NCERT Solutions Class 10 Science Chapter 4 – Carbon and its Compounds
NCERT Solutions for Class 10 Science Chapter 4: Carbon and its Compounds
This chapter introduces organic chemistry – the study of carbon compounds. It covers covalent bonding, hydrocarbons, functional groups, and important carbon compounds like ethanol and ethanoic acid. This chapter typically carries 5-7 marks in board exams.
In-text Questions and Answers
Page 61 Questions
Q1. What would be the electron dot structure of carbon dioxide which has the formula CO₂?
Answer: Carbon dioxide has a linear structure where carbon shares two electrons each with two oxygen atoms (double bonds).
O::C::O or O=C=O
Carbon has 4 valence electrons, and each oxygen has 6. Carbon forms two double bonds with oxygen atoms to complete octet of all atoms.
Q2. What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur?
Answer: Sulphur exists as S₈ molecule in a ring structure. Each sulphur atom shares one electron with two neighboring sulphur atoms, forming a puckered ring structure called crown-shaped or cyclo-octasulphur.
Page 68-69 Questions
Q1. How many structural isomers can you draw for pentane?
Answer: Pentane (C₅H₁₂) has three structural isomers:
- n-Pentane: CH₃-CH₂-CH₂-CH₂-CH₃ (straight chain)
- Isopentane (2-methylbutane): CH₃-CH(CH₃)-CH₂-CH₃ (branched)
- Neopentane (2,2-dimethylpropane): C(CH₃)₄ (highly branched)
Q2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
Answer:
- Catenation: The unique ability of carbon atoms to form bonds with other carbon atoms, creating long chains, branched chains, and ring structures. This is due to the strong C-C bond (bond energy 83 kcal/mol).
- Tetravalency: Carbon has 4 valence electrons, allowing it to form 4 covalent bonds with other atoms. This enables carbon to bond with hydrogen, oxygen, nitrogen, halogens, and other carbon atoms in various combinations.
Q3. What will be the formula and electron dot structure of cyclopentane?
Answer: Cyclopentane is a cyclic hydrocarbon with formula C₅H₁₀. It forms a five-membered ring where each carbon is bonded to two hydrogen atoms and two adjacent carbon atoms.
Exercise Questions
Q1. 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
Explanation: Ethane has 1 C-C bond and 6 C-H bonds (3 for each carbon), totaling 7 covalent bonds.
Q2. Butanone is a four-carbon compound with the functional group:
(a) Carboxylic acid (b) Aldehyde (c) Ketone (d) Alcohol
Answer: (c) Ketone
Explanation: Butanone (CH₃-CO-CH₂-CH₃) contains the ketone functional group (C=O) bonded to two carbon atoms. The suffix “-one” indicates a ketone.
Q3. 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
Explanation: Incomplete combustion produces carbon (soot) which deposits as black residue on the vessel. This happens due to insufficient oxygen supply or improper air-fuel ratio.
Homologous Series
| Series | General Formula | Functional Group | Examples |
|---|---|---|---|
| Alkanes | CₙH₂ₙ₊₂ | None (saturated) | Methane, Ethane, Propane |
| Alkenes | CₙH₂ₙ | C=C (double bond) | Ethene, Propene, Butene |
| Alkynes | CₙH₂ₙ₋₂ | C≡C (triple bond) | Ethyne, Propyne, Butyne |
| Alcohols | CₙH₂ₙ₊₁OH | -OH (hydroxyl) | Methanol, Ethanol, Propanol |
| Carboxylic Acids | CₙH₂ₙ₊₁COOH | -COOH (carboxyl) | Methanoic acid, Ethanoic acid |
Important Chemical Reactions
1. Combustion Reaction
CH₄ + 2O₂ → CO₂ + 2H₂O + Heat + Light
2. Addition Reaction (for unsaturated hydrocarbons)
CH₂=CH₂ + H₂ → CH₃-CH₃ (in presence of Ni catalyst)
3. Substitution Reaction (for saturated hydrocarbons)
CH₄ + Cl₂ → CH₃Cl + HCl (in presence of sunlight)
Key Takeaways
- Carbon forms covalent bonds due to its tetravalency (4 valence electrons)
- Catenation allows carbon to form long chains, branched chains, and rings
- Saturated hydrocarbons have single bonds; unsaturated have double/triple bonds
- Homologous series members differ by -CH₂- unit and have similar chemical properties
- IUPAC nomenclature follows: Prefix + Root word + Suffix
- Ethanol and Ethanoic acid are important carbon compounds with industrial uses
- Soaps are sodium/potassium salts of long-chain fatty acids
Key Concepts: Carbon and its Compounds
Carbon is the basis of all life on Earth. This chapter explains why carbon forms such a vast number of compounds — the study of which is called organic chemistry.
