Question

Define decomposition reaction, state its types define each and give example.​

Answer 1

Decomposition Reactions

The chemical change in which a compound splits up into two or more simpler substances (elements or compounds) by the absorption of energy is called Decomposition Reaction.

A chemical change involving decomposition brought about by the action of heat is called thermal decomposition, whereas that brought about by the passage of electric current is called electrolytic decomposition.

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Some examples of decomposition reactions are as follows :

$\sf Thermal \ Decomposition \ Reactions$

$\small\text{(i) Decomposition of mercuric oxide into mercury and oxygen by absorbing heat}$

$\sf \underset{Mercuric \: Oxide}{2HgO} \xrightarrow{Heat} \underset{Mercury}{2Hg} + \underset{Oxygen}{{O}_{2}}$

$\small\text{(ii) Decomposition of lead nitrate into lead oxide, nitrogen dioxide and oxygen by absorbing heat}$

$\sf \underset{Lead \: nitrate}{2Pb({NO}_{3})_2} \xrightarrow{Heat} \underset{Lead \: Oxide}{2PbO} + \underset{Nitrogen \: dioxide}{4{NO}_{2}} + \underset{Oxygen}{{O}_{2}}$

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Electrolytic Decomposition Reactions :

$\small\text{(i) Decompositon of water into hydrogen and oxygen by absorbing electrical energy or electrolysis.}$

$\sf 2{H}_{2} O \underset{H_2SO_4}{\xrightarrow{Electric \: current}} \underset{Hydrogen}{ 2H_2} + \underset{Oxygen}{O_2}$

$\small\text{ (ii) Decomposition of molten sodium chloride into sodium and chlorine by electrolysis.}$

$\sf \underset{Sodium \: chloride \: (molten)}{2NaCl} \xrightarrow{Electric \: current} \underset{Sodium}{2Na} + \underset{Chlorine}{Cl_2}$

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$\mathbb{NOTE:}$

1.) The digestion in our body involves a series of decomposition reactions in presence of various enzymes. The food which we eat like wheat, rice , potato contains starch which after a series of decomposition reaction finally changes to glucose. The proteins are decomposed to amino acids.

2.) A series of the decomposition reaction of food (which we use for living) is carried out in our body by enzymes.

$\sf Starch \xrightarrow{Diastase} Maltose \xrightarrow{Maltase} \: Glucose \: \xrightarrow{Zymase} \: Ethanol \underset{bacilli}{\xrightarrow{Mycoderma}} \: Acetic \: Acid.$

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Photochemical Decomposition Reactions :

A chemical reaction involving decomposition by the action of light is called a photochemical decomposition reaction.

1. Photochemical decomposition of silver chloride : Silver chloride on exposing to sunlight undergoes photochemical decomposition forming silver metal and chlorine gas. Silver chloride being white in color, silver is grey and chlorine is greenish yellow. During photochemical decomposition of silver chloride, white color changes to grey. Greenish yellow color of Cl₂ can also be noticed.

$\sf \underset{Silver \: chloride \: (white)}{2AgCl(s)} \xrightarrow{Sunlight} \underset{Silver \: metal \: (grey)}{2Ag(s)} + \underset{Chlorine}{Cl_2 (g)}$

2.) Photochemical decomposition of silver bromide : Silver bromide on exposing to sunlight also undergoes photochemical decomposition to give silver metal (grey) and bromine vapours (brown) .

$\sf \underset{Silver \: bromide \: (white)}{2AgBr(s)} \xrightarrow{Sunlight} \underset{Silver \: metal \: (grey)}{2Ag(s)} + \underset{Bromine}{Br_2 (g)}$

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$\mathbb{NOTE:}$

1. Electrolytic decomposition is used in extraction of metals and other industrially important products.
2. Photochemical decomposition of AgBr is used in black and white photography.

However, anhydrous copper II sulphate (white) changes to blue hydrated copper II sulphate on exposure to moist air.

$\sf \underset{white}{CuSO_4} \overset{\scriptsize \sf Moisture} \leftrightharpoons\underset{Blue}{CuSO_4 \cdot 5H_2O}$

Hope it helps.