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A decompositon reaction is a reaction that takes place when one reactant breaks down into two or more simpler substances. The decomposition reaction can be represented by the general equation: AB → A + B. Most of the decomposition reactions require an input of energy in the form of heat, light or electricity.
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Read Also: Class 10 Chemical Reactions
The topic Decomposition Reaction is covered in the unit 1 chapter 1 of NCERT Class 10 Science. As per the latest updates, in the revised syllabus of CBSE, no topics have been excluded from the above mentioned chapter. The entire unit 1 i.e., Chemical Substances- Nature and Behaviour will carry a weightage of 26 marks in CBSE Class 10 Board Examination.
Define Decomposition Reaction
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The reaction in which a single substance decomposes to produce more than one substance is known as the decomposition reaction. The key characteristic of a decomposition reaction is that one substance acts as a reactant forming more than one product.
For example, in the decomposition of sodium hydrogen carbonate three products are formed - sodium carbonate, carbon dioxide, and water.
2 NaHCO3(s) → Na2CO3(s) + CO2(g) + H2O(?)
Explanation
Heat
2FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g)
In this reaction, a single reactant breaks down into simpler products. This is a decomposition reaction. During heating of Ferrous Sulphate, crystals (FeSO4, 7H2O) lose water, and the color of the crystals changes. Then it decomposes to produce ferric oxide (Fe2O3), sulfur dioxide (SO2), and sulfur trioxide (SO3). Ferric oxide is in solid-state, while SO2, and SO3 are in the gaseous state. Few examples for decomposition reactions are as follows:
- Decomposition of calcium carbonate on heating produces calcium oxide and carbon dioxide. This decomposition reaction is used in various industries. Calcium oxide is known as lime or quicklime. One of its uses is to manufacture cement. The decomposition reaction, which is carried out by heating, is known as thermal decomposition.
Heat
CaCO3(s) → CaO(s) + CO2(g)
- On heating nitrate salt emission of brown fumes is observed. These brown fumes are of nitrogen dioxide (NO2).
Heat
2Pb(NO3)2(s) → 2PbO(s)+ 4NO2(g) + O2(g)
- White silver chloride turns grey in sunlight which is an example of a decomposition reaction. In the presence of light silver chloride breaks into silver and chlorine.
Sunlight
2AgCl(s) → 2Ag(s) + Cl2(g)
From the examples above, we can see that for a decomposition reaction to take place, some form of energy is required as an external source of the reaction. As energy is required in the case of the decomposition reaction, it may be exothermic or endothermic but the former type is more common in day-to-day life.
Other examples of decomposition reaction are:
- Electrolysis of water which breaks it into oxygen and hydrogen:
2H2O(I) → 2H2 + O2
- Carbonates are decomposed when heated:
H2CO3 → H2O + CO2
- Decomposition of Metal chlorates in presence of heat:
2MClO3 → 2MCl+ 3O2
Types of Decomposition Reaction
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There are three types of decomposition reactions, which are discussed as under:
i) Thermal decomposition reaction
A thermal decomposition reaction is considered as a decomposition reaction which is activated by thermal energy. In this type of decomposition reaction, energy is supplied to the reactants in the form of heat energy. As energy is required to break the chemical bond of the reactants, these are endothermic. A common example of a thermal decomposition reaction is the decomposition of carbonate salt.
CaCO3 → CaO + CO2
On heating, calcium carbonate decomposes to produce calcium oxide and carbon dioxide. Quicklime is produced by this process which is an important substance for many industries.
Read more: Balanced Chemical Equations
ii) Electrolytic decomposition reaction
When the reactant is broken down into multiple products by receiving energy from electricity, it is known as an Electrolytic decomposition reaction. Electrolysis of water is a simple example for the same:
2H2O → 2H2 + O2
iii) Photochemical decomposition
When the reactant is broken down into several products using the energy from the light source is termed photochemical decomposition. Breaking down of silver chloride in presence of sunlight is an example of photochemical decomposition:
Sunlight
2AgCl(s) → 2Ag(s) + Cl2(g)
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Uses of Decomposition Reactions
- Manufacture of cement from calcium carbonate.
- Extraction of metals from their ores using electrolytic decomposition.
- Breaking up of starch and other sugars in the body for proper digestion.
- Thermite welding process.
Double Decomposition reactions
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A double decomposition reaction is a type of decomposition reaction involving 2 reactants that exchange their cations and anions to form 2 new compounds.
General form of double decomposition reaction is:
AB + CD→CB + AD
Example:
- i) AgNO3(aq)+NaCl(aq)→AgCl(s)+NaNO3(aq)
- ii) HCl(aq)+NaOH(aq)→NaCl(aq)+HOH(l)
Sample Questions
Ques: Are all decomposition reactions endothermic?(2 marks)
Ans. All decomposition reactions may not necessarily be endothermic.
