Ans. There is six hydrogen atoms is presented in a compound containing the four carbon atoms has a triple bond in it.
Example
H-C-C -C-H
H 1 H
(butyne)
Q.3 Why the alkanes are called paraffin?
Ans. Alkanes are saturated hydrocarbons. In these compounds all the bonds of carbon atoms are single that mean valencies of carbon atoms are fully satisfied (saturated therefore they are least reactive. That is the reason alkanes are called paraffins.
Q.4 What do you know about hydrogenation of alkenes?
Ans. Hydrogenation means addition to an unsaturated hydrocarbon in presence of a catalyst (Ni, Pt) to form saturated compound.
H.C CH,+ H
Ni
2.50-300 C H,C-CH
Q.5 Why the alkanes are 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 is take place in automobile. combustion engines, domestic heaters and cooking appliances. It is highly exothermic reaction and because of its alkanes are used as fuel.
CH+20 CO,+2H O+heat
Q.6 How can you prepare ethene from alcohol.
Ans. Ethene is prepared by heating a mixture of ethanol and excess of cone sulphuric acid at 180°C in first step ethyl hydrogen sulphate is formed which decomposes on heating to produce ethane which is collected over water
Q.7. Indentify propene from propane with a chemical test.
Ans. Pass the two gases through bromine water separately. Propene will decolourise reddish brown colour of bromine but propane cannot. Reaction is CH, -CH=CH, +Br, CH, -CH-CH,
Br Br
redish-brown in colour
Colourless
Q.8 Why alkenes are called olefins?
Ans. alkenes are called olefins. Because first members of alkenes form oily products when react with halogens.
Q.9 Why alkane can't be oxidized with KMnO4 solution?
Ans. Alkanes are saturated hydrocarbons. They are least reactive at high temperatures that are why alkenes can't be oxidized with KMnO solution.0.10 What are addition reactions? Explain with an example Ans. Addition of substance to an unsaturated hydrocarbon is called addition reaction Example:
Addition of hydrogen to an unsaturated hydrocarbon in the presence of catalyst (Ni. Pt).
H.C CH. +H, 250-wwe->H,C-CH,
Q.11 Justify that alkanes give substitution reactions.
Ans. A reaction in which one or hydrogen atoms of a saturated compound are replaced with some other atoms is called a substitution reaction these reactions are characteristic property of alkanes. For example in diffused sunlight alkanes react fairly with halogens.
In these reactions at each step one hydrogen atom is substituted by halogen atom.
Q.12 Both alkenes and alkynes are unsaturated hydrocarbons. State the one most significant difference between them
Ans. Alkynes have greater carbon to hydrogen ratio. So they give smokier flames but alkanes and alkenes do not.
Q.13 Write the molecular, dot and cross and structural formula of ethyne Ans. (i) Molecular formula of ethyne CH
(ii) Structural formula
H-CEC
(iii) Cross and dot formula
Hx CxC.xH
Q.14 Why hydrocarbons are soluble in organic solvents?
Ans. Hydrocarbons are soluble in organic solvents because they are non polar.
Q.15 Give the physical properties of alkanes.
Ans.
() Alkanes are non-polar, therefore they insoluble in water but soluble in organic solvents ii) The density of alkanes increases gradually with the increases of molecular size (ii) The alkanes become more viscous as their molecular size increase
Q.12 Both alkenes and alkynes are unsaturated hydrocarbons. State the one most significant difference between them
Ans. Alkynes have greater carbon to hydrogen ratio. So they give smokier flames but alkanes and alkenes do not.
Q.13 Write the molecular, dot and cross and structural formula of ethyne Ans. (i) Molecular formula of ethyne CH
(ii) Structural formula
H-CEC
(iii) Cross and dot formula
Hx CxC.xH
Q.14 Why hydrocarbons are soluble in organic solvents?
Ans. Hydrocarbons are soluble in organic solvents because they are non polar.
Q.15 Give the physical properties of alkanes.
Ans.
