Chemical reactions can be classified in following types:
In this part of the lesson, you will read about combination reaction and decomposition reaction. You will learn about various types of decomposition reaction, like thermal decomposition and photo-decomposition. You will also learn about decomposition reaction.
Combination Reaction: Reactions in which two or more reactants combine to form one product are called COMBINATION REACTION.
A general combination reaction can be represented by the chemical equation given here.
A + B ⇨ AB
In this imaginary reaction, A and B are reactants. They undergo combination reaction to form the product AB.
Example: When magnesium is burnt in air (oxygen), magnesium oxide is formed. In this reaction, magnesium combines with oxygen to give magnesium oxide.
Mg + O2 ⇨ 2MgO
Magnesium + Oxygen ⇨ Magnesium oxide
Example: When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon combines with oxygen to give carbon dioxide.
C + O2 ⇨ CO2
Carbon + Oxygen ⇨ Carbon dioxide
Example: When hydrogen reacts with chlorine, hydrogen chloride is formed. Int this reaction, hydrogen combines with chlorine to give hydrogen chloride.
H2 + Cl2 ⇨ 2HCl
Hydrogen + Chlorine ⇨ Hydrogen chloride
Example: When calcium oxide reacts with water, calcium hydroxide is formed. In this reaction, calcium combines with hydroxide ion to give calcium hydroxide.
CaO + H2O ⇨ Ca(OH)2
Calcium oxide + Water ⇨ Calcium hydroxide
Example: When carbon monoxide reacts with oxygen, carbon dioxide is formed. In this reaction, carbon monoxide combines with oxygen to give carbon dioxide.
2CO + O2 ⇨ 2CO2
Carbon monoxide + Oxygen ⇨ Carbon dioxide
Decomposition Reaction: Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.
A general decomposition reaction can be represented as follows:
AB ⇨ A + B
In this imaginary reaction, AB is a reactant. It undergoes decomposition to give two products, i.e. A and B.
Example: When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide
CaCO3 ⇨ CaO + CO2
Calcium carbonate ⇨ Calcium oxide + Carbon dioxide
Example: When ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3 ⇨ Fe2O3 + 3H2O
Ferric hydroxide ⇨ Ferric oxide + Water
Example: When lead nitrate is heated, it decomposes into lead oxide, nitrogen dioxide and oxygen.
2Pb(NO3)2 ⇨ 2PbO + 4NO2 + O2
Lead nitrate ⇨ Lead oxide + Nitrogen oxide + Oxygen
In above examples, compound is decomposed because of heating, so, these reactions are called THERMAL DECOMPOSITION REACTION.
Reactions in which compounds decompose into simpler compounds because of passage of electricity, are known as ELECTROLYTIC DECOMPOSITION. In other words, decomposition because of electric current is called electrolytic decomposition. This is also known as ELECTROLYSIS.
Example: When electricity is passed in water, it decomposes into hydrogen and oxygen.
2H2O ⇨ 2H2 + O2
Reactions in which a compound decomposes because of sunlight are known as PHOTOLYSIS or PHOTO DECOMPOSITION REACTION. In other words, decomposition due to light is called photo-decomposition.
Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl ⇨ 2Ag + Cl2
Similarly, when silver bromide is put under sunlight, it decomposes into silver metal and bromine gas.
2AgBr ⇨ 2Ag + Br2
Photographic paper has coating of silver chloride, which turns into grey when exposed to sunlight. It happens because silver chloride is colourless while silver is a grey metal. In this era of digital cameras, many of you may not be aware about photographic film. Now-a-days, such films are used for taking X-ray of internal organs.
Reactions in which atoms or ions move from one compound to other to form new compound are known as DISPLACEMENT REACTION. Displacement reaction is also known as Substitution Reaction or Single displacement /Replacement Reaction.
A general displacement reaction can be represented using chemical equation as follows:
A + BC ⇨ AC + B
In this imaginary reaction, A and B are metals and BC is a compound of metal B. Since A is more reactive than B so it displaces B to make a new compound AC.
Displacement reaction takes place only when A is more reactive than B. If B is more reactive than A, then A will not displace B from BC and reaction will not be taken place.
Example: When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.
Zn + 2HCl ⇨ ZnCl2 + H2
Example: When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.
Zn + CuSO4 ⇨ ZnSO4 + Cu
Example: When silver metal is dipped in copper nitrate, no reaction takes place because silver metal is less reactive than copper.
Ag + Cu(NO3 )2 ⇨ No reaction takes place
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