Solution :: Chapter 4: Carbon and its Compounds

Carbon and Its Compounds

Page no-61 Questions and Answers

Question 1:

What would be the electron dot structure of carbon dioxide (CO₂)?

Answer:

• Count the Valence Electrons:
  • Carbon (C): 4 valence electrons
  • Oxygen (O): 6 valence electrons each
  • Total = 4 + (2 × 6) = 16 electrons
• Arrange the Atoms:

  Place carbon in the center with an oxygen atom on each side.

• Form the Bonds:

  To satisfy the octet rule, carbon forms two double bonds (one with each oxygen). Each double bond uses 4 electrons (2 pairs).

• Place the Lone Pairs:

  After bonding, each oxygen atom has 4 electrons remaining as 2 lone pairs.

• Lewis Structure Diagram:
  :Ö::C::Ö:

  Here, each “::” represents a double bond (i.e., two shared pairs of electrons). Each oxygen atom shows two lone pairs (represented by “..” above and below).

Question 2:

What would be the electron dot structure of a molecule of sulphur (S₈) made up of eight atoms joined together in the form of a ring?

Answer:

• Valence Electrons for Sulphur:

  Sulphur (S) has 6 valence electrons.

• Structure of S₈:

  In S₈, the eight sulphur atoms form a closed ring (often described as a crown-like or puckered ring). Each S atom forms two single bonds with its two neighboring S atoms in the ring.

• Lone Pairs:

  After forming two single bonds (using 1 electron per bond), each sulphur atom has 4 electrons remaining which appear as two lone pairs.

• Lewis Structure Diagram:
  S — S / \ S S | | S S \ / S — S

  Note: This diagram represents the ring of eight sulphur atoms with each S atom bonded to two others. Although the lone pairs on each sulphur are not explicitly drawn in this skeletal diagram, remember that each sulphur atom carries two lone pairs.

Page no-68 Questions and Answers

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

Answer:

There are 3 structural isomers of pentane (C₅H₁₂):

• n-Pentane (straight chain of 5 carbons)
• Isopentane (methylbutane; a branch on the second carbon)
• Neopentane (dimethylpropane; a central carbon bonded to four methyl groups)

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

Answer:

• Tetravalency – Each carbon atom can form four covalent bonds.
• Catenation – Carbon atoms can bond to each other to form long chains and rings.

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

Answer:

Formula: C₅H₁₀

Structure: Five carbon atoms in a ring, each bonded to two neighboring carbons and two hydrogens.

C — C / \ C C \ / C

Each C is also bonded to 2 H, giving a total of 10 H.

4. Draw the structures for the following compounds:

(i) Ethanoic acid (Acetic acid)

Formula: CH₃COOH

H O \ // C / \ H OH

or more simply CH₃–COOH.

(ii) Bromopentane (C₅H₁₁Br)

Example: 1-bromopentane: CH₃–CH₂–CH₂–CH₂–CH₂Br

Are structural isomers possible for bromopentane?

Yes. You can place the Br atom on different carbons (e.g., 1-bromopentane, 2-bromopentane, 3-bromopentane).

(iii) Butanone (C₄H₈O)

Structure: CH₃–CO–CH₂–CH₃ (Also called methyl ethyl ketone.)

(iv) Hexanal (C₆H₁₂O)

Structure: CH₃–CH₂–CH₂–CH₂–CH₂–CHO (An aldehyde with six carbons.)

5. How would you name the following compounds?

(i) CH₃—CH₂—Br

Name: Bromoethane (common name: ethyl bromide)

(ii) H—C=O

Properly written as H₂C=O
IUPAC Name: Methanal
Common Name: Formaldehyde

(iii)

H H H H \ | | / H — C — C — C — C — H / | | \ H H H H

This is a four-carbon alkane with the formula C₄H₁₀.
Name: Butane.

