12th Class Chemistry Chapter 8 Aliphatic Hydrocarbons Short Questions Answer

Chemistry short QA

12th Class Chemistry Chapter 8 Aliphatic Hydrocarbons Short Questions Answer

1.What are hydrocarbons?Give one example of each type of hydrocarbon?
Those organic compounds which are made up of carbon and hydrogen only are known as hydrocarbons.When we keep in view their cyclic or open chain structures.there are two types. i)Acylic ethane ethene and ethyne are acyclic hydrocarbons. ii) Cyclic:Cyclopropane cyclobutane cyclohexane and benzene ets are cyclic hydrocarbons.
2.Give the general formulas for alkanes alkenes and alkynes?
General formulas are CnH2n+2 for alkanes CnH2n for alkenes and CnH2n-2 for alkynes.
3.Briefly tell why alkanes are less reactive than alkenes?
Alkanes do not have any functional group.All the carbon atoms are sp3 hybridized.C-C bonds are o in nature.There bonds are chemically inert.There are no  multiple bonds.Due to these resoons alkanes have least tendency to react with acids bases oxidizing agents and reducing agents.Alkennes have carbon carbon  double bonds.The electrons of the π bond are diffused alkenes give the addition reactions while alkanes give subsitution reactions.
4.What is the effect of branching on the meltin points of alkanes?
Branched chain alkanes have low boiling points as compared to linear chain alkanes.
5.How many alkyl groups can be generated by an alkane with molecular formula C4H10?Give the name of alkyl groups?
6.Why some hydrocarbons are saturated and other unsaturated?What type of reactions are characteristics of them?
A saturated gydrocarbon has sufficient hydrogens in it  but in unsaturated  hydrocarbons double and truple bonds are there.They give addition reaction reactions.While saturated gydrocarbons give subsititutions reactions.
7.How do you justify that the bond angles in propane and butane between carbon-carbon bonds are greater than 109.5?
8.What is Sabatier-Sendern’s reaction?
9.What is the important of hydrogenation of unsaturated compounds?
a) We can do the quantitative measurement of unsaturation in the compound and find the number of double bonds in it. b) Oils are converted into vegetable ghee by the saturation process.
10.What are the reduction producs of alkyl halides.Indicate the important reducing agents purpose?
Alkyl halides upon reduction give corresponding alkanes.The important reducing  agents are Zn+HCI,C2H5OH+Na,HI,P+HI and Zn –Cu couple +HCI.
11.What is hydrogenolysis?
12.What is the importance of Wurtz synthesis for the manufacture of alkanes?
13.What do you mean decarboxylation of carboxylc acids?
14.Give the nature of the products when sodium salt of propanoic acid is electrolysed in Kolbe’s electrosynthesis?
15.Write the equation for the conversions of aldehyde and ketones into corresponding alkanes?
Clemmensen’s reduction and Wolf-Kishner reduction convert the carbonyl groups of ketones and aldehdes into- CH2– group.So ketones and aldehdes are converted into corresponding alkanes.They are carried out in the presence of Zn/Hg + HCI and N2H4+ KOH respectively.
16.How the reactions of Grignard’s reagent with active hydrogen compounds give alkanes with the same number of carbon aoms as present in the alkyl group of Grignard’s reagent?
The active hydrogen compounds are HOH,ROH,RCOOH etc.They give their hydrogen to the alkyl group of Grignard’sreagent to give  alkanes and the carbon atoms do not increase.
HO H+R-    Mg – X ……………..R-H +Mg      X
17.Reason out the low reactivity of alkanes among the hydrocarbon family?
Lkanes are saturated compounds having all sigma bonds.Sigma bonds are very stable.So alkanes are less reactive than alkenes and alkynes.These families have weaker π bonds in them.
18.Why the saturated hdrocarbons act as good fuels?
The reaction of alkanes with oxygen are highly exothermic.The amount of heat evolved which is called the heat of combustion of alkanes is used for many purposes.So they act as fuels.
19.Give the nature of products in the chemical equations by catalytic oxidation of methane?
20.The nitration of higher hydrocarbons breaks the carbon carbon bond..How HNO3 reacts with CH4?
When higher alkanes say butane and pentane are reacted with conc.HNO3 at 400-500 ᵒC the nitro-derivatives are obtained by the substitution of hydrogen of alkanes by nitro groups.Due to high temperature carbon carbon bond is  also broken.But in case of CH4,no C-C bond breaks and following reaction happens.
21.The reaction of chlorine with CH4 has a free radical mechanism.Justify it?
The reaction happens in the presence of U.V light diffused sunlight  or at 300-400ᵒC temperature.Molecule of CI2 is broken up into its free radicals which further produce the free redicals.In this way chloroderivatives of methane are produced.
22.Why alkenes are called olefins?
Alkenes gives oily liquids when treated with halognes like CI2 and Br2.The word olefiant means oil forming.
23.What are monoenes dienes and polyenes?Give one example of each?

The alkenes having only one double bond like ethene are called monoenes.The alkenes having two double bonds like CH2=CH =CH2are called dienes.the alkenes having more than two double bonds are called polyenes.

