Carbon and Its Compounds
CBSE Class 10 Science Notes — Chapter 4 (Complete Exhaustive Notes
Carbon is an incredibly important element found in many things we use every day, like food, clothes, medicines, and books. Even though there's only a tiny bit of carbon in the Earth's crust (0.02%) and in the atmosphere (0.03% as carbon dioxide), it plays a huge role in our lives. This chapter explores why carbon is so crucial and what makes it so special.
1. Bonding in Carbon - The Covalent Bond
Carbon compounds are substances made primarily of carbon atoms bonded with other elements. These compounds exhibit specific properties that distinguish them from other types of compounds.
Properties of Carbon Compounds:
Poor Conductivity: Most carbon compounds do not conduct electricity well. This means they do not allow electric current to flow through them easily.
Low Melting and Boiling Points: Carbon compounds generally have low melting and boiling points in comparison to ionic compounds. This indicates that the forces holding the molecules together are relatively weak.
Weak Intermolecular Forces: The forces of attraction between molecules in carbon compounds are not very strong, leading to their low melting and boiling points.
Non-Conductivity: Since carbon compounds are mostly non-conductors of electricity, it suggests that the bonding within these compounds does not result in the formation of ions.
Carbon has 6 electrons total. These are distributed as 2 in the first shell (K-shell) and 4 in the second shell (L-shell).
Carbon has 4 valence electrons since these are the electrons in its outermost shell.
Elements aim to achieve a stable outer shell similar to noble gases. Carbon needs 4 more electrons to achieve a stable octet configuration.
Note: Carbon can achieve stability by two theoretical paths, both of which are energetically unfavorable:
(a) Gaining 4 Electrons: Carbon could gain 4 electrons to become C⁴⁻ (anion). However, it would be extremely difficult for the nucleus with 6 protons to hold on to 10 electrons.
(b) Losing 4 Electrons: Carbon could lose 4 electrons to become C⁴⁺ (cation). But it would require a massive amount of energy to remove 4 electrons, leaving behind a carbon cation with 6 protons holding only 2 electrons.
Carbon overcomes this problem by sharing its valence electrons with other carbon atoms or with atoms of other elements. This sharing of outermost electrons helps each atom achieve a stable noble gas configuration.
What is a Covalent Bond?
Covalent bonds are formed when two atoms share one or more electron pairs, resulting in strong intramolecular bonds within molecules but weak intermolecular forces between molecules.
Properties of Covalent Compounds:
Covalent compounds generally have low melting and boiling points.
In water and other polar solvents, covalent compounds are often insoluble or less soluble.
When fused or dissolved in solution, these materials are poor conductors of electricity due to absence of free ions.
(i) Hydrogen Molecule (H₂): Hydrogen has an atomic number of 1 with 1 electron in its outer shell. Two hydrogen atoms share their single valence electrons to form a hydrogen molecule (H₂), giving each atom the stable helium configuration.
(ii) Oxygen Molecule (O₂): Oxygen has 6 valence electrons and needs 2 more. Two oxygen atoms share two pairs of electrons to form a double bond (O=O) between them.
(iii) Methane (CH₄): Methane is one of the simplest carbon compounds. Carbon (valency 4) shares its 4 outer electrons with 4 hydrogen atoms (valency 1) to form 4 single covalent bonds.
Classification of Covalent Bonds
(i) Single Covalent Bond: Formed by sharing one pair of electrons between two atoms (e.g., H₂, HCl).
(ii) Multiple Covalent Bonds: Involve sharing more than one pair of electrons:
• Double Bond: Shares two pairs of electrons (e.g., O₂, CO₂).
• Triple Bond: Shares three pairs of electrons (e.g., N₂).
1.1 Allotropes of Carbon
Allotropy is the property due to which an element exists in two or more distinct physical forms having identical chemical properties. The different physical forms are called Allotropes.
Carbon exists in several crystalline allotropes (Diamond, Graphite, Fullerene) as well as amorphous forms (coke, charcoal, lampblack, gas carbon):
2. Versatile Nature of Carbon
Carbon forms millions of organic compounds. This vast versatility is primarily attributed to three unique structural characteristics:
(i) Catenation (Self-Linking Ability): Carbon possesses the remarkable ability to form stable covalent bonds with other carbon atoms, resulting in long straight chains, branched chains, or cyclic rings.