Why Carbon Forms So Many Compounds
- Catenation: Carbon atoms can bond with other carbon atoms to form long chains, branched chains, or rings. This is unique to carbon.
- Tetravalency: Carbon has 4 valence electrons and can form 4 covalent bonds — with H, O, N, S, Cl, or other carbon atoms.
Homologous Series
A series of compounds with the same functional group and similar chemical properties, where each member differs from the next by a CH2 group. Example: Alkanes — CH4, C2H6, C3H8, C4H10…
Functional Groups
- Alcohols (–OH): Example: Ethanol (C2H5OH) — used in beverages, sanitizers, fuel
- Aldehydes (–CHO): Example: Methanal (HCHO, formaldehyde) — preservative
- Ketones (–C=O): Example: Propanone (acetone) — nail polish remover
- Carboxylic Acids (–COOH): Example: Ethanoic acid (CH3COOH, acetic acid) — vinegar
- Halogens (–Cl, –Br): Example: Chloromethane (CH3Cl)
Important Carbon Compounds
- Ethanol (C2H5OH): Alcohol used in beverages. Burns with a blue flame. Reacts with sodium to release H2. Oxidised to ethanoic acid using alkaline KMnO4 or K2Cr2O7.
- Ethanoic Acid (CH3COOH): Glacial acetic acid when pure. Turns blue litmus red. Forms esters with alcohols. Melts at 16.6°C (freezes slightly below room temperature).
- Soaps vs Detergents: Soaps are sodium/potassium salts of long-chain fatty acids — they do not work in hard water (form scum). Detergents are sulphonate salts — they work in hard water. Both have a hydrophilic head and hydrophobic tail (micelle formation cleans oil/grease).
Chemical Reactions of Carbon Compounds
- Combustion: Hydrocarbons burn in oxygen to give CO2 + H2O. Limited oxygen = CO (carbon monoxide, poisonous). Example: CH4 + 2O2 → CO2 + 2H2O
- Oxidation: Ethanol oxidised to ethanoic acid using acidified KMnO4 or K2Cr2O7
- Addition Reaction: Unsaturated compounds (alkenes, alkynes) add H2, Cl2, or HCl across the double/triple bond in the presence of a catalyst (Ni/Pd). Example: CH2=CH2 + H2 → CH3-CH3 (used in hydrogenation of vegetable oils)
- Substitution Reaction: Saturated hydrocarbons react with Cl2 in the presence of sunlight — H atoms replaced by Cl atoms. CH4 + Cl2 → CH3Cl + HCl
- Esterification: Alcohol + Carboxylic Acid → Ester + Water (in presence of concentrated H2SO4). Esters have a fruity smell and are used in perfumes and food flavouring.
- Saponification: Ester + NaOH → Soap + Alcohol (the reverse of esterification — soap making)
Important Board Exam Questions
- What is catenation? Why is it a special property of carbon?
- Write the structural formula and IUPAC name of the first four alkanes.
- What happens when ethanol reacts with sodium? Write the equation.
- Explain the process of saponification.
- What is the difference between soaps and detergents?
- Why do soaps not work in hard water?
- What is esterification? Give one example with an equation.
- Differentiate between saturated and unsaturated hydrocarbons.
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