Chapter 4 Carbon and Its Compounds

 

Question 1
Ethane, with the molecular formula C₂H₆ has
(a) 6 covalent bonds
(b) 7 covalent bonds
(c) 8 covalent bonds
(d) 9 covalent bonds
Answer:
(b) 7 covalent bonds.

Question 2
Butanone is a four-carbon compound with the functional group
(a) carboxylic acid
(b) aldehyde
(c) ketone
(d) alcohol
Answer:
(c) Ketone.

Question 3
While cooking, if the bottom of the vessel is getting blackened on the outside, it means that
(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuel is wet.
(d) the fuel is burning completely.
Answer:
(b) The fuel is not burning completely.

Question 4
Explain the nature of the covalent bond using the bond formation in CH₃Cl.
Answer:
Covalent bond is formed by sharing of electrons so that the combining atoms complete their outermost shell.

In CH₃Cl : C = 6, H = 1 and Cl = 17 And their electronic configuration is C – 2,4, H – 1 and Cl – 2, 8, 7

Three hydrogen atoms complete their shells by sharing three electrons (one electron each) of carbon atom.
Chlorine completes its outer shell by sharing its one out of seven electrons with one electron of carbon atom.
Thus carbon atom shares all its four electrons with three hydrogen atoms and one of chlorine atom and completes its outermost shell and single covalent bonds are formed in CH₃Cl.

Question 5
Draw the electron dot structures for
(a) ethanoic acid
(b) propanone
(c) H₂S
(d) F₂.
Answer:

 

Question 6
What is a homologous series? Explain with an example.
Answer:
Homologous series : A homologous series is a group of organic compounds having
similar structures and similar chemical properties in which the successive compounds differ by -CH₂ group.

Characteristics of homologous series :
(i) All members of a homologous series can be represented by the same general formula. For example, the general formula of the homologous series of alkanes is C_nH_2n+2, in which ‘n’ denotes number of carbon and hydrogen atoms in one molecule of alkane.
(ii) Any two adjacent homologues differ by one carbon atom and two hydrogen atoms in their molecular formulae.
(iii) The difference in the molecular masses of any two adjacent homologues is 14u.
(iv) All the compounds of a homologous series show similar chemical properties.
(v) The members of a homologous series show a gradual change in their physical properties with increase in molecular mass.

For example, general formula of the homologous series of alkanes is C_nH_2n+2, in which ‘n’ denotes number of carbon atoms in one molecule of alkane. Following are the first five members of the homologous series of alkanes (general formula C_nH_2n+2).

Value of n	Molecular formula	Name of compound
1	CH4	Methane
2	C2H6	Ethane
3	C3H8	Propane
4	C4H10	Butane
5	C5H12	Pentane

Question 7
How can ethanol and ethanoic acid he differentiated on the basis of their physical and chemical properties?
Answer:
Difference on the basis of physical properties

Property	Ethanol	Ethanoic acid
(i) State	Liquid	Liquid
(ii) Odour	Sweet smell	Pungent vinegar-like smell
(iii) Melting point	156 K	290 K
(iv) Boiling point	351 K	391 K

Difference on the basis of chemical properties

Test	Ethanol	Ethanoic acid
(i) Litmus test	No change in the colour of litmus solution.	Blue litmus solution turns red.
(ii) Sodium hydrogen carbonate test	C2H5OH + NaHCO3 → No reaction No brisk effervescence.	CH3COOH + NaHCO3 → CH3COONa + H2O + CO2 Brisk effervescence due to evolution of CO2.
(iii) Alkaline potassium permanganate	On heating, pink colour disappears.	Does not happen so.

Question 8
Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?
Answer:
Micelle formation takes place when soap is added to water because the hydrocarbon chains of soap molecules are hydrophobic (water repelling) which are insoluble in water, but the ionic ends of soap molecules are hydrophilic (water attracting) and hence soluble in water.
Such micelle formation will not be possible in other solvents like ethanol in which sodium salt of fatty acids do not dissolve.

Question 9
Why are carbon and its compounds used as fuels for most applications?
Answer:
Carbon and its compounds give a large amount of heat per unit weight and are therefore, used as fuels for most applications.

Question 10
Explain the formation of scum when hard water is treated with soap.
Answer:
Hard water contains salts of calcium and magnesium. Calcium and magnesium on reacting with soap form insoluble precipitate called scum. The scum formation lessens the cleansing property of soaps in hard water.

Question 11
What change will you observe if you test soap with litmus paper (red and blue)?
Answer:
Red litmus will turn blue because soap is alkaline in nature. Blue litmus remains blue in soap solution.

Question 12
What is hydrogenation? What is its industrial application?
Answer:
The addition of hydrogen to an unsaturated hydrocarbon to obtain a saturated hydro-carbon is called hydrogenation. The process of hydrogenation takes place in the presence of nickel (Ni) or palladium (Pd) metals as catalyst.

Application : The process of hydrogenation has an important industrial application. It is used to prepare vegetable ghee (or vanaspati ghee) from vegetable oils.

Question 13
Which of the following hydrocarbons undergo addition reactions:
C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄
Answer:
Addition reactions take place only in unsaturated hydrocarbons. So addition reaction take place only in C₃H₆ and C₂H₂.

Question 14
Give a test that can be used to differentiate chemically between butter and cooking oil.
Answer:
Butter is a saturated carbon compound while cooking oil is an unsaturated carbon compound. An unsaturated compound decolourises bromine water, while a saturated compound cannot decolourise it. So we can distinguish chemically between a cooking oil and butter by the bromine water. Add bromine water to a little of cooking oil and butter taken in separate test-tubes.

• Cooking oil decolourises bromine water showing that it is an unsaturated compound.
• Butter does not decolourise bromine water showing that it is a saturated compound.

Question 15
Explain the mechanism of the cleaning action of soaps.
OR
Explain the cleansing action of soaps
Answer:
When a dirty cloth is put in water containing dissolved soap, then the hydrocarbon end of the soap molecules in micelle attach to the oil or grease particles present on the surface of dirty cloth. In this way the soap micelle entraps the oily or greasy particles by using its hydrocarbon ends. The ionic ends of the soap molecules in the micelles, however, remain attached to water. When the dirty cloth is agitated in soap solution, the oily and greasy particles present on its surface and entrapped by soap micelles get dispersed in water due to which the soap water becomes dirty but the cloth gets cleaned. The cloth is cleaned thoroughly by rinsing in clean water a number of times.