Chemistry class 10thClass Matric Part 2 Notes

Class 10th Chemistry Unit 12 Question And Answer

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Unit 12

Q1. A compound consisting of four carbon atoms has a triple bond in it. How many hydrogen atoms are present it?

Ans: There is six hydrogen atoms is presented in a compound containing the four carbon atoms has a triple bond in it.

Q2. 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 paraffin’s.

Q3. 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.

Q4. 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 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.

Q5. 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.

Q7. Identify propene from propane with a chemical test.

Ans: Pass the two gases through bromine water separately. Propene will decolorize reddish brown colour of bromine but propane cannot. Reaction is:

Q8. Why alkenes are called olefins?

Ans: Alkenes are called olefins, because first members of halogens. falkenes form oily products when react with

Q9. Why alkane cannot be oxidized with KMnO, Solution?

Ans: Alkanes are saturated hydrocarbons. They are least reactive at high temperatures that are why alkenes cannot be oxidized with KMnO, solution

Q10. 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)

Q11. 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.

Q12. 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 14. Why hydrocarbons are soluble in organic solvents?

Ans: Hydrocarbons are soluble in organic solvents because they are non-polar.

Q15. Give the physical properties of alkanes.

Ans: Physical properties of alkanes are: (i) 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. (iii) 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 discharged at once. to ethane in an inert solvent like carbon tetrachloride, its colour is

Q17. 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:

Q18. State one important use of each: (i) Ethene (ii) Acetylene (iii) Chloroform (iv) Carbon tetrachloride

Ans. Ethene: It is used for manufacturing of polythene Acetylene: It is used to prepare alcohols acetaldehyde and acids Chloroform: It is used as a solvent for rubber, waxes and used for anesthesia. Carbon tetra-chloride: It is used an industrial solvent and in dry cleaning

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 compound.

Q 21. What is the difference between a straight and a branched chain hydrocarbon

Ans: Straight Chain Hydrocarbons: Straight chain hydrocarbons straight chata compound are these in which carbon atoms link with each other through a single double or triple bond forming a straight chain. Example:

Q 27. Give a few uses of methane?

Ans: Few major uses of methane are: 1.Natural gass is chiefly methane is used as domestic fuel. 2.Compressed natural gas (CNG) is used as automobile fuel.

Q 28. What are Cyclic hydrocarbons?

Ans: Compounds having ring of carbon atoms in their molecule are called closed chain or cyclic hydrocarbons.

Q 29. What are alkenes? Give its general formula?

Ans: Alkenes are unsaturated hydrocarbons having double bond between two carbon atoms the General formula is CH

Q30. What are alkynes? Give its general formula?

Ans: The hydrocarbon in which two carbon atoms are linked by a triple bond are called alkynes.

Q31. Write the sources of alkanes?

Ans: The Main sources of alkanes are The main sources of Falkanes are petroleum and natural gas. Methane forms about 85% of Natural gas. Fuel gases obtained from coal gas contain alkanes in small amount.

Q 32. Why the burning of alkanes require sufficient supply of oxygen?

Ans: Because in the limited supply of oxygen there is incomplete combustion. As a result, carbon monoxide is produced that creates suffocation and causes death. As shown in chemical reaction equation.

Q33. How can you prepare propene from propyl alcohol?

Ans: By Dehydration of Alcohols: Alcohols when dehydrated in the presence of a catalyst give alkene. The best procedure is to pass vapours of alcohol over heated alumina.

Q34. Give a test to identify unsaturation of an organic compound.

Ans: Oxidation with KMnO: When unsaturated compounds oxidized with KMnO, the pink colour discharged.

Q 35. How is tetra-bromoethane prepared from acetylene?

Ans: Preparation of Tetra-bromoethane from Acetylene: Tetra-bromoethane can be prepared by the addition of halogens to the acetylenes what bromine water added to acetylene, red brown colour of bromine water is discharged rapidly due to formation of colorless tetra-bromoethane. Chemical Equation:

Q37. 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 any jar you can set fire to it.

Q38. Which chemicals were produced by orchids to attract the insects for their pollination?

Ans: Orchids are beautiful ornamental and colorful flowers. Some orchids produce alkanes to attract bees to pollinate their flowers.

Q 39. 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 vaporize easily when the pressure is released. Cylinders of butane are used in the homes. Butane is also used in portable torches and gas lighters.

Q 40. Give few physical properties of alkenes. Ans:

The physical properties of alkenes are as under: The first member of alkenes is ethane. It is a colorless gas with pleasant odour. Alkenes are non-polar therefore, they are insoluble in water but soluble in organic solvent. The first member of the series ethane is slightly less dense than air.

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