the Various Theories of Acids & Bases
Traditional approach
Substance that have certain properties
Acid: sour taste, turns litmus red;
Base: bitter taste, turns litmus to blue
Arrhenius
Acid : give H+ in aqueous solution
Base : give OH- in aqueous solution
At neutrality: [H+] = [OH-]
Bronsted-Lowry
Acid : H+ donor
Base : H+ acceptor
Conjugate acid-base pairs
No concept of neutrality
Lewis
Acid : a potential electron-pair acceptor
Base : a potential electron-pair donor
Usanovich
Acid: a substance that donates a cation, or accepts
an anion or an electron
Base: a substance that donates an anion, or accepts a
cation.
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