Page no-71 Questions and Answers

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

Answer:

The conversion of ethanol (CH₃CH₂OH) to ethanoic acid (CH₃COOH) involves the addition of oxygen to the molecule. This process is carried out using oxidizing agents like alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇).

The chemical reaction is:
CH₃CH₂OH + [O] → KMnO₄/K₂Cr₂O₇ CH₃COOH

Since oxidation is defined as the addition of oxygen or removal of hydrogen, and here oxygen is added to ethanol, it is an oxidation reaction.

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:

Ethyne (C₂H₂) burns incompletely in air because air contains only 21% oxygen, leading to the formation of a yellow, sooty flame due to unburnt carbon particles. However, when pure oxygen is used, ethyne undergoes complete combustion, producing a very hot blue flame (~3000°C):

2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O + heat

This high-temperature flame is essential for welding metals efficiently, whereas a sooty, incomplete combustion flame (with air) is not hot enough for welding.

Page no-71 Questions and Answers

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

Answer:

You can distinguish between an alcohol and a carboxylic acid using the following tests:

• a) Litmus Test:
  • Alcohols: No change in litmus paper.
  • Carboxylic Acids: Turn blue litmus red (since they are acidic).
• b) Sodium Bicarbonate (NaHCO₃) or Sodium Carbonate (Na₂CO₃) Test:
  • Alcohols: No reaction.
  • Carboxylic Acids: Effervescence (bubbling) due to CO₂ gas evolution.
  CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂

  Test for CO₂: Passing the gas through lime water (Ca(OH)₂) turns it milky.

• c) Esterification Test:

  React both with ethanoic acid and conc. H₂SO₄, then warm.

  • Alcohols: Produce sweet-smelling esters.
  • Carboxylic Acids: No ester formation unless an alcohol is added.

2. What are oxidising agents?

Answer:

Oxidising agents are substances that oxidize (remove electrons from) other substances while themselves getting reduced. They help convert:

• Alcohols → Carboxylic Acids
• Ethanol → Ethanoic Acid using alkaline potassium permanganate (KMnO₄) or acidified potassium dichromate (K₂Cr₂O₇).

Example of Oxidation Reaction:

C₂H₅OH + [O] → KMnO₄/K₂Cr₂O₇ CH₃COOH + H₂O

Common Oxidizing Agents:

• Acidified KMnO₄ (Potassium permanganate, purple) → Turns colorless
• Acidified K₂Cr₂O₇ (Potassium dichromate, orange) → Turns green

Page no-76 Questions and Answers

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

Answer:

No, you cannot check if water is hard using a detergent. Detergents do not form scum with calcium (Ca²⁺) and magnesium (Mg²⁺) ions in hard water. They lather (foam) easily in both soft and hard water. Soap, however, can be used to check hardness:

• If soap forms a white curdy precipitate (scum), the water is hard.
• If soap forms foam easily, the water is soft.

2. Why is agitation necessary to get clean clothes?

Answer:

Agitation helps remove dirt by:

• Breaking up dirt particles – Vigorous movement dislodges dirt trapped in fabric fibers.
• Enhancing soap/detergent action – Ensures that soap molecules reach all parts of the fabric.
• Forming micelles effectively – Soap/detergents lift and suspend grease and dirt in water.
• Preventing dirt from reattaching – Keeps dirt particles moving, so they do not settle back.

Methods of Agitation:

• Beating on a stone or with a paddle – Loosens dirt physically.
• Scrubbing with a brush – Helps remove tough stains.
• Washing machine agitation – Mechanically stirs clothes for even cleaning.

EXERCISES

1. Ethane, with the molecular formula C₂H₆, has

Answer:

(b) 7 covalent bonds.

Ethane (C₂H₆) consists of:

• One C-C single bond
• Six C-H single bonds

Total 7 covalent bonds

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

Answer:

(c) Ketone.

Butanone (C₄H₈O) contains the -CO- (ketone) functional group.

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

Answer:

(b) The fuel is not burning completely.

