Chapter 12 Chemistry 10th Class Notes

Ans: Saturated Hydrocarbons 

Unsaturated Hydrocarbons 

Hydrocarbons in which two carbon atoms are linked by a single bond. 

Hydrocarbons in which two carbon atoms are linked 

by a double or a triple bond. 

Example: CH₃―CH₃ 

Example: CH₂ = CH₂ 

Ans: CH₃― CH₂―C = CH and CH₃―C = C―CH₃ The compound has six hydrogen atoms. 

Ans: They are least reactive. That’s why alkanes are called “Paraffin’s”. 

Ans: “The addition of hydrogen across a double bond is called Hydrogenation”. 

Example: CH₂ = CH₂ + H₂ CH₃― CH₃ 

Ans: Alkyl halides are reduced to alkanes with nascent hydrogen which is obtained by the action of Zn with dilute HCl. 

R―X + 2[H] R―H + HX 

Ans: Because burning of alkanes in the presence of excess of air or oxygen produces a lot of heat. 

Ans: (i) CH₃―CH₂―OH + HOSO₃H CH₃―CH₂―OSO₃H + H₂O CH₃―CH₂―OSO₃H CH₂=CH₂ + H₂SO₄ 

(ii) CH₃―CH₂―Br + KOH CH₂=CH₂ + KBr + H₂O 

Ans: Propene reacts with cold alkaline KMnO₄ solution to give a colourless 1,2-propanediol. Propane does not react with KMnO₄. 

OH OH 

3CH₃―CH=CH₂ + 2KMnO₄ + 4H₂O 3CH₃―CH ― CH₂ + 2MnO₂ + 2KOH 

Q9: Why the alkenes are called olefins? 

Ans: Since lower members of alkene form oily products on treatment with Cl₂ or Br₂, therefore, alkenes are also called olefins. 

Ans: In alkanes because all the valencies of carbon atoms are fully satisfied by single bonds. They are saturated. The single bonds are highly strong. Therefore, alkanes cannot be oxidized with KMnO₄ solution. 

Ans: “A reaction in which two atoms or groups are attached to each carbon of the double or triple bond is called Addition Reaction”. Br Br 

Example: CH=CH₂ + Br₂ CH₂―CH₂ 

Ans: Since in alkanes all bonds are single bonds which are very strong. Therefore, alkanes give only substitution reactions. 

Ans: The difference is that alkynes are capable of adding two molecules of the reagent while alkenes are capable of a adding only one molecule of the reagent. 

Structural formula of ethyne: H―C = C―H Dot and Cross formula of ethyne: H C : : C H 

Q15: Why Hydrocarbons are soluble in Organic solvent? 

Ans: Like dissolves like, therefore, hydrocarbons being non-polar in character are more soluble in non-polar organic solvents such as ether, carbon tetra chloride, etc. These are insoluble in water and other polar solvents. 

Ans: Physical Properties of Alkanes: 

(i) First four members are gases, C₅ to C₁₇ are liquids while higher members of alkanes are solids. 

(ii) They are non-polar and are soluble in non-polar organic solvents. 

(iii) The density of alkanes increases gradually with the increase of molecular mass. 

(iv) The melting and Boiling points of alkanes increase with the increase of molecular size. 

(v) They become less flammable with the increase of molecular size. 

Ans: Ethene decolourize the pink colour of cold, dilute, alkaline solution of potassium permanganate due to formation of colourless 1,2-ethanediol. 

OH OH 

3CH₂=CH₂ + 2KMnO₄ + 4H₂O 3CH₂―CH₂ + 2MnO₂ + 2KOH 

Ans: The reddish brown colour of bromine water discharges on a addition of ethene in it due to formation of colourless 1,2-dibromoethane. 

Br Br 

CH₂=CH₂ + Br₂ CH₂―CH₂ 

Ans: (i) Ethene 

Ethene is used for manufacturing of polythene plastic. 

(ii) Acetylene 

Acetylene is used in Welding and Cutting of metals. 

(iii) Chloroform 

Chloroform is used for general anaesthesia. 

(iv) Carbon tetra chloride 

Carbon tetrachloride is used as an industrial solvent.