Engineering Chemistry S Datta

186 ENGINEERING CHEMISTRY
Role of Solvent
Acid
Conjugate base
HClO H + + ClO –
H 2
O H + + OH –
HSO
–
4
H + + SO
=
4
HCO
–
3
H + + CO
=
3
NH
+
4
H + + NH 3
[Cr(H 2
O) 6
] +3 H + + [Cr(H 2
O) 5
OH] +2
Solvent makes to act the substances to behave as an acid or a base. Solvent behaves as
a base towards acid and vice versa.
Acid I
+ Base II
Acid II
+ Base I
CH 3
COOH + H 2
O H 3
O + + CH 3
COO –
HCl + CH 3
COOH CH 3
COOH
+
2
+ Cl –
CH 3
COOH + NH 3
NH
+
4
+ CH 3
COO –
HF + CH 3
COOH CH 3
COOH
+
2
+ F –
H 2
O + H 2
O H 3
O + + OH –
(Autoprotolysis of water)
Acid I
and Base I
are conjugate pair and Acid II
and Base II
are conjugate pair.
Highlights:
• HClO 4
, HBr, H 2
SO 4
, HCl and HNO 3
are of same strengths in water; but in glacial
acetic acid their strengths are as follows:
HClO 4
> HBr > H 2
SO 4
> HCl > HNO 3
This phenomenon in water is known as levelling effect of water.
• Neutralisation reaction can be explained with the help of the equation:
Acid I
+ Base II
Acid II
+ Base I
CH 3
COOH + OH – H 2
O + CH 3
COO –
• Hydrolysis reaction is explained as follows according to Bronsted Lowry concept:
Aqueous solution CH 3
COONa is alkaline because CH 3
COONa ionises completely
in solution as it is a salt and water being weak electrolyte ionises incompletely.
CH 3
COONa ⎯⎯→ CH 3
COO – + Na + ; H 2
O H + + OH –
According to Bronsted, CH 3
COO – is a strong base as its conjugate acid, acetic acid
is weak. Therefore, CH 3
COO – ions will readily accept protons from water molecules
setting OH – ions free. As a result, the solution will get enriched with OH – ions and
will behave alkaline. And water will remain undissociated being a weak electrolyte.
Classification of Solvents
Solvent
Example
(i) Protophilic (Proton seeking) H 2
O, NH 3
(ii) Protogenic (Proton generating)
H 2
O, CH 3
COOH, HF
(iii) Amphiprotic (Both proton seeking
H 2
O, EtOH
and generating)
(iv) Aprotic (Independent of proton C 6
H 6
, CCl 4
.
seeking and generating)