Aqueous Solutions and Chemical Equilibria:
Acid-base titration
Complexometric titration
Separations
Electrochemistry
Etc., etc.
Illustration: acid-base titrations
Concepts and determination of pH during course of titration
Acid-base titrations involve several simultaneous
equilibria. To fully understand changes in pH during
titrations involving strong acids/strong bases, weak
acids/strong bases, multiprotic acids/strong bases, etc. we
must be able to treat the following cases:
Solution pH of strong acid or base
Solution pH of weak acid or base
Solution pH of salts
Solution pH of buffers
Four possible levels of approximation apply depending
upon whether or not we can 1) ignore autoprotolysis of
water and 2) assume the concentration of the acid or base in
solution is either zero (very strong acid or base) or the same
as the analytical concentration of the acid or base (very
weak acid or base).
We will first deal with the estimation of pH for these
various cases and apply these approaches to understanding
the shapes of titration curves.
Compounds that dissolve in water to form solvated
ions are called electrolytes:
Solubility is determined by the energies associated
with disrupting solute bonds and solvent bonds, the
formation of new solvent-solute bonds, and entropy.
Entropy always favors disorder (dissolving).
Acids and Bases
We will use the Brønsted-Lowry definition of acids
and bases:
Acid = proton donor
Base = proton acceptor
H2O
Acid1
+
ABase1
OHBase2
+
HA
Acid2
Acid1 (H2O) and Base2 (OH-) are called a conjugate
acid/base pair. H2O is the conjugate acid of OH- and
OH- is the conjugate base of H2O.
A- is the conjugate base of HA and HA is the
conjugate acid of A-.
All proton transfer reactions involve two conjugate
acid/base pairs.
H2O
Base1
+
HA
Acid1
H3O+
Acid2
+
ABase2
Amphiprotic molecules, like H2O, can act as either
an acid or as a base.
Amphiprotic solvents undergo autoprotolysis:
H2O
Acid1
+
H2O
Base1
H3O+
Acid2
+
OHBase2
NH3
Acid1
+
NH3
Base1
NH4+
Acid2
+
NH2Base2
CH3OH
Base1
CH3OH2+ + CH3OAcid2
Base2
CH3OH +
Acid1
The autoprotolysis behavior of a solvent plays a very
important role in acid/base chemistry in the solution.
It can serve as a differentiating solvent, in which
differences in acid strengths are readily apparent, or
as a leveling solvent, in which differences in inherent
acidities are not readily apparent.
e.g., HClO4 dissociated 5000X more than HCl in glacial acetic
acid; both are 100% dissociated in H2O
HA + solvent
solventH+ + A-
Whenever solvent is a much stronger base than A-,
HA is completely dissociated.
H2O is a much stronger base than either Cl- or ClO4-.
Therefore, HCl and HClO4 are both strong acids (≈
100% dissociated).
HC2H3O2 is much more basic than ClO4“
is not much more basic than ClHence, in glacial acetic acid,
HClO4 ≈ 100% dissociated
HCl << 100% dissociated
H3O+ is referred to as the hydronium ion
Why H3O+? Remember the special characteristic of H+, it
has a very large charge density due to its small size.It will
stick to virtually anything and therefore is always solvated.
Proton affinity of compound X = energy to remove a
proton from XH+ proton affinity of water is 165
kcal/mol
Sodium ion affinity of compound X = energy to remove a proton
from XNa+ sodium ion affinity of water = 25 kcal/mole
Acids produce H3O+, the hydronium ion, in aqueous solution. In
reality, the proton is even more heavily solvated…
H3O+
Acid1
+
NH3
Base1
H2O
Base2
+
NH4+
Acid2
Whether the solvent water acts as an acid or a base
depends upon the solute.
Proton donors that dissociate nearly 100% are
referred to as strong acids.
H
..
..
+ H:X
..:
O
H
[H
..
O
+
..
] +[:X..:]
H
-
H
e.g., HCl is a strong acid and is essentially fully
dissociated in water
Note:
strong acids have weak conjugate bases and strong bases have weak
conjugate acids
Proton donors that dissociate less than 1% are
referred to as weak acids.
H2O
Base1
+
HA
Acid1
e.g., HCN is a weak acid
H3O+
Acid2
+
ABase2
Organic acids:
e.g.
H
H
CH3COOH + H2O
acetic acid
(vinegar)
O
C
C
O
H
H
+
CH3COO- + H3O+
acetate anion
H2O
H3O
+
+
Acetic acid is an example of a carboxylic acid:
O
OH
O
C
C
H
Acidic H
C
H
H
Carboxyl group
O-
HCl(aq) is a monoprotic acid….some acids are polyprotic:
H2SO4(aq) = sulfuric acid is diprotic
H3PO4(aq) = phosphoric acid is triprotic
Polyprotic acids are associated with multiply-charged anions:
e.g.
H2SO4 + 2H2O Æ 2H3O+ + SO42H3PO4 + 3H2O Æ 3H3O+ + PO43-
In practice, polyprotic acids are dissociated in step-wise fashion:
e.g. 1
H2SO4 + H2O
H3O+ + HSO4-
HSO4- + H2O
H3O+ + SO42__________________________________
2H3O+ + SO42-
net reaction
H2SO4 + 2H2O
e.g. 2
H3PO4 + H2O
H3O+ + H2PO4-
H2PO4- + H2O
H3O+ + HPO42-
H3O+ + PO43HPO42- + H2O
__________________________________
net reaction
H3PO4 + 3H2O
3H3O+ + PO43-