Carbon and its Compounds - Solutions

Chapter Questions
Exercise Questions

Chapter Questions

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

The electron dot structure of carbon dioxide (CO2) is:

:O::C::O:

In CO2, carbon shares two pairs of electrons with each oxygen atom, forming two double bonds. Carbon has 4 valence electrons and each oxygen has 6 valence electrons. Carbon shares two electrons with each oxygen atom, and each oxygen shares two electrons with carbon, resulting in stable octets for all atoms.

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.)

The electron dot structure of S8 (sulphur molecule) is a ring structure where each sulphur atom is bonded to two neighboring sulphur atoms:

Each S atom has 6 valence electrons and forms single bonds with two adjacent S atoms, completing its octet.

In the S8 molecule, each sulphur atom forms two single covalent bonds with adjacent sulphur atoms, creating a crown-shaped ring structure. Each sulphur atom has 8 electrons in its valence shell (6 from itself + 1 from each of the two bonds).

3. How many structural isomers can you draw for pentane?

Pentane (C5H12) has three structural isomers:

  1. n-Pentane: Straight chain of five carbon atoms
  2. Isopentane (2-methylbutane): Four carbon chain with one methyl group on the second carbon
  3. Neopentane (2,2-dimethylpropane): Three carbon chain with two methyl groups on the middle carbon
4. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

The two key properties of carbon that lead to the formation of a huge number of compounds are:

  1. Catenation: Carbon has the unique ability to form bonds with other carbon atoms, giving rise to large molecules with long chains, branched chains, or rings.
  2. Tetravalency: Carbon has a valency of four, which allows it to bond with four other atoms (carbon or other elements), resulting in a wide variety of compounds.
5. What will be the formula and electron dot structure of cyclopentane?

Cyclopentane has the molecular formula C5H10.

Its electron dot structure shows a pentagonal ring of five carbon atoms, each bonded to two other carbon atoms and two hydrogen atoms:

Each carbon atom forms two single bonds with adjacent carbon atoms in the ring and two single bonds with hydrogen atoms.
6. Draw the structures for the following compounds.

(i) Ethanoic acid: CH3COOH

(ii) Bromopentane: C5H11Br (multiple structural isomers possible)

(iii) Butanone: CH3COCH2CH3

(iv) Hexanal: CH3(CH2)4CHO

*Structural isomers are possible for bromopentane because the bromine atom can be attached to different carbon atoms in the pentane chain.

7. How would you name the following compounds?

(i) CH3-CH2-Br: Bromoethane

(ii) H-C=O: Methanal (Formaldehyde)

(iii) The structure appears to be incomplete in the text, but based on the description it might be an alkene like butene.

8. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?

The conversion of ethanol to ethanoic acid is an oxidation reaction because:

  • Oxygen is added to the ethanol molecule
  • Hydrogen is removed from the ethanol molecule
  • The reaction involves an increase in the oxygen content of the compound
  • Oxidizing agents like alkaline KMnO4 or acidified K2Cr2O7 are used

The reaction: CH3CH2OH → CH3COOH involves the addition of oxygen and removal of hydrogen, which characterizes oxidation.

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

A mixture of ethyne and air is not used for welding because:

  • Air contains only about 21% oxygen, which is insufficient for complete combustion of ethyne
  • Incomplete combustion produces a sooty flame with lower temperature
  • Pure oxygen with ethyne produces a much hotter flame (about 3000°C) suitable for welding
  • The oxy-acetylene flame provides the high temperature needed to melt metals for welding
10. How would you distinguish experimentally between an alcohol and a carboxylic acid?

We can distinguish between an alcohol and a carboxylic acid experimentally by:

  1. Sodium bicarbonate test: Carboxylic acids react with NaHCO3 to produce CO2 gas with effervescence, while alcohols do not.
  2. Litmus test: Carboxylic acids turn blue litmus red, while alcohols do not affect litmus paper.
  3. Ester test: Carboxylic acids react with alcohols in presence of acid catalyst to form esters with fruity smell, while alcohols alone do not produce this reaction.
11. What are oxidising agents?