() Alkanes are non-polar, therefore they insoluble in water but soluble in organic solvents ii) The density of alkanes increases gradually with the increases of molecular size (ii) The alkanes become more viscous as their molecular size increase(iv) Alkanes become less flammable i.e. difficult to burn with the increase of molecular sizes
Q.16 How can you identify ethane from ethene?
Ans. When bromine water is added to ethane in an inert solvent like carbon tetrachloride, its colour is discharged at once.
H.C=CH, +Br, Br-CH,-CH,-Br
(red brown colour)
(colourless)
Q.17 Why colour of bromine water discharges on addition of ether in it?
Ans. Because in the reaction double bond of ethane is converted into single bond by addition of a molecule of bromine
H,C CH +Br Br-CH-CH-Br
(red brown colour
(colourless)
Q.18 State one important use of each:
(i) Ethene (ii) Acetylene (iii) Chloroform (iv) Carbon tetrachlorideAns.
(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 an industrial solvent and in dry cleaning
Q.19 Give the structural formula of isobutane and isopentane.
Ans.(a) Isobutene
Q.20 Why hydrocarbons are considered as parent organic compounds?
Ans. Because mostly organic compounds are derived from hydrocarbons by the replacement of one or more hydrogen atoms by other atoms or group of atoms that why hydrocarbons are considered as a parent organic compounds.
Q.22 Define unsaturated hydrocarbons? Give example Ans. The hydrocarbons in which two carbon atoms are linked by a double or a triple bond are called unsaturated hydrocarbons
Example:
HC CH
HC CH
Ethene
Ethyne
Q.23 What do you mean by halogenations? Give the reaction of methane with chlorine in bright sunlight.
Ans. Halogenation means addition of halogen like chlorine or bromine to unsaturated hydrocarbons. In bright sunlight, the reaction of with chlorine is explosive and carbon is deposited
CH+Cl
C+ 411C1
Q24. Why alkenes are reactive?
Ans. Alkenes are very reactive compounds because the electrons of the double bond are easily available for reaction. The compounds have tendency to react readily by adding other atoms to become saturated compounds.
Q.31 What are aliphatic hydrocarbons?
Ans. These are the compounds in which the first and the last carbon are not directly joined to each other. These are may be straight or branched.
Examples:
H&C-CH2-CH2-CH
(n butane)
H,C CH -CH,
CH, Isobutane)
Q.32 Give a few uses of methane?
Ans. i. Natural gas is chiefly methane is used as domestic fuel.
ii. Compressed natural gas (CNG) is used as automobile fuel.
Q.33 What are Cyclic hydrocarbons?
And Compounds having ring of carbon atoms in their molecule are called closed chain or cyclic hydrocarbons.
Q.34 What are alkenes? Give its general formula?
Ans. Alkenes are unsaturated hydrocarbons having double bond between two carbon atoms the
General formula:
C.H2
Example: H2C CH2 (Ethene)
Q.35 What are alkynes? Give its general formula?
Ans. The hydrocarbon in which two carbon atoms are linked by a triple bond are called alkynes.
Q.42 Why methane is known as marsh gas?
Ans. Methane as a Marsh Gas: Poke around with a stick in the muddy bottom of a pond or marsh. You may see bubbles coming out of the mud. These bubbles are the hydrocarbon gas methane, which is sometimes called 'marsh gas. If you collect the gas in a jam jar you can set fire to it.
Q.43 Which chemicals were produced by orchids to attract the insects for their pollination?
Ans. Orchids: Orchids are beautiful ornamental and colourful flowers. Some orchids produce alkanes to attract bees to pollinate their flowers.
Q.44 Why butane is used in portable torches and gas lighters?
Ans. Propane and butane burn with very hot flames and are sold as liquefied petroleum gas (LPG). They are kept as liquids under pressure, but they vapourize easily when thepressure is released. Cylinders of butane are used in the homes. Butane is also used in portable torches and gas lighters.
Q.45 Give few physical properties of alkenes.
Ans.
(1) The first member of alkenes is ethane. It is a colourless gas with pleasant odour (ii) Alkenes are non-polar therefore, they are insoluble in water but soluble in organic solvent.
(iii) The first member of the series ethane is slightly less dense than air.
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