Incomplete combustion produces carbon (soot), blackening the vessel.

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

Answer:

CH₃Cl (Methyl chloride) is formed by covalent bonding between:

• Carbon (C) – shares 4 valence electrons with 3 hydrogen (H) atoms and 1 chlorine (Cl) atom.
• Chlorine (Cl) – shares one electron with carbon to complete its octet.
• Hydrogen (H) – shares one electron with carbon to complete its duplet.

Electron Dot Structure of CH₃Cl:

H | H — C — Cl | H

Nature of the bond:

• Pure covalent bonds: C-H bonds (same electronegativity).
• Polar covalent bond: C-Cl bond (Cl is more electronegative).

5. Draw the electron dot structures for:

(a) Ethanoic acid (CH₃COOH)

O || CH₃— C — OH

Contains covalent bonds between C, H, and O.

(b) Hydrogen sulfide (H₂S)

H — S — H

S shares two electrons with two H atoms.

(c) Propanone (C₃H₆O) – Acetone

O || CH₃— C — CH₃

Ketone (-CO-) functional group.

(d) Fluorine (F₂)

F — F

Single covalent bond between two fluorine atoms.

6. What is a homologous series? Explain with an example.

Answer:

A homologous series is a group of organic compounds with the same functional group and similar chemical properties but differ by -CH₂ in molecular formula.

Example: Alkanes (CₙH₂ₙ₊₂)

• Methane (CH₄)
• Ethane (C₂H₆)
• Propane (C₃H₈)

7. How can ethanol and ethanoic acid be differentiated based on physical and chemical properties?

Answer:

Property	Ethanol (C₂H₅OH)	Ethanoic Acid (CH₃COOH)
Odor	Pleasant, alcoholic smell	Strong, vinegar-like smell
Taste	Burning taste	Sour taste
pH	Neutral	Acidic
Reaction with Na₂CO₃	No reaction	Effervescence (CO₂ gas)
Reaction with NaOH	No reaction	Forms sodium acetate

8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in ethanol?

Answer:

In water: Soap molecules form micelles (hydrophobic tails face oil/dirt, hydrophilic heads face water). Helps in removing grease and dirt.

In ethanol: No micelle formation because ethanol is non-polar and does not support micelle structure.

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

Answer:

• High calorific value (release large energy per gram).
• Efficient combustion (burn completely in oxygen).
• Easily available (coal, petroleum, natural gas).

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

Answer:

Hard water contains Ca²⁺ and Mg²⁺ ions. Soap reacts with these ions to form insoluble precipitates (scum). Reduces soap’s effectiveness.

2C₁₇H₃₅COONa + Ca²⁺ → (C₁₇H₃₅COO)₂Ca↓ + 2Na⁺

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

Answer:

Soap is basic in nature.

• Red litmus → Turns blue.
• Blue litmus → No change.

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

Answer:

Hydrogenation: Addition of H₂ to unsaturated hydrocarbons using Ni/Pt as a catalyst.

CH₂=CH₂ + H₂ → CH₃−CH₃

Industrial Application: Converts vegetable oil (liquid) → ghee (solid fat).

13. Which hydrocarbons undergo addition reactions?

Answer:

Addition reactions occur in unsaturated hydrocarbons (Alkenes and Alkynes).

Undergo addition reactions: C₃H₆ (propene), C₂H₂ (ethyne).

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

Answer:

Bromine water test:

• Unsaturated hydrocarbons (alkenes, alkynes): Decolorize brown bromine water.
• Saturated hydrocarbons (alkanes): No reaction, bromine water remains brown.

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

Answer:

Soap molecules have two parts:

• Hydrophobic tail (repels water, attaches to grease).
• Hydrophilic head (dissolves in water).

Cleaning Process:

• Soap molecules surround grease particles and form micelles.
• Hydrophobic tails stick to grease, and hydrophilic heads remain in water.
• Agitation (rubbing or washing) removes grease.