Oxidising agents are substances that:

  • Cause oxidation of other substances
  • Accept electrons during a chemical reaction
  • Themselves get reduced in the process
  • Add oxygen to or remove hydrogen from other substances

Examples include alkaline KMnO4, acidified K2Cr2O7, and oxygen gas.

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

No, we cannot check if water is hard by using a detergent because:

  • Detergents do not form insoluble precipitates (scum) with calcium and magnesium ions present in hard water
  • Detergents work equally well in both soft and hard water
  • To test for water hardness, we need to use soap, which forms a white curdy precipitate (scum) with hard water
13. 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?

Agitation is necessary to get clean clothes because:

  • It helps to loosen the dirt particles from the fabric
  • It facilitates the formation of micelles by soap molecules
  • It helps the soap micelles to surround and emulsify the oily dirt
  • It enables the dirt trapped in micelles to be washed away with water
  • Without agitation, the cleaning action of soap would be less effective

Exercise Questions

1. Ethane, with the molecular formula C2H6 has

(b) 7 covalent bonds

In ethane (C2H6), there is one C-C bond and six C-H bonds, making a total of 7 covalent bonds.

2. Butanone is a four-carbon compound with the functional group

(c) ketone

Butanone (CH3COCH2CH3) contains the carbonyl group (C=O) attached to two carbon atoms, which is characteristic of ketones.

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

(b) the fuel is not burning completely

When the fuel doesn't burn completely, it produces soot (carbon particles) which deposits on the bottom of the vessel, blackening it.

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

In CH3Cl (chloromethane):

  • Carbon has 4 valence electrons and needs 4 more to complete its octet
  • Each hydrogen has 1 valence electron and needs 1 more to complete its duplet
  • Chlorine has 7 valence electrons and needs 1 more to complete its octet
  • Carbon shares one electron each with three hydrogen atoms, forming three C-H single covalent bonds
  • Carbon shares one electron with chlorine, forming a C-Cl single covalent bond
  • All atoms achieve stable electronic configurations through sharing of electrons

This sharing of electron pairs between atoms constitutes covalent bonding.

5. Draw the electron dot structures for

(a) Ethanoic acid (CH3COOH):

H:O:
|
H-C-C:::O:H
H

(b) H2S:

H:S:H

(c) Propanone (CH3COCH3):

H H
| |
H-C-C-C-H
| ||
H O

(d) F2:

:F:F:
6. What is an homologous series? Explain with an example.

A homologous series is a family of organic compounds that:

  • Have the same functional group
  • Show similar chemical properties
  • Have successive members differing by a -CH2 group
  • Show a gradation in physical properties with increasing molecular mass

Example: The alkane series - methane (CH4), ethane (C2H6), propane (C3H8), butane (C4H10), etc.

Each successive member differs from the previous one by a -CH2 unit, and they all have the general formula CnH2n+2.

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

Physical properties:

  • Odor: Ethanol has a characteristic alcoholic smell, while ethanoic acid has a pungent vinegar-like smell.
  • Boiling point: Ethanoic acid has a higher boiling point (391K) than ethanol (351K).

Chemical properties:

  • Litmus test: Ethanoic acid turns blue litmus red, while ethanol has no effect on litmus.
  • Sodium bicarbonate test: Ethanoic acid produces CO2 gas with effervescence when added to NaHCO3, while ethanol does not.
  • Ester test: Ethanoic acid reacts with alcohols to form esters with fruity smell, while ethanol alone does not produce this reaction.
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?

Micelle formation takes place when soap is added to water because:

  • Soap molecules have a hydrophilic (water-attracting) ionic head and a hydrophobic (water-repelling) hydrocarbon tail
  • In water, these molecules arrange themselves such that the hydrophobic tails point inward away from water, and hydrophilic heads point outward toward water
  • This arrangement forms spherical structures called micelles
  • Micelles help in emulsifying oily dirt and suspending it in water

Micelles will not form in solvents like ethanol because:

  • Ethanol can dissolve both hydrophilic and hydrophobic parts of soap molecules
  • There is no driving force for the molecules to arrange in micelles
  • The soap molecules remain uniformly distributed in ethanol
9. Why are carbon and its compounds used as fuels for most applications?

