Page 48 - Chemistry ICSE Class IX

36 ICSE Chemistry – 9
The properties of water are altogether different from those of hydrogen
and oxygen.
So this is called a chemical reaction or chemical change.
In a chemical reaction, the substances which react together are called
reactants and the new substances formed are called products.

How are chemical reactions grouped on the basis of their
nature

Chemical reactions can be grouped into various types on the basis of their
nature. Some common types of chemical reactions are:
ƒ Combination reactions or synthesis reactions
ƒ Decomposition reactions
ƒ Displacement reactions
ƒ Double-displacement reactions
ƒ Precipitation reactions
ƒ Oxidation-reduction (or Redox) reactions
ƒ Exothermic and endothermic reactions.

What is a combination reaction
A reaction in which two or more substances combine together to form a
new substance is called a combination reaction. The following reactions
are combination reactions:
(a) Sulphur dioxide and oxygen combine to form sulphur trioxide.

A н o A 4SO (g) + 2O (g) o 4SO (g)
2
2
3
ŽŵďŝŶĂƟŽŶ ƌĞĂĐƟŽŶ sulphur dioxide oxygen sulphur trioxide
ͻ
DĂŐŶĞƐŝƵŵ ƌŝďďŽŶ ǁŚĞŶ ŚĞĂƚĞĚ (b) Calcium oxide and carbon dioxide combine to form calcium carbonate.
ŝŶ Ăŝƌ͕ ďƵƌŶƐ ǁŝƚŚ ĚĂnjnjůŝŶŐ ůŝŐŚƚ ƚŽ
3
2
ƉƌŽĚƵĐĞ Ă ǁŚŝƚĞ ƉŽǁĚĞƌ ǁŚŝĐŚ ŝƐ CaO(s) + CO (g) o CaCO (s)
ŵĂŐŶĞƐŝƵŵ ŽdžŝĚĞ͘ calcium oxide carbon dioxide calcium carbonate
ͻ Many combustion (or burning) reactions are also combination
ŽŵďŝŶĂƟŽŶ ƌĞĂĐƟŽŶƐ ŝŶ ǁŚŝĐŚ reactions. For example,
Ă ĐŽŵƉŽƵŶĚ ŝƐ ĨŽƌŵĞĚ ĨƌŽŵ ŝƚƐ
burn
ĐŽŶƐƟƚƵĞŶƚ ĞůĞŵĞŶƚƐ ĂƌĞ ĐĂůůĞĚ C(s) + O (g) o CO (g)
2
2
synthesis reacƟons͘ carbon oxygen carbon dioxide
burn
2H (g) + O (g) o 2H O(l)
2
2
2
hydrogen oxygen water
burn
dŚĞ ƐůƵƌƌLJ ŽĨ ůŝŵĞ ; ĂKͿ ŝŶ ǁĂƚĞƌ ŝƐ 2Mg(s) + O (g) o 2MgO(s)
2
ƵƐĞĚ ĨŽƌ ǁŚŝƚĞǁĂƐŚŝŶŐ ďƵŝůĚŝŶŐƐ͘ >ŝŵĞ magnesium oxygen magnesium oxide
;Žƌ ƋƵŝĐŬůŝŵĞͿ ĐŽŵďŝŶĞƐ ǁŝƚŚ ǁĂƚĞƌ The combination reactions in which a compound is formed from its
ƚŽ ĨŽƌŵ ƐůĂŬĞĚůŝŵĞ Ă;K,Ϳ ;sͿ͘ dŚŝƐ
Ϯ
ŝƐ ĚŝƐƉĞƌƐĞĚ ŝŶ ŵŽƌĞ ǁĂƚĞƌ ƚŽ ŐĞƚ ƚŚĞ constituent elements are called synthesis reactions. It should be noted here
ƐůƵƌƌLJ ĨŽƌ ǁŚŝƚĞǁĂƐŚŝŶŐ͘ that all synthesis reactions are also combination reactions but all combination
ĂK;sͿ н , K;lͿ reactions may not be synthesis reactions.
Ϯ
ƋƵŝĐŬůŝŵĞ ǁĂƚĞƌ
;Žƌ ůŝŵĞͿ ;ůŝŵŝƚĞĚ Some typical synthesis reactions are:
ǁŚŝƚĞ ƐŽůŝĚ ƋƵĂŶƟƚLJͿ ƒ Synthesis of hydrogen chloride (HCl)
ĚŝƐƐŽůǀĞ ǁŝƚŚ
ŚŝƐƐŝŶŐ ƐŽƵŶĚ sunlight
H (g) + Cl (g) o 2HCl(g)
2
2
o
Ă;K,Ϳ Ϯ н , Ğ Ă ƚ hydrogen chloride hydrogen chloride
ƐůƵƌƌLJ ŽĨ
ƐůĂŬĞĚůŝŵĞ ƒ Synthesis of water (H O)
2
; Ƶ Ɛ Ğ Ě Ĩ Ž ƌ
2
2
2
ǁŚŝƚĞǁĂƐŚŝŶŐͿ 2H (g) + O (g) o 2H O(l)
hydrogen oxygen water