Page 72 - Chemistry ICSE Class X

60 ICSE Chemistry – 10
General Methods of Preparation of Acids

Acids can be prepared by four general methods.
ƒ By synthesis method
ƒ By the action of water on the oxides of nonmetals.
ƒ By the displacement of more volatile acid from its salt by a less
volatile acid.
ƒ By the oxidation of nonmetals.

Preparation of acids by synthesis (or by direct
combination)

Hydracids (HX) are obtained by the direct combination of hydrogen and a
dŚĞ ĐŽŵƉŽƵŶĚƐ ŽĨ ŚLJĚƌŽŐĞŶ ǁŝƚŚ nonmetal under suitable conditions. For example,
ŚŝŐŚůLJ ĞůĞĐƚƌŽŶĞŐĂƟǀĞ ;ŶŽŶŵĞƚĂůůŝĐͿ sunlight +H O
2
ĞůĞŵĞŶƚƐ ŽƚŚĞƌ ƚŚĂŶ ŽdžLJŐĞŶ ĂƌĞ H (g) + Cl (g) o 2HCl(g) o 2HCl(aq)
2
2
ĐĂůůĞĚ hydracids͘ hydrogen chlorine hydrogen chloride hydrochloric acid
+H 2 O
, ů͕ , ^͕ , ƌ͕ ĞƚĐ͘ ĂƌĞ ŚLJĚƌĂĐŝĚƐ͘ H (g) + Br (g) o 2HBr(g) o 2HBr(aq)
heat
2
2
2
hydrogen bromine Pt catalyst hydrogen bromide hydrobromic acid

+H O
2
H (g) + S(g) o H S(g) o H S(aq)
2
2
2
hydrogen boiling sulphur hydrogen sulphide (saturated solution
of H S is weakly
2
acidic in nature)
Preparation of acids by the action of water on acidic
dŚĞ ĂĐŝĚ ǁŚŝĐŚ ĐŽŶƚĂŝŶƐ ĂŶ ŽdžLJŐĞŶͲ oxides (or oxides of nonmetals)
ĐŽŶƚĂŝŶŝŶŐ ĂŶŝŽŶ ;ĐĂůůĞĚ ŽdžŽĂŶŝŽŶͿ Oxoacids are obtained by the action of water on the acidic oxides (or oxides
ĂƌĞ ĐĂůůĞĚ oxoacids ;ĞĂƌůŝĞƌ ĐĂůůĞĚ
ŽdžLJĂĐŝĚƐͿ͘ dŚƵƐ͕ ŽdžŽĂĐŝĚƐ ĂƌĞ ƚŚĞ of nonmetals).
ĐŽŵƉŽƵŶĚƐ ǁŚŝĐŚ ĐŽŶƚĂŝŶ ŚLJĚƌŽŐĞŶ͕ For example,
ŽdžLJŐĞŶ ĂŶĚ ĂŶŽƚŚĞƌ ŶŽŶŵĞƚĂůůŝĐ CO (g) + H O(l) o H CO (aq)
ĞůĞŵĞŶƚ͘ 2 2 2 3
carbon dioxide carbonic acid
, SO ͕ ,EK ͕ , WK ͕ , CO ĂƌĞ
3
4
3
3
4
2
2
ŽdžŽĂĐŝĚƐ͘ SO (g) + H O(l) o H SO (aq)
2
2
3
2
sulphur dioxide sulphurous acid
SO (g) + H O(l) o H SO (aq)
4
2
2
3
tŚĞŶ ĂŶ ŽdžŝĚĞ ŽĨ Ă ŶŽŶŵĞƚĂů ŝƐ sulphur trioxide sulphuric acid
ĚŝƐƐŽůǀĞĚ ŝŶ ǁĂƚĞƌ͕ ĂŶ ĂĐŝĚ ŝƐ ĨŽƌŵĞĚ͘ 2P O (g) + 6H O(l) o 4H PO (aq)
dŚŝƐ ĐĂŶ ďĞ ƚĞƐƚĞĚ ďLJ ĂĚĚŝŶŐ Ă ĚƌŽƉ 2 5 2 3 4
ŽĨ ďůƵĞ ůŝƚŵƵƐ ƚŽ ƐƵĐŚ Ă ƐŽůƵƟŽŶ͘ phosphorus pentoxide phosphoric acid
ĐŝĚƐ ĐŚĂŶŐĞ ƚŚĞ ĐŽůŽƵƌ ŽĨ ďůƵĞ N O (g) + H O(l) o 2HNO (aq)
ůŝƚŵƵƐ ƚŽ ƌĞĚ͘ 2 5 2 3
nitrogen pentoxide nitric acid
Preparation of acids by the displacement of more volatile
acid from their salts by less volatile acid
Conc. sulphuric acid (conc. H SO ) has high boiling point (338°C). It is less
4
2
volatile as compared to other mineral acids, e.g., HCl and HNO . Therefore,
3
conc. H SO displaces HCl, HNO from their salts. For example,
3
4
2
+H O
NaCl(s) + conc. H SO o NaHSO (s) + HCl(g) o HCl(aq)
heat
2
4
4
2
sodium chloride sulphuric acid <200°C sodium hydrogen sulphate hydrogen chloride hydrochloric acid

+H O
heat
KNO (s) + conc. H SO o KHSO (s) + HNO (g) o HNO (aq)
2
4
3
3
2
3
4
potassium nitrate <200°C potassium hydrogensulphate nitric acid vapour nitric acid