What is the difference between combination reaction and decomposition reaction? Explain with suitable examples.

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Solution

Combination reaction:
Decomposition reaction:
a. Reaction in which two or more reactants combine to form a new product. A + B → C a. Reaction in which reactant is broken down into two or more products in the presence of heat, light or electricity. $A \to B + C$
b. They are generally exothermic reactions. b. They are generally endothermic reactions.
c. Exothermic reactions are reactions in which heat energy is given out. c. Endothermic reactions are reactions in which energy is absorbed in form of heat.
d. Example:
$CaO (s) + H {}_{2}O (l) \to Ca (OH) {}_{2}(aq) + Heat$
Calcium oxide Water Calcium hydroxide
(Quick lime) (Slaked lime)
It is a combination reaction in which calcium oxide combines with water and forms calcium hydroxide and it is a highly exothermic reaction.
d. Example:
Reaction
${FeSO}_{4} . 7 H_{2} O (s) \overset{heat}{\to} {FeSO}_{4} (s) + 7 H_{2} O (g)$
(Ferrous sulphate hydrated) (Ferrous sulphate) (Water)
Green
$2 {FeSO}_{4} (s) \overset{heat}{\to} {Fe}_{2} O_{3} (s) + {SO}_{2} (g) + {SO}_{3} (g)$
(Ferrous sulphate) (Ferric oxide) (Sulphur dioxide) (Sulphur trioxide)
Reddish-brown
On heating green colour hydrated ferrous sulphate crystals water molecules get evaporated and on further heating, they decompose into a ferric oxide which is reddish-brown in colour and two gases are released sulphur dioxide and sulphur trioxide. The choking odour of sulphur is of ${SO}_{2} (g)$ .
So, this is a decomposition reaction since on heating ${FeSO}_{4}$ is getting decomposed into several products.
Since decomposition in taking place in presence of heat so it is known as thermal decomposition.

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Combination reaction:	Decomposition reaction:
a. Reaction in which two or more reactants combine to form a new product. A + B → C	a. Reaction in which reactant is broken down into two or more products in the presence of heat, light or electricity. $A \to B + C$
b. They are generally exothermic reactions.	b. They are generally endothermic reactions.
c. Exothermic reactions are reactions in which heat energy is given out.	c. Endothermic reactions are reactions in which energy is absorbed in form of heat.
d. Example: $CaO (s) + H {}_{2}O (l) \to Ca (OH) {}_{2}(aq) + Heat$ Calcium oxide Water Calcium hydroxide (Quick lime) (Slaked lime) It is a combination reaction in which calcium oxide combines with water and forms calcium hydroxide and it is a highly exothermic reaction.	d. Example: Reaction ${FeSO}_{4} . 7 H_{2} O (s) \overset{heat}{\to} {FeSO}_{4} (s) + 7 H_{2} O (g)$ (Ferrous sulphate hydrated) (Ferrous sulphate) (Water) Green $2 {FeSO}_{4} (s) \overset{heat}{\to} {Fe}_{2} O_{3} (s) + {SO}_{2} (g) + {SO}_{3} (g)$ (Ferrous sulphate) (Ferric oxide) (Sulphur dioxide) (Sulphur trioxide) Reddish-brown On heating green colour hydrated ferrous sulphate crystals water molecules get evaporated and on further heating, they decompose into a ferric oxide which is reddish-brown in colour and two gases are released sulphur dioxide and sulphur trioxide. The choking odour of sulphur is of ${SO}_{2} (g)$ . So, this is a decomposition reaction since on heating ${FeSO}_{4}$ is getting decomposed into several products. Since decomposition in taking place in presence of heat so it is known as thermal decomposition.