Matric Notes Chemistry 10th Chapter 12 Hydrocarbons Exercise Short Questions

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1. Differentiate between saturated and unsaturated hydrocarbons.

Ans. Saturated Hydrocarbons

• The hydrocarbons in which all the four valencies of carbon atoms are fully satisfied by single bond with other carbon atoms and hydrogen atoms are called saturated hydrocarbons.
• They are also called alkanes.
• The general formula of saturated hydrocarbons is CnH2n+2
• Examples CH4, C2H6, C3H8

Unsaturated Hydrocarbons

• The hydrocarbons in which two carbon atoms are linked by a double or a triple bond are called unsaturated hydrocarbons.
• They are also called alkenes and alkynes.
• The general formula of alkenes is CnH2n and that of alkynes is CnH2n-2
• Examples alkenes: C2H4, Alkynes C2H2

2. A compound consisting of four carbon atoms has a triple bond in it. How many hydrogen atoms are present in it?

Ans. There are six hydrogen atoms in a compound containing the four carbon atoms and having a triple bond in it. It is butyne (C4H6)

3. Why are the alkanes called paraffins?

Ans. Alkanes are saturated hydrocarbons. In these compounds all the bonds of carbon atoms are single that means valencies of carbon atoms are fully satisfied. Therefore they are least reactive. That is the reason alkanes are called paraffins.

4. What do you know about hydrogenation of alkenes?

Ans. Hydrogenation means addition of molecular hydrogen to an unsaturated hydrocarbon in presence of a catalyst. (Ni, Pt) to form saturated compound. 

e.g. H2C=CH2 + H2 ----> H3C-CH3

5. How are alkyl halides reduced?

Ans. Reduction of alkyl halides:

Reduction means addition of nascent hydrogen. In fact, it is a replacement of a halogen atom with a hydrogen atom. This reaction takes place in the presence of Zn metal and HCl. e.g.

CH₃Br + 2[H] ---> CH₄ + HBr

CH₃CH₂BR + 2[H] ---> CH₃-CH₃ + HBr

6. Why are the alkanes used as fuel?

Ans. Alkanes burn in the presence of excess of air or oxygen to produce a lot of heat, carbon dioxide and water. This reaction takes place in automobile combustion engines, domestic heaters and cooking appliances. It is highly exothermic reaction and because of it alkanes are used as fuel. e.g.

CH₄ + 2O₂ ----> CO₂ + 2H₂O + heat

7. How can you prepare ethene from alcohol and ethyl bormide?

Ans. i) By Dehydration of alcohols:

Dehydration is removal of water. Ethene is prepared by heating a mixture of ethanol and excess of concentrated sulphuric acid at 180oC. In first step, ethyl hydrogen sulphate is formed which decomposes on heating to produce ethene, which is collected over water.

CH₃-CH₂OH + H₂SO₄ ----> CH₃CH₂-OSO₃H + H₂O

CH₃-CH₂OSO₃H -----> H₂C=CH₂ + H₂SO₄

ii) By Dehydrohalogenation of ethlyl bromide:

On heating ethyl bromide with alcoholic KOH, ethene is formed. Removal of hydrogen and halogen takes place from adjacent carbon atoms to create a double bond.

H₃C-CH₂-Br + KOH ----> H₂C=CH₂ + KBr + H₂O

8. Identify propane from propene with a chemical test.

Ans. Pass the two gases through bromine water separately. Propene will decolorize reddish brown color of bromine but propane cannot. Reaction is:

CH₃-CH₂-CH₃ + Br₂ ----> No Reaction

CH₃-CH=CH₂ + Br₂ ----> CH₃-CHBr-CH₂Br 

9. Why are alkenes called olefins?

Ans. Alkenes are called olefins because first members of alkenes form oily products when react with halogens.

10. Why alkanes can’t be oxidized with KMnO₄ solution?

Ans. Alkanes are saturated hydrocarbons. They are least reactive that is why alkanes can’t be oxidized with KMnO₄ solution.

11. What are addition reactions? Explain with an example.

Ans. These are the reaction in which the products are formed by the addition of a substance like H₂, Cl₂, etc to an unsaturated hydrocarbon.

Example: Addition of hydrogen to an unsaturated hydrocarbon in the presence of catalyst (Ni, Pt).

H₂C=CH₂ + H₂ ----> H_#C-CH₃

12. Justify that alkanes give substitution reactions.

Ans. A reaction is which one or more hydrogen atoms of a saturated compound are replaced with some other atoms is called a substitution reaction. These reaction are characteristic property of alkanes. For examples in diffused sunlight alkanes react fairly with halogens. In these reactions at each step one hydrogen atom is substituted by halogen atom.

CH₄ + Cl₂ ---> CH₃Cl +HCl

CH₃Cl + Cl₂ ----> CH₂Cl2 +HCl

CH₂Cl2 + Cl₂ ---> CHCl₃ +HCl

CHCl3 + Cl₂ ----> CCl₄ +HCl

13. Both alkenes and alkynes are unsaturated hydrocarbons. State one most significant difference between them.

Ans. Alkynes have triple covalent bond between two carbon atoms and Alkenes have double bond. Alkynes have greater carbon to hydrogen ration. So they give smokier flames that that of alkenes.

14. Write the molecular, dot and cross and structural formula of ethyne.

Ans. i) Molecular formula of ethyne C₂H₂

ii) Structural formula H-C≡C-H

iii) Cross and dot formula 

15. Why are hydrocarbons soluble in organic solvents?

Ans. Hydrocarbons are soluble in organic solvents because they are non polar in nature.

16. Give the physical properties of alkanes.

Ans. i) Alkanes are non-polar therefore they are insoluble in water but soluble in organic solvents.

ii) The density of alkanes increases gradually with the increase of molecular size.

iii) The alkanes become more viscous as their molecular size increases.

iv) Alkanes become less flammable i.e. difficult to burn with the increase of molecular sizes.

17. How can you identify ethane from ethene?

Ans. When bromine water is added to ethene in an inert solvent like carbon tetrachloride, its color is discharged at once but ethane does not change the reddish brown color of bromine.

H₂C=CH₂ + Br₂ ----> Br-CH₂-CH₂-Br

18. Why does color of bromine water discharge on addition of ethene in it?

Ans. In the reaction double bond of ethene is converted into single bond by addition of a molecule of bromine so color of bromine water discharges.

H₂C=CH₂ + Br₂ ----> Br-CH₂-CH₂-Br

19. State one important use of each:

i) Ethene ii) Acetylene iii) Chloroform iv) Carbon tetrachloride

i) Ethene

It is used for manufacturing of polythene.

ii) Acetylene

It is used to prepare alcohols, acetaldehyde and acids.

iii) Chloroform

It is used as a solvent for rubber, waxes and used for anaesthesia.

iv) Carbon tetrachloride

It is used as an industrial solvent and in dry cleaning.