(ii) Tetravalency: With 4 valence electrons, carbon can form strong covalent bonds with 4 other monovalent atoms or polyvalent atoms (H, O, N, S, halogens).
(iii) Small Atomic Size: Due to its small atomic size, the nucleus holds shared electron pairs firmly, making carbon-carbon and carbon-heteroatom bonds exceptionally strong and stable.
2.1 Saturated and Unsaturated Carbon Compounds (Hydrocarbons)
Compounds composed exclusively of carbon and hydrogen are called Hydrocarbons. They are divided into two main classes:
2.2 Chains, Branches, and Rings (Structural Isomerism)
Carbon skeletons can assemble into three major structural types:
(i) Straight Chains: Carbon atoms linked continuously in a single linear chain (e.g., Propane C₃H₈, Butane C₄H₁₀).
(ii) Branched Chains & Structural Isomerism: Compounds having identical molecular formulas but different structural arrangements are called structural isomers, and the phenomenon is called structural isomerism. Example: Pentane (C₅H₁₂) has 3 structural isomers (n-pentane, isopentane, neopentane).
(iii) Cyclic Structures: Carbon atoms linked in closed ring structures (e.g., Cyclohexane C₆H₁₂ [saturated], Benzene C₆H₆ [unsaturated]).
2.3 Heteroatoms and Functional Groups
Elements such as halogens (Cl, Br, I), oxygen, nitrogen, and sulfur that replace hydrogen atoms in a hydrocarbon chain are called Heteroatoms. An atom or group of atoms that confers specific chemical properties to the organic compound, regardless of the length and nature of the carbon chain, is called a Functional Group.
2.4 Homologous Series
A Homologous Series is a family of organic compounds having the same general molecular formula, similar chemical properties, and identical functional groups, in which successive members differ by a -CH₂- unit.
Key Characteristics of a Homologous Series:
All members share the same general molecular formula (e.g., Alkanes: CₙH₂ₙ₊₂).
Any two adjacent homologues differ in molecular formula by a -CH₂- unit.
Any two adjacent homologues differ in molecular mass by 14 u (Atomic mass: C=12 u, H=1 u).
All members exhibit similar methods of preparation and chemical properties due to the presence of the same functional group.
Physical properties (melting point, boiling point, density, solubility) show a gradual gradation as molecular mass increases.
2.5 Nomenclature of Carbon Compounds (IUPAC Rules)
Systematic IUPAC naming follows clear sequential steps:
1. Carbon Chain Length (Root Word): Count number of carbon atoms (1 C: Meth-, 2 C: Eth-, 3 C: Prop-, 4 C: But-, 5 C: Pent-, 6 C: Hex-).
2. Saturation Suffix: If all single bonds: '-ane'; double bond present: '-ene'; triple bond present: '-yne'.
3. Functional Group Suffix/Prefix: Add appropriate suffix or prefix. If suffix begins with a vowel (a, e, i, o, u), drop the final 'e' from the carbon chain name (e.g., Propane + -ol = Propanol; Propane + -one = Propanone).
3. Chemical Properties of Carbon Compounds
3.1 Combustion Reaction
Carbon and its compounds burn in the presence of oxygen to produce carbon dioxide, water vapor, heat, and light energy. Saturated hydrocarbons burn with a clean blue non-sooty flame. Unsaturated hydrocarbons burn with a yellow sooty flame due to incomplete oxidation caused by a higher carbon percentage.
3.2 Oxidation Reaction
Oxidation involves the addition of oxygen or removal of hydrogen. Alcohols are oxidized to carboxylic acids using strong oxidizing agents such as alkaline Potassium Permanganate (KMnO₄) + heat or acidified Potassium Dichromate (K₂Cr₂O₇) + heat:
3.3 Addition Reaction (Hydrogenation)
Unsaturated hydrocarbons add hydrogen across double or triple bonds in the presence of catalysts such as Nickel (Ni) or Palladium (Pd) to form saturated hydrocarbons. This reaction is commercially used in the hydrogenation of vegetable oils to form vegetable ghee:
(Vegetable Oil → Vegetable Ghee)
3.4 Substitution Reaction
In the presence of sunlight, alkanes undergo halogenation where chlorine radicals replace hydrogen atoms one by one in a rapid chain reaction:
CH₄ + Cl₂ ──[ Sunlight ]──>CH₃Cl + HCl (Methane → Chloromethane + Hydrogen Chloride)
4. Some Important Carbon Compounds: Ethanol & Ethanoic