24.Justify that dehydrohalaogenation of alkyl halides give alkenes?
When we remove halogens from α carbon and one of the hydrogens from B-carbon a double bond is created between C-α and C-B.In this way hydrogens and halogens are removed and alkene is produced.
CH2-CH2– CI………………..CH2=CH2+H2O
25.What are the most important dehydrating reagents to convert alcohols into alkenes?
26.Justify that the order of reactivity of alcohols for dehydration purposes is as follows:Ter.Alcohol>Sec.alcohol>Prim.alcohol.
The dehydration of alchols completes through the formation of carbonium ions.Tertiary alcohols give a stable carbonium ions than taht of secondary than primary.So this order determines the reactivity of alcohols.
27.Give the reaction conditions and the reagents which are necessary for dehalogenation of vicinal dihalides?
28.How Kolbe’s electrosynthesis gives us ethane from sodium succinate?
When the sodium salt of succinic acid is electrolysed  in solution decarboxylation takes place and the double  bond is created between  two carbon atoms.Ethene is collected  at the anode while hydrogen is collected at the cathode.
2CH3COONa+H2O………….. C2H6+2CO2+NaOH+H2
29.How do you convert alkynes into cis and trans alkenes?
When alkynes are reacted with hydrogen in the presence of Lindlar catalyst then cis alkenes are produced.When they are treated with sodium in liquid ammonia then trans alkenes are produced at-33ᵒC.
30.Why the π bond is more rective than o-bond?
The electrons of the π bonds are more diffused as compared to the electrons of the o-bonds.The reagents which want to react with alkenes attack the π electrons and break it down.The electrons of o-bond are on the joining line of two nuclei and are held tightly.
31.What is Raney nickel and where is it used?
Raney nickel is prepared by treating an alloy of Ni and AI with caustic soda.Ni is obtained in the powder form and has high surface activity.This is called as Raney pickel.It functions better at 100ᵒC and pressure of 3 atm.
32.What are the applications of hydrogenation of unsaturated cpmpounds to saturated compounds in the labortary and industry?
In the laboratory we use this reaction as one of the synthetic methods to convert unsaturated hydrocarbons into saturated ones.It is a quantitative reaction and helps us to measure be amount of unsaturation in the alkenes.On industrial scale vegetable oils are converted into vegetable ghee.
33.Indicate the mechanism for the addition of a bromine on propene?
34.What is Markownikoff’s rule?Give its applications for the addition of HBr on propene?
When an unsymmetrical molecule is added to unsymmetrical alkene the positive part of adding molecule goes  to that carbon atom which has already greater number of hydrogen atoms.
35.How the alkenes can be converted into alcohols?
Alkenes add one water molecule in the presence of dil.H2SO4 at 100ᵒC to give alcohols.The reaction takes place according to Markownikoff’s rule.
36.How the alkenes are converted into epoxide and what are their appliications?
37.What is Baeyer’s test to check the presence of carbon carbon double bond?
38.Give the mechanism of ozonalysis of propene?
39.Give the equations and necessary conditions for the polymerization of ethene into polythene?
40.How mustard gas is prepared?Give its properties and uses.
41.How vicinal dihalldes are converted into corresponding alkynes?
When vicinal dihalides are reacted with alc.KOH two molecules of halogen acid are eliminated and triple bond is produced.
CH2-CH2+KOH……..alcoholic……… CH=CH+2HBr
Br     Br 
42.How ethyne can be prepared by Kolbe’s electrosynthesis?
When we do the electrolysis of potassium salts of maleic acid acetylene is collected at the anode while KOH at the cathode.
43.What is the role of combustion of ethyne in daily life?
Ethyne burns in oxygen to give oxyacetylene flame which is used for welding purposes and maintains the temperature of 3500ᵒC.
44.How chloroprene is produced by the polymerization of acetylene?
Two molecules of acetylene give viny acetylene in the presence in the presence of Cu2CI2 and NH4CI.Vinyl acetylene adds HCI to give chloroprene which on polymerization gives synthetic rubber.
45.Benzene can be prepared commercially from acetylene.Give reaction conditions?
46.How does sodamide react with alkynes?What are the applications of this reaction?
Sodamide substitutes the acidic hydrogens by sodium metal to give sodium salts of alkynes.They are used to get higher alkynes by reacting with alkyl halides.
47.How ammonicaql solution of AgNO3 can be used to distnguish betweenb 1- butyne and 2-butyne?
Ammonical solution of AgNO3 reacts only with those alkynes which have acidic hydrogen in them.1-butyne will give the white precipitate of silver salt but 2-butyne will not.
CH3-C=CH-CH3+AgNO3+NH4OH……………..No reaction
48.How ammonical solution of Cu2CI2 can be used to distinguish between 1-butyne and 2-butyne?
1-Butyne has acidic hydrogen which gives red ppt.with this reagent but 2-butyne does not give the reaction.
49.Give the uses of acetylene?
Acetylene is used for welding purposes preparing many organic compounds manufacture of synthetic polymer and for the artificial ripening of fruits.
50.How do you distinguish between ethene and ethyne?
Ammonical solution of Cu2CI2 and of AgNO3 give red and white precipitates  respectively  with ethyne but not with ethene.

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