Page 49 - Chemistry ICSE Class IX

Chemical Changes and Reactions 37
ƒ Synthesis of magnesium oxide (MgO)
2Mg(s) + O (g) o 2MgO(s)
2
magnesium from air magnesium oxide
ƒ Synthesis of ammonia (NH )
3
N (g) + 3H (g) q 2NH (g)
2
2
3
nitrogen hydrogen ammonia A o A н
ĞĐŽŵƉŽƐŝƟŽŶ ƌĞĂĐƟŽŶ
An important industrial application of combination (or synthesis)
reaction is the manufacture of hydrogen chloride gas from hydrogen
and chlorine. Hydrogen chloride gas when dissolved in water gives
hydrochloric acid.
o
H (g) + Cl (g) o 2HCl(g) +H O(l) HCl(aq)
2
2
2
hydrogen chlorine hydrogen hydrochloric
chloride acid ĚĞĐŽŵƉŽƐŝƟŽŶ ƌĞĂĐƟŽŶ ŝƐ
ŽƉƉŽƐŝƚĞ ŽĨ ƚŚĞ ĐŽŵďŝŶĂƟŽŶ
What is a decomposition reaction ƌĞĂĐƟŽŶ͘
A reaction in which a substance is broken down into two or more simpler
substances is known as decomposition reaction.
Decomposition reactions take place only when some energy in the
form of heat, light or electricity is supplied to the substance.
Various kinds of decomposition reactions are:
ƒ Thermal decomposition. Decomposition reaction caused by heating
is called thermal decomposition.
ƒ Photodecomposition. Decomposition reaction caused by light is
called photodecomposition or photolysis.
ƒ Electrolytic dissociation. Decomposition reaction caused by electricity >ŝŵĞƐƚŽŶĞ ;ĐĂůĐŝƵŵ ĐĂƌďŽŶĂƚĞͿ
is called electrolytic decomposition or electrolysis.
Some typical decomposition reactions are:
ƒ Limestone (CaCO ) when heated strongly gives quicklime (CaO) and ŝŐĞƐƟŽŶ ŽĨ ĨŽŽĚ ŝŶ ƚŚĞ ďŽĚLJ ŝƐ
3
carbon dioxide (CO ). ĂŶŽƚŚĞƌ ĞdžĂŵƉůĞ ŽĨ ĚĞĐŽŵƉŽƐŝƟŽŶ
2
heat
CaCO (s) o CaO(s) + CO (g) ƌĞĂĐƟŽŶ͘
3
2
limestone quicklime carbon dioxide ƵƌŝŶŐ ĚŝŐĞƐƟŽŶ͕ ĐŽŵƉůĞdž
ŵŽůĞĐƵůĞƐ ƉƌĞƐĞŶƚ ŝŶ ƚŚĞ ĨŽŽĚ ĂƌĞ
ƒ Potassium chlorate, when heated, decomposes to give potassium ďƌŽŬĞŶ ĚŽǁŶ ŝŶƚŽ ƐŵĂůůĞƌͬƐŝŵƉůĞƌ
chloride (KCl) and oxygen (O ). ŵŽůĞĐƵůĞƐ ďLJ ĞŶnjLJŵĞƐ͘
2
heat
2KClO (s) o 2KCl(s) + 3O (g)
2
3
potassium chlorate potassium chloride oxygen
ƒ Ferrous sulphate crystals are light green in colour. When heated
gently, they decompose to lose water and their colour change to dirty
white.
heat
FeSO .7H O o FeSO (s)+ 7H O(g)
2
2
4
4
ferrous sulphate crystals anhydrous water
(light green) ferrous sulphate
(dirty white)
When anhydrous ferrous sulphate is heated strongly, it decomposes
to give ferric oxide (Fe O ) and oxides of sulphur.
2
3
heat strongly
2FeSO (s) o Fe O (s)+ SO 2 + SO 3
2
3
4
ferrous sulphate ferric oxide oxides of sulphur
(dirty white) (dark brown) (smelling of burning sulphur)
ƒ Water is decomposed when electricity is passed through acidulated
water. Fig. 2.1 When electricity is passed
electricity
2H O(l) o 2H (g) + O (g) through acidulated water, it decomposes
2
2
2
water hydrogen oxygen to produce hydrogen and oxygen