Carbon and its Compounds

Intext Questions 1

Question 1

What would be the electron dot structure of carbon dioxide which has the formula CO₂?

Answer

Electron dot structure of carbon dioxide (CO₂) is shown below:

Question 2

What would be the electron dot structure of a molecule of Sulphur which is made up of eight atoms of sulphur? (Hint – The eight atoms of Sulphur are joined together in the form of a ring).

Answer

Electron dot structure of a molecule of Sulphur with eight atoms is shown below:

Intext Questions 2

Question 1

How many structural isomers can you draw for pentane?

Answer

Three structural isomers of pentane can be drawn.

Pentane [n-pentane]

2-Methyl butane [iso-pentane]

2,2 Dimethyl Propane [neo-pentane]

Question 2

What are the two properties of carbon which lead to the huge number of carbon we see around us?

Answer

Two properties of carbon which lead to the huge number of carbon we see around us are :

(a) Catenation — Carbon atoms possess a unique property to link together (self linking) to form very long chains. This property is known as catenation. Catenation is shown by other elements also but Carbon exhibits this property to the maximum extent. This is because carbon-carbon bond is very strong and hence stable.

(b) Tetravalency of carbon atoms — Compounds of carbon are formed with oxygen, hydrogen, nitrogen, sulphur, chlorine and many other elements giving rise to compounds with specific properties. The bonds that carbon forms with most other elements are very strong due to its small size hence making these compounds exceptionally stable.

Question 3

What will be the formula and electron dot structure of cyclopentane?

Answer

Formula of cyclopentane — C₅H₁₀

Electron dot structure of cyclopentane

Question 4

Draw the structures for the following.

(i) Ethanoic acid

(ii) Bromopentane*

(iii) Butanone

(iv) Hexanal

*Are structural isomers possible for Bromopentane?

Answer

(i) Ethanoic acid

(ii) Bromopentane

(iii) Butanone

(iv) Hexanal

* Yes, structural isomers of Bromopentane are possible. A few of them are:

Bromopentane

2-bromopentane

3-bromopentane

Question 5

How would you name the following compounds?

Answer

(i) Bromoethane

(ii) Methanal

(iii) 1–hexyne

Intext Questions 3

Question 1

How is the conversion of ethanol to ethanoic acid an oxidation reaction?

Answer

The conversion of ethanol to ethanoic acid involves the removal of the hydrogen atom and the addition of oxygen. Hence, it is an oxidation reaction.

$\underset{\text{Ethanol}}{\text{C}_2\text{H}_5\text{OH}} +[\text{O}]\xrightarrow[\text{acidified}]{\text{K}_2\text{Cr}_2\text{O}_7} \underset{\text{[Acetaldehyde]} }{\text{CH}_3\text{CHO}} +[\text{O}]\xrightarrow[\text{acidified}]{\text{K}_2\text{Cr}_2\text{O}_7} \underset{\text{[Ethanoic acid]} }{\text{CH}_3\text{COOH}}$

In the first step, an H₂ molecule is removed from ethanol to form ethanal. As the loss of hydrogen is oxidation, so the reaction is an oxidation reaction.

In the second step, an oxygen atom is added to form ethanoic acid from ethanal. As the gain of oxygen is called oxidation, the reaction is an oxidation reaction.

Question 2

A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?

Answer

Welding requires higher temperatures and a combination of oxygen and ethyne is preferred for it. This is because the complete combustion of oxygen and ethyne generates higher temperatures compared to a mixture of air and ethyne, which yields a sooty flame due to unburnt particles and consequently provides less heat.

Intext Questions 4

Question 1

How would you distinguish experimentally between an alcohol and a carboxylic acid?

Answer

Add Sodium Carbonate (Na₂CO₃) / Sodium Hydrogen Carbonate (NaHCO₃) solution to alcohol (ethanol) and carboxylic acid (ethanoic acid). With carboxylic acid it will produce brisk effervescence of carbon dioxide (CO₂) gas whereas no effervescence is seen with alcohol.

The chemical equation for reaction between Sodium Carbonate (Na₂CO₃) / Sodium Hydrogen Carbonate (NaHCO₃) and Ethanoic Acid is given below:

2CH₃COOH + Na₂CO₃ ⟶ 2CH₃COONa + H₂O + CO₂↑

CH₃COOH + NaHCO₃ ⟶ CH₃COONa + H₂O + CO₂↑

Question 2

What are oxidising agents?

Answer

Oxidising agents are substances which add oxygen atoms to others compounds or remove hydrogen atoms from others compounds. Oxidising agents accept electrons and themselves undergo reduction.
Examples: alkaline potassium permanganate, acidified potassium dichromate, etc.

Intext Questions 5

Question 1

Would you be able to check if water is hard by using a detergent?

Answer

Detergent are unaffected by hardness of water as their calcium and magnesium salts are soluble in water so they do not form scum and cleaning action is easily done. Therefore, it is not possible to check if the water is hard by using a detergent.

Question 2

People use a variety of methods to wash clothes. Usually, after adding the soap, they 'beat' the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?

Answer

Soap molecules have two parts namely hydrophilic which dissolves in water and hydrophobic, that dissolves in hydrocarbons. With the help of these, it attaches to the grease or dirt particle and forms a cluster called micelle. These micelles remain suspended as a colloid, on the clothes and on being beaten or agitated, the particles are removed from the clothes surface and go into the water, thus cleaning the clothes.

