12th Class Chemistry Chapter 8 Short Questions with Answers Solved

Question: 1

What is heat of combustion?

Answer: 1

1-25

Burning of alkane in presence of oxygen is known as combustion,Complete combustion of an alkene yield CO₂,H₂O and heat.The amount to heat evolved when one mole of a hydrocarbon is brunt to CO₂and H₂O is called heat of combustion.

Question: 2

What is effect of branching on boiling of alkanes?

Answer: 2

2-25

The boiling points of alkanes having branced chain structure are lower than their isomeric normal chain alkanes,e.g n-butane has higher boiling point than isobutene.

Question: 3

How ethene can be prepared by Kolb's electrolysis?

Answer: 3

3-25

When a concentrated solution of sodium or potassium of a monocarboxylic acid is electrolyzed an alkane is produced this method is only suitable foe the preparation of symmetrical alkanes.

Question: 4

Why are some hydrocarbons called saturated and other unsaturated?

Answer: 4

4-25

Those compounds in which the four valencies of carbons are satisfied by single bonds to either other carbon atom or hydrogen atom are called saturated hydrocarbons,Those compounds in which the four valencies of carbon atom are not satisfied by single bonds,however satisfied by double or triple bond are called unsaturated hydrocarbons.

Question: 5

Explain Markovnikov rule with one example,

Answer: 5

5-25

In the addition of an unsymmetrical reagent to an unsymmetrical alkene the negative part of the adding reagent goes to that carbon,constituting the double bond which has least number of hydrogen atom is called Markovnikov rule.

Question: 6

How common name of alkenes are derived?Give common names of CH₂=CH₂and CH₃-CH₂=CH_2?

Answer: 6

6-25

For alkanes the word is derived from the Greek or Latin numerals indicating the number of carbons atom in molecule and the name is completed by adding ane as a suffix.Alkenes are similarly named by replacing the ending "ane" of the name of alkane with "ylene".

Question: 7

Why are alkenes also called olefins?

Answer: 7

7-25

Alkenes also known as olefins(derived from Latin word olefiant meaning oil forming ) because its lower members from oily products on treatment with chlorine or bromine.

Question: 8

Define Ramy nickel.Give its use.

Answer: 8

8-25

Rany nickel is a catalyst which is prepared by treating a Ni-Al alloy with caustic soda.Most alkanes are hydrogenated over Raney nickel at about 100 degree and upto 3-atmosphere pressure.

Question: 9

Give three use of methane?

Answer: 9

9-25

Methane is used as fuel as an illuminating gas.
used for the preparation of methyl chloride ,methylene chloride,chloroform.
for the preparation of carbon black used in paints ,prints printing inks and automobiles tyres.

Question: 10

Why pie bond is more reactive than alpha bond?

Answer: 10

10-25

In the formation of pie bond,the partially filled p-orbitals overlap in a parallel fashion,the probability of finding electron is thus away from the line joining the two nuclei,due to this reason pei electron are less fimely helf between the nuclei.

Question: 11

The sigma bonds are inert in alkanes.Explain?

Answer: 11

11-25

In the alpha bond the electrons are very tightly held between the nuclei which make it very stable bond,A lot of energy is required to break it.The alkanes or paraffins under ordinary condition are inert towards acid,alkalis,oxidizing and reducing agents.

Question: 12

Describe nitration of methane.

Answer: 12

12-25

It is substitution reaction of alkanes in which a hydrogen atom of an alkane is replaced by nitro group .Alkanes undergo vapour -phase nitration under dractic condition to give nitroalkanes.

Question: 13

Mention four uses of ethene.

Answer: 13

13-25

i) For the manufacture of polythene,a plastic material used of making toys,cables,bags,boxes etc.

Question: 14

What is Hydrogenolysis?Give an example.

