Page 70 - Chemistry ICSE Class X

58 ICSE Chemistry – 10

Weak acids
The acids which are partially dissociated or ionised when dissolved in
dŚĞ ĚĞŐƌĞĞ ŽĨ ĚŝƐƐŽĐŝĂƟŽŶͬŝŽŶŝƐĂƟŽŶ
;DͿ ĨŽƌ ǁĞĂŬ ĂĐŝĚƐ ŝƐ ůĞƐƐ ƚŚĂŶ Ϭ͘ϯ Žƌ water are called weak acids.
ϯϬй͘ The solutions of weak acids in water are poorly conducting due to
+
+
low concentration of H O (or H ) ions in them.
3
Acetic acid or ethanoic acid (CH COOH), carbonic acid (H CO ),
3
2
3
hydrocyanic acid (HCN), sulphurous acid (H SO ), hydrogen sulphide
3
2
(H S) are weak acids.
2
Phosphoric acid (H PO ) and oxalic acid [H C O or (COOH) ] are
4
2
3
2
4
2
moderately weak acids.
How are acids classified on the basis of their basicity
+
The number of replaceable hydrogen ions (H ) in a molecule of an acid is
called its basicity.
+
The number of free hydrogen ions (H ) produced by one molecule of
an acid is called its basicity. Or,
The basicity of an acid is always a whole number, e.g., 1, 2, and 3.
ƒ The acids having basicity of one (1) are called monobasic acids.
ƒ The acids having basicity of two (2) are called dibasic acids.
ƒ The acids having basicity of three (3) are called tribasic acids.
ƒ The acids which on dissociation/ionisation in solution produce only
+
one H ion per molecule of the acid are called monobasic acids.
Š Monobasic acids ionise/dissociate in one step only. For example,
+
–
,LJĚƌŽĐŚůŽƌŝĐ ĂĐŝĚ ;, ůͿ͕ EŝƚƌŝĐ ĂĐŝĚ HCl(aq) + H O(l) U H (aq) + Cl (aq)
2
;,EK Ϳ͕ ĐĞƟĐ ĂĐŝĚ ; , KK,Ϳ ĂƌĞ Š Monobasic acids form only one normal salt. For example, HCl forms
3
3
ƚLJƉŝĐĂů monobasic acids͘
sodium chloride which is a normal salt.
ƒ The acids which on dissociation/ionisation in solution produce
+
two (2) H ions per molecule of the acid are called dibasic acids.
Š Dibasic acids dissociate/ionise in two steps. For example,
+
–
H SO (l) + H O(l) U H (aq) + HSO (aq)
2
4
4
2
sulphuric acid hydrogen sulphate ion
Solid : KdžĂůŝĐ ĂĐŝĚ ;, C O Ϳ
2 2 4
Liquid : ^ƵůƉŚƵƌŝĐ ĂĐŝĚ ;, SO Ϳ ĂŶĚ HSO (aq) + H O(l) U H (aq) + SO 2–
–
+
4
2
4
ĐĂƌďŽŶŝĐ ĂĐŝĚ ;, CO Ϳ ĂƌĞ ƚLJƉŝĐĂů 4 2 sulphate ion
3
2
dibasic acids͘
Š Dibasic acids form one normal and one acid salt. For example,
normal and acid salts of sulphuric acid are,
Normal salt: Na SO 4 (Sodium sulphate)
2
Acid salt: NaHSO 4 (Sodium hydrogensulphate)
ƒ The acids which on dissociation/ionisation in solution produce
+
three (3) H ions per molecule of the acid are called tribasic acids.
Š Tribasic acids dissociate/ionise in three steps. For example,
H PO + H O U H O + + H PO 4 –
4
2
3
3
2
phosphoric acid dihydrogen
phosphate ion
–
H PO + H O U H O + + HPO 4 2–
4
2
3
2
monohydrogen
phosphate ion
HPO 4 2– + H O U H O + + PO 4 3–
3
2
phosphate ion