Page 242 - Chemistry ICSE Class X

228 ICSE Chemistry – 10
VJG ƀCUM D[ RTGUUKPI VJG VGCV QH VJG FTQRRGT 6JKU TGCFKN[ FKUUQNXGU 0*
3
ICU VJGTGD[ NQYGTKPI VJG RTGUUWTG KPUKFG VJG ƀCUM 6Q OCMG WR HQT VJKU
FKHHGTGPEG QH RTGUUWTG YCVGT HTQO VJG DGCMGT TWUJGU KPVQ VJG ƀCUM KP VJG
form of fountain changing its colour to blue.
ƒ Density. Ammonia gas is lighter than air and is therefore collected by
the downward displacement of air.
ƒ Liquefaction. #OOQPKC ICU ECP DG GCUKN[ NKSWGſGF CV TQQO
VGORGTCVWTG D[ CRRN[KPI C RTGUUWTG QH CDQWV Ō CVOQURJGTG
.KSWKF COOQPKC HTGG\GU CV Ō u% VQ IKXG C YJKVG ET[UVCNNKPG UQNKF
ƒ Boiling point. .KSWKF COOQPKC DQKNU CV Ō u% WPFGT QPG CVOQURJGTG
pressure. It has a high latent heat of vaporisation and is therefore
used in refrigeration plants.
What are the chemical properties of ammonia
Fig. 9.5 Fountain experiment to illustrate
high solubility of ammonia gas in water Some main chemical properties of ammonia are described below.
ƒ Thermal stability. Ammonia is a highly stable compound. When
passed over heated metallic catalysts or when electric discharge is
passed, it decomposes to give nitrogen and hydrogen.
heat
2NH (g) o N (g) + 3H (g)
2
2
3
(metallic catalyst)
ƒ Burning (combustibility). Ammonia is not combustible in air and
does not support combustion. However, it burns in an atmosphere of
QZ[IGP YKVJ C ITGGPKUJ [GNNQY ƀCOG
ITGGPKUJ Ō [GNNQY ƀCOG
4NH (g) + 3O (g) o 6H O(g) + 2N (g)
2
2
3
2
ammonia oxygen
ƒ Action with water. Ammonia is highly soluble in water. In aqueous
–
+
solution, it ionises to form ammonium (NH ) and hydroxide (OH )
Fig. 9.6 Burning of ammonia gas 4
ions.

+
–
or NH (g) + H O U NH OH(aq) U NH (aq) + OH (aq)
3
4
4
2
ammonia ammonium ion hydroxide ion
Ammonia ionises to a very small extent in aqueous solution because
dŚĞ ďĂƐŝĐ ĐŚĂƌĂĐƚĞƌ ŽĨ ĂŵŵŽŶŝĂ ŝƐ it is a weak base.
ĚƵĞ ƚŽ ƚŚĞ ƉƌĞƐĞŶĐĞ ŽĨ Ă ůŽŶĞ ƉĂŝƌ
ŽĨ ĞůĞĐƚƌŽŶƐ ŽŶ ŶŝƚƌŽŐĞŶ ĂƚŽŵ ŝŶ ŝƚƐ ƒ Basic character of ammonia. Ammonia molecule accepts a proton
–
+
ŵŽůĞĐƵůĞ͘ from water molecule to produce NH and OH ions.
4
The aqueous solution of ammonia acts as a weak base due to the
–
presence of OH ions in the solution. Therefore, ammonia turns red
litmus to blue.
–
+
NH (g) + H O(l) U NH (aq) + OH (aq)
3
2
4
free hydroxide ion
makes the solution basic
Action of ammonia on some indicators is described below:
Colour change
Indicators
From To
Litmus (moist) Red Blue
Phenolphthalein Colourless Pink
Methyl orange Orange Yellow