Chapter 4 Chemistry Class 10 Important Questions

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• Important Multiple Choice Questions of Chapter Number 12: Hydrocarbons of Chemistry class 10th Urdu Medium.
• Important Short Questions of Chapter Number 12: Hydrocarbons of Chemistry class 10th Urdu Medium.
• ALKENES: The simplest alkene is ethene having formula C2H4. These compounds are also known as olefins (a Latin word meaning oil forming) because first members form oily products when react with halogens.
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• Occurrence: (i) Alkenes being more reactive than alkanes, seldom occur free in nature. (ii) Lower alkenes occur in coal gas in minute quantities. (iii) Ethylene is present in natural gas sometimes to the extent of 20%. (iv) Alkenes are produced in large amounts by cracking of petroleum.
• Preparation of Alkenes: Alkenes are prepared by the removal of small atoms (H,OH,X) from the adjacent carbon atoms of the saturated compounds, so as to create a double bond between carbon atoms. 
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• Dehydration of Alcohols: Dehydration is removal of water. Ethene is prepared by heating a mixture of ethanol and excess of concentrated sulphuric acid at 180°C. In first step, ethyl hydrogen sulphate is formed which decomposes on heating to produce ethene, which is collected over water.
• Dehydrohalogenation of Alkyl Halides: 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.
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• Physical Properties of Alkenes: (i) The first member of the alkenes is ethene. It is a colourless gas with pleasant odour. (ii) Alkenes are nonpolar, therefore, they are insoluble in water but soluble in organic solvents. (iii) The first member of the series ethene is slightly less dense than air. (iv) Alkenes are flammable hydrocarbons. On complete combustion, they form carbon dioxide and water with release of energy. However, their flame is smokier than alkanes having a similar number of carbon atoms. (v) Their melting and boiling points gradually increase with the increase of molecular sizes of the compounds in the series. 
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• Uses of Ethene (Ethylene): Ethene is used: (i) for artificial ripening of fruits; (ii) as a general anaesthetic; (iii) for manufacture of polythene: Polythene is a plastic material used in packaging, toys, bags, etc; (iv) as a starting material for the manufacture of a large number of compounds such as ethylene oxide, ethyl alcohol, ethylene glycol,diethyl ether, etc.; ethylene oxide is used as a fumigant, ethylene glycol is used as an anti-freeze, diethyl ether and ethyl alcohol are used as solvents and (v) for making poisonous mustard gas which is used in chemical warfare.
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• ALKYNES: The simplest alkyne is acetylene, with molecular formula C2H2. Alkynes are also called acetylenes because of the name of the first member of the series is acetylene. Molecular, condensed, structural and dot and cross formulae of a few alkynes are:
• Preparation of Alkynes: Alkynes are prepared by the following methods. 
• Dehydrohalogenation of Vicinal Dihalides: When a vicinal dihalide is heated with alcoholic KOH, two hydrogen atoms along with two halogen atoms are removed from two adjacent carbon atoms with the formation of a triple bond between the adjacent carbons
• Physical Properties of Alkynes:  1. Alkynes also form a series of compounds. Its first member is acetylene. It is a colourless gas with faint garlic odour. 2. Acetylene is slightly soluble in water but soluble in organic solvents such a benzene, alcohol, acetone, ether, etc. 3. Acetylene is slightly lighter than air. 4. Alkynes are also flammable. They produce smokier flames than those of alkanes and alkenes.
• Uses of Acetylene:  (i) Acetylene produces oxy-acetylene flame with oxygen. It is a highly exothermic reaction. Heat released is used for welding purposes. (ii) Acetylene is used to prepare other chemicals, such as alcohols, acetaldehyde and acids. (iii) It is used for the ripening of fruits. (iv) It is used for the manufacturing of polymer products like polyvinyl chloride, polyvinyl acetate and synthetic rubber like neoprene.