Answer: 14

14-25

Hydrogenolysis is a chemical reaction whereby a carbon -carbon or carbon-heteroatom single bond is cleaved or undergoes lysis by hydrogen.The heteroatom may vary,but it usually is oxygen,nitrogen or sulfur.A related reaction is hydrogenation,where hydrogen is added to the molecule,without cleaving bonds.
R-X + H-H-----> R-H + HX

Question: 15

Define Hydrogenation,

Answer: 15

15-25

Hydrogenation is a process in which molecule of hydrogen added to an alkene in the presence of a catalyst and at moderate pressure (1-5 atm ) to give a saturated compound.

Question: 16

Why Alkynes are less reactive than alkenes towards Electrophilic Regents?

Answer: 16

16-25

A sigma-bond in alkenes is not only weak but its electrons are more expose to an attack by an electrophilic reagent.Both these acts make the alkenes a very reactive class of compounds. Alkynes although contain two sigma-bond are less reactive than alkenes towards electrophilic reagents this is because the bond distance between the two triple bonded carbon atoms is very short and hence the sigma-electrons are not available to be attacked by electrophilic reagents.

Question: 17

Explain acidic nature of ethyne?

Answer: 17

17-25

Ethnelnethyne,the hydrogen atom is bonded to the carbon atom with sp-s overlap.The sp hybridized carbon atom of ethyne pulls the electrons more strongly making the attached hydrogen atom slightly acidic as:H-C=C^-8-H⁺⁸

Question: 18

Identify the actual product ,when HBr is added to propene.

Answer: 18

18-25

Propene is an unsymmetrical alkene,According to Markovnikov rule.the negative part of the adding reagent goes to that carbon,constituting the double bond,which has least number of hydrogen atom.

Question: 19

How can ethyne be prepared commercially from calcium carbide?

Answer: 19

19-25

On industrial scale ethyne is prepared by the reaction of calcium carbide (CaC₂) with water.Calcium carbide is prepared by heating lime (CaO) and coke (C) at a very high temperature in an electric furnace.
CAO + C------>CaC₂+ CO
CaC₂+2H₂O------->Ca(OH)₂+ HC=CH

Question: 20

Alkanes are less reactive than alkenes. Comment?

Answer: 20

20-25

The alkanes or paraffins(Latin: parum=little,affins=affinity) under ordinary condition are inert towards acids,alkalis,oxidizing and reducing agents.The un-reactivity of alkanes can also be explained on the basis of inertness of a o-bond.In a o-bond the electrons are very tightly held between the nuclei which makes it is very stable bond.A lot of energy is required to break it.

Question: 21

Distinguish between ethane & ethyne by a chemical test?

Answer: 21

21-25

When ethyne passed through ammonical solution of AgNO₃,it forms white precipitate of di-silver acetylide,while ethene does not give this test.
CH=CH + 2AgNO₃+2NH₄OH---->AgC=CAg + 2NH₄NO₃+ 2H₂O

Question: 22

Distinguish between Ethene and Acetylene?

Answer: 22

22-25

Ethene and acetylene both are un-saturated compounds. Ethene contains sigma-bond,having partially fiiled p-orbitals overlap in a parallel fashion.sigma-electron are less firmly held between the nuclie. In ethene a sigma-bond is a weak bond.

Question: 23

Describe a test for the presence of unsaturation in organic molecules.

Answer: 23

23-25

When alkenes are treated with mild oxidizing reagents like dilute alkaline KMnO₄solution (Bayer's Reagent) at low temperature,hydroxylation of double bond occur resulting in the formation of dihydroxy compounds known as vicinal glycols. The pink colouration of KMnO₄solution is discharged.
3CH₂=CH₂+ 2KMnO₄+ 4H₂O---->HO-CH₂-CH₂-OH+2MnO₂+2KOH

Question: 24

Why ethene is more reactive than ethyne towards additions reactions?

Answer: 24

24-25

Ethene contains sigma-bond having partially filled p-orbitals overlap in a parallel fashion.sigma-electrons are less firmly held between the nulei. In ethene a sigma-bond is a weak bond.

Question: 25

Write structural formulas for two compound;i) Vinyl acetylene ii) But-3-en-yne

Answer: 25

25-25

i) Vinyl acetylene: CH₂=CH-C=CH

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12th Class Chapter 8 Chemistry Short Questions

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