For example decomposition of NO into N2 & O2 is exothermic and the decomposition of ozone (O3) to oxygen (O2) is exothermic.
Ques: Why is respiration an exothermic process?(2 marks)
Ans. Sugar breaks down in the cells of the body and releases the energy necessary for the reactions in the body. Respiration is the process by which the sugar is broken down in the cells, hence it is an exothermic reaction.
Ques: Identify which equation is a composition reaction, a decomposition reaction, or none between the two.
Fe2O3 + 3SO3 → Fe2(SO4)3
NaCl + AgNO3 → AgCl + NaNO3
(NH4)2Cr2O7 → Cr2O3 + 4H2O + N2 (2 marks)
Ans.
- In this equation, two substances combine to produce a single substance. So, it is a composition reaction.
- In this equation, two different substances react to make two new substances. It is neither a composition reaction nor a decomposition reaction, so it is neither. It is a double-replacement reaction.
- It is a decomposition reaction because a single substance breaks to form multiple substances.
Ques. Potassium chlorate (KClO3) on heating forms potassium chloride and oxygen. Explain whether it is a decomposition reaction or not?(3 marks)
Ans. Potassium chlorate (KClO3) on heating forms potassium chloride and oxygen. It is a decomposition reaction because one substance breaks to form two new substances.
- In this equation, two substances combine to produce a single substance. So, it is a composition reaction.
- In this equation, two different substances react to make two new substances. It is neither a composition reaction nor a decomposition reaction, so it is neither. It is a double-replacement reaction.
- It is a decomposition reaction because a single substance breaks to form multiple substances.
- Potassium chlorate (KClO3) on heating forms potassium chloride and oxygen. Explain whether it is a decomposition reaction or not?
- Potassium chlorate (KClO3) on heating forms potassium chloride and oxygen. It is a decomposition reaction because one substance breaks to form two new substances.
2KClO3 (s) → 2KCl (s) + 3O2 (g)
Ques: How decomposition reaction helps in the extraction of ores?(2 marks)
Ans: One of the major applications of decomposition reactions in industries is in the extraction of metals from their ores on a large scale. For example, zinc is extracted from calamine by a decomposition reaction. Sodium is also obtained from sodium chloride (NaCl) by decomposition reaction.
Previous Year’s Questions
Ques: 2g of ferrous sulphate crystals are heated in a dry boiling tube.
(i) List any two observations
(ii) Write the type of chemical reactions taking place.
(iii) Write the chemical equation of the reaction. (2015) (3 marks)
Ans: (i) The two observations are
- Green colour of FeSO4 disappears and reddish brown solid is formed.
- Smell of burning sulphur.
(ii) The type of chemical reaction is decomposition reaction.
Ques: Name the type of chemical reactions that are represented by the following equations: (2015) (3 marks)?
Ans: (i) Combination reaction
(ii) Double displacement reaction
(iii) Decomposition reaction
Ques: Which products will obtain when lead nitrate is heated simply. Write the balanced chemical equation for the reaction and state the type of chemical reaction that occur in the change. (2013) (3 marks)
Ans: Lead monoxide, nitrogen dioxide and oxygen gas will be liberated when lead nitrate is heated simply.
The type of chemical reaction that occur in the change is the thermal decomposition reaction.
Ques: What is the colour of ferrous sulphate crystals. State the way in which the colour changes after heating. (2012) (2 marks)
Ans: The colour of ferrous sulphate is pale green. On heating, the colour changes to reddish brown due to the formation of iron oxide.
Ques: Give an example each for thermal decomposition photochemical decomposition reactions. Write the relevant balanced chemical equations also. (2012) (2 marks)?
Ans: The thermal decomposition reaction:
The photochemical decomposition reaction
Ques: (i) Explain two ways by which the food industries prevent rancidity.
(ii) Discuss any three important points of decomposition reaction in metal industry. (2015) (5 marks)
Ans: (i) The two ways by which the food industries prevent rancidity are,
- Rancidity can be prevented by adding antioxidants to food containing fat as well as oil, such as butylated hydroxy anisole is added to butter in the form of antioxidant.
- It can be prevented by packaging fat and oil containing foods in nitrogen gas.
(ii) The three important points of decomposition reaction in metal industry are as follows,
- Molten NaCl is electrolytically is decomposed in order to form sodium metal.
- By electric decomposition of bauxite ore mixed with cryolite, aluminium is obtained.
- Carbonate ores are thermally decomposed to give metal oxide that on reduction give metal.
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