Frank Solutions for Chapter 11C Unsaturated Hydrocarbons Class 10 Chemistry ICSE


1. The molecules of alkene family are represented by a general formula CnH2n .
Answer the following:
(i) What do n and 2n signify?
(ii) What is the name of alkene when n = 4 ?
(iii) What is the molecular formula of the alkene, if there are six hydrogen atoms in it?
(iv) Write the molecular and structural formula of the first member of alkene family.
(v) Write the molecular formulae of lower and higher homologues of an alkene which contains four carbon atoms. 
Answer 
(i) The n signifies number of carbon atoms where as 2n signifies Number of hydrogen atoms. 
(ii) Butene 
(iii) C3H6 
(iv) Molecular formula of first member of alkene is : C2H4 

(v) Lower Homologues of alkene which contains four carbon atoms is : C3H6 
Higher Homologues of alkene which contains four carbon atoms is C5H10 

2. How is ethylene prepared in laboratory ? Give balanced equation and procedure. How is the gas prepared and dried ? 
Answer 
Ethylene is prepared in the laboratory by dehydration of ethyl alcohol. 
Reaction : 

Procedure:
  1. Take one part of ethyl alcohol in a flask. Add  two parts of concentrated sulphuric acid and heat to 170°C. Ethylene is evolved and small amount of aluminium sulphate is also added to the flask to avoid frothing. 
  2. Gas is prepared by reacting ethyl alcohol in presence of sulphuric acid and is collected by downward displacement of water. 
  3. The gas is dried by passing the gas through sodium hydroxide solution . 

3. How would you convert : 
(a) Ethene into 1, 2 - dibromoethane. 
(b) Ethene to bromoethane 
Answer
(a) Ethene into 1,2 - dibromoethene : 
When bromine gas is passed into inert solvent like CCl4 containing dissolved ethene at room temperature, one molecule of Br2 adds across the double bond to give ethylene dibromide. 
(b) Ethene to bromoethane : 
Ethene reacts with halogen acids to form alkyl halides. 

4. Write the physical properties of ethene. 
Answer 
Physical properties:
  • State: colourless gas, neutral to litmus, faint sweet odour
  • Solubility: slightly soluble in water, highly soluble in organic solvents.
  • Density: slightly lighter than air
  • Melting point: -169°C
  • Boiling point: -104°C 

5. Write the balanced equations to convert : 
(a) Ethanol to ethene 
(b) Ethene to ethanol 
Answer 

6. How will you differentiate that whether the given gas is ethane or ethene ? 
Answer 
When bromine solution is added to ethane and ethene, In case of ethene the orange colour of bromine disappears where as in case of ethane colour does not disappear. 

7. What is hydrogenation ? How dydrogenation of ethene occurs ? Give one use of hydrogenation. 
Answer 
The addition of hydrogen across the double bond is called hydrogenation. 
Ethene reacts with hydrogen gas when heated in presence of catalyst like nickel, to give ethane. 

Use of hydrogenation : 
Hydrogenation of Oils : The oils can be converted into ghee which are semi solid at room temperature. 

8. Give two equations to show the conversion of a unsaturated compound to saturated compound. 
Answer
Two equations : 

9. What are addition reactions ? Why ethene undergo addition reactions? Give two examples. 
Answer 
Addition Reaction : The reactions in which molecules of the attacking reagent add across the double or triple bond of an unsaturated compound to yield saturated compound. 
Ethene is a reactive compound due to presence of double bond. Hence undergo addition reaction . 
Two examples : 

10. Give three uses of ethene. 
Answer 
Three uses of ethene are : 
  1. It is used for the manufacture of polythene 
  2. For the artificial ripening of fruits. 
  3. As a general anaesthetic. 

11. Write the observation and name of the product for the following :
(i) When ethene is passed in bromine solution in CCl4 .
(ii) When Baeyer's reagent reacts with ethene. 
Answer 
(i) When ethene is passed in bromine solution is CCl4 , the orange colour of bromine disappears due to formation of colourless ethylene bromide. 

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