Page 240 - Chemistry ICSE Class X

226 ICSE Chemistry – 10

Why cannot conc. sulphuric acid, phosphorus pentoxide
and fused calcium chloride be used for drying ammonia
gas
Ammonia being basic cannot be dried by passing through concentrated
ŽŶĐ͘ ƐƵůƉŚƵƌŝĐ ĂĐŝĚ͕ ƉŚŽƐƉŚŽƌƵƐ sulphuric acid or phosphorus pentoxide (P O ) because it reacts with them
2
5
ƉĞŶƚŽdžŝĚĞ ĂŶĚ ĨƵƐĞĚ ĐĂůĐŝƵŵ ĐŚůŽƌŝĚĞ to form ammonium sulphate and ammonium phosphate, respectively.
ĐĂŶŶŽƚ ďĞ ƵƐĞĚ ĨŽƌ ĚƌLJŝŶŐ ĂŵŵŽŶŝĂ
ŐĂƐ͘ 2NH (g) + H SO o (NH ) SO 4
4 2
4
3
2
ͻ ammonia sulphuric acid ammonium sulphate
,ŝŐŚĞƌ ƌĂƟŽ ŽĨ ƐůĂŬĞĚůŝŵĞ ŝƐ ƵƐĞĚ 6NH (s) + P O 5 + 3H O o 2(NH ) PO 4
3
4 3
2
2
ƚŽ ƉƌĞǀĞŶƚ ĂŶLJ ůŽƐƐ ŽĨ ĂŵŵŽŶŝƵŵ ammonia phosphorus ammonium phosphate
ĐŚůŽƌŝĚĞ ĚƵĞ ƚŽ ƐƵďůŝŵĂƟŽŶ͘ pentoxide
Anhydrous calcium chloride also cannot be used for drying ammonia
gas because it forms ammoniates with calcium chloride.
CaCl 2 0 * 3 (g) o CaCl 0* 3
2
calcium chloride ammonia calcium chloride
ammoniate
How is aqueous ammonia solution prepared
Aqueous solution of ammonia is prepared by dissolving ammonia gas in
water. Ammonia gas is highly soluble in water. So, to avoid back-suction
of water, a funnel-attachment is used for preparing aqueous ammonia.
A glass funnel is attached to the open end of the delivery tube. Mouth
of the funnel is just dipped into the water taken in a container.
Ammonia is highly soluble in water. Since the rate of dissolution of
ammonia is faster than the rate of its formation, low pressure is created
in the funnel. This pressure difference forces the water to rush into the
funnel. As a result, rim of the funnel loses contact with the water and no
OQTG YCVGT ECP DG UWEMGF KPVQ VJG ƀCUM 5KPEG COOQPKC KU DGKPI RTQFWEGF
the pressure of ammonia inside the funnel increases. This forces the water
(rather, the solution of ammonia in water) out. Thus, the funnel attachment
prevents the back-suction of water into the apparatus.

Industrial Preparation of Ammonia—Haber’s Process

Fig. 9.3 The funnel attachment How is ammonia manufactured by Haber’s process
Ammonia is prepared on industrial scale by Haber’s process. In this
process, ammonia is produced by direct reaction between nitrogen and
hydrogen.
N (g) + 3H (g) U 2NH (g) – Heat (= 95.3 kJ)
2
3
2
1 vol 3 vol 2 vol
The reaction is
ƒ Reversible
ƒ Exothermic
ƒ Proceeds with a decrease in volume.
ƒ Reaction conditions. According to the Le-Chatelier’s principle, the
favourable conditions for this reaction are
Š .QY VGORGTCVWTG QH Ō u%
Š *KIJ RTGUUWTG DGVYGGP Ō CVO
Š Higher concentrations of reactants.
Š Use of a proper catalyst. Finely divided iron with alumina or
molybdenum as promoter.