Carbon and its compounds are used as fuels because:

  • They have high calorific value, releasing large amounts of heat energy on combustion
  • They burn with a clean flame producing mainly CO2 and H2O
  • They are readily available from natural sources like petroleum, coal, and natural gas
  • They are easy to store and transport
  • They can be used in various forms - solid (coal), liquid (petrol, diesel), and gas (LPG, CNG)
  • The combustion reaction is controllable
10. Explain the formation of scum when hard water is treated with soap.

When soap is treated with hard water, scum formation occurs because:

  • Hard water contains calcium (Ca2+) and magnesium (Mg2+) ions
  • Soap molecules are sodium or potassium salts of long-chain fatty acids
  • When soap is added to hard water, the calcium and magnesium ions displace sodium or potassium ions
  • This forms insoluble calcium and magnesium salts of fatty acids, which appear as white curdy precipitate called scum
  • The reaction can be represented as:
    2C17H35COONa + Ca2+ → (C17H35COO)2Ca + 2Na+

This scum reduces the cleaning efficiency of soap and requires more soap to be used.

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

When soap is tested with litmus paper:

  • Blue litmus paper: No change (remains blue)
  • Red litmus paper: Turns blue

This indicates that soap solution is basic in nature. Soaps are salts of strong bases (NaOH or KOH) and weak fatty acids, so their aqueous solutions are alkaline.

12. What is hydrogenation? What is its industrial application?

Hydrogenation is a chemical reaction in which hydrogen (H2) is added to unsaturated compounds in the presence of a catalyst (usually nickel, palladium, or platinum) to form saturated compounds.

Industrial application: The most important industrial application of hydrogenation is in the hydrogenation of vegetable oils to produce vanaspati ghee (vegetable ghee).

  • Vegetable oils are unsaturated and exist as liquids at room temperature
  • When hydrogenated, they become saturated and solidify
  • This process converts liquid oils into solid fats, which have longer shelf life and are suitable for cooking and baking
  • The reaction: Unsaturated oil + H2 → Saturated fat
13. Which of the following hydrocarbons undergo addition reactions: C2H6, C3H8, C3H6, C2H2 and CH4?

Unsaturated hydrocarbons undergo addition reactions. Among the given compounds:

  • C3H6 (propene) - alkene with double bond
  • C2H2 (ethyne) - alkyne with triple bond

These unsaturated hydrocarbons undergo addition reactions. The saturated hydrocarbons C2H6 (ethane), C3H8 (propane), and CH4 (methane) do not undergo addition reactions.

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

The bromine water test can be used to differentiate between saturated and unsaturated hydrocarbons:

  • Unsaturated hydrocarbons (alkenes and alkynes) decolorize bromine water (reddish-brown color disappears)
  • Saturated hydrocarbons (alkanes) do not decolorize bromine water (color remains reddish-brown)

This is because unsaturated hydrocarbons undergo addition reaction with bromine, while saturated hydrocarbons do not react with bromine under normal conditions.

15. Explain the mechanism of the cleaning action of soaps.

The cleaning action of soaps involves the following mechanism:

  1. Soap molecules have a unique structure with a hydrophilic (water-loving) ionic head and a hydrophobic (water-hating) hydrocarbon tail.
  2. When soap is added to water, the molecules arrange themselves at the water surface with their hydrophilic heads in water and hydrophobic tails protruding out of water.
  3. Inside water, soap molecules form clusters called micelles, where hydrophobic tails point inward away from water, and hydrophilic heads point outward toward water.
  4. When soap solution comes in contact with oily dirt, the hydrophobic tails of soap molecules dissolve in the oil droplet.
  5. The hydrophilic heads remain oriented outward, forming an emulsion of oil in water.
  6. This arrangement breaks the oil into small droplets that are suspended in water and can be easily rinsed away.
  7. The micelles with trapped dirt particles remain suspended in water and do not come together due to ion-ion repulsion between the hydrophilic heads.

This mechanism allows soap to effectively remove oily dirt from clothes and other surfaces.