Exercises

Question 1

Ethane, with the molecular formula C₂H₆, has

1. 6 covalent bonds
2. 7 covalent bonds
3. 8 covalent bonds
4. 9 covalent bonds

Answer

7 covalent bonds

Reason — Ethane, with the molecular formula C₂H₆, has 7 covalent bonds.

Question 2

Butanone is a four-carbon compound with the functional group

1. carboxylic acid
2. aldehyde
3. ketone
4. alcohol

Answer

ketone

Reason — Butanone [C₄H₈O] has the functional group ketone.

Question 3

While cooking, if the bottom of the vessel is getting blackened on the outside, it means that

1. the food is not cooked completely
2. the fuel is not burning completely
3. the fuel is wet
4. the fuel is burning completely

Answer

the fuel is not burning completely

Reason — While cooking, if the bottom of the vessel is getting blackened on the outside it means that the fuel is not burning completely because certain substances burn partially due to limited supply of oxygen, producing carbon monoxide and soot. The black mark on the vessel is due to the deposition of soot.

Question 4

Explain the nature of the covalent bond using the bond formation in CH₃Cl.

Answer

Carbon [2, 4] in CH₃Cl completes its octet configuration by sharing its 3 electrons with 3 hydrogen [1] atoms and one electron with chlorine [2, 8, 7]. Each of the participating elements attain a stable electronic configuration. Hence, the bonding in CH₃Cl is a covalent bonding.

Covalently bonded CH₃Cl has strong bonds within the molecule but intermolecular forces are weak. Due to this, CH₃Cl has low melting and boiling points. As the electrons are shared between atoms, CH₃Cl lacks ions, hence, it is a poor conductor of electricity.

Question 5

Draw the electron dot structures for

(a) ethanoic acid

(b) H₂S

(c) propanone

(d) F₂

Answer

(a) Ethanoic acid [CH₃-COOH]

(b) H₂S

(c) Propanone [CH₃-CO-CH₃]

(d) F₂

Question 6

What is a homologous series? Explain with an example.

Answer

A homologous series is a group of organic compounds having a similar structure and similar chemical properties in which the successive compounds differ by a CH₂ group.

For example,

• Ethene [C₃H₆], Butene [C₄H₈], Propene [C₅H₁₀] etc., are part of the alkene homologous series.

• They differ by a -CH₂- unit.

• The general formula of this series is C_nH_2n where n = 2, 3, 4 etc.

Question 7

How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?

Answer

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

Soaps are molecules in which the two ends have differing properties, one is hydrophilic (water attracting), that is, it interacts with water, while the other end is hydrophobic (water repelling), that is, it interacts with hydrocarbons. Inside water, the soap molecules arrange themselves in a cluster in such a way that the hydrophobic tails are in the interior of the cluster and the hydrophilic (ionic) ends are on the surface of the cluster. This formation is called a micelle.

No, micelle formation does not take place in ethanol because the hydrocarbon chains (hydrophobic) of soap becomes soluble in ethanol.

Question 9

Why are carbon and its compounds used as fuels for most applications?

Answer

Carbon and its compounds are used as fuels because of the following reasons:

1. They have high calorific value and give out a large amount of heat on combustion due to high percentage of carbon and hydrogen.
2. They are easily combustible due to their optimum ignition temperature.
3. Their combustion can be controlled.

Question 10

Explain the formation of scum when hard water is treated with soap.

Answer

Hardness of water is due to the presence of calcium and magnesium salts. When soap molecules react with these salts, they form a precipitate which appears as a floating off-white layer over water and is called as scum.

Question 11

What change will you observe if you test soap with litmus paper (red and blue)?

Answer

Soap is alkaline/basic in nature. Hence, it will turn red litmus blue but no colour change will be observed with blue litmus paper.

Question 12

What is hydrogenation? What is its industrial application?

Answer

The addition of hydrogen to unsaturated hydrocarbons in the presence of catalyst such as nickel or palladium to obtain saturated hydrocarbons is called hydrogenation.

One of its industrial applications is preparation of vanaspati ghee by catalytic hydrogenation of vegetable oils.

Question 13

Which of the following hydrocarbons undergo addition reactions:

C₂H₆, C₃H₈, C₃H₆, C₂H₂ and CH₄?

Answer

C₃H₆, C₂H₂ will undergo addition reaction as they are unsaturated hydrocarbons.

Question 14

Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.

Answer

When bromine is passed through saturated and unsaturated hydrocarbons, incase of unsaturated hydrocarbon, brown colour of bromine is discharged whereas in case of saturated hydrocarbon brown colour remains brown.

Question 15

Explain the mechanism of the cleaning action of soaps.

Answer

Most dirt is oily in nature and oil does not dissolve in water. The molecules of soap are sodium or potassium salts of long-chain carboxylic acids. The ionic-end of soap interacts with water while the carbon chain interacts with oil. The soap molecules, thus form structures called micelles where one end of the molecule is towards the oil droplet while the ionic end faces outside. This forms an emulsion in water. The soap micelle thus helps in pulling out the dirt in water and we can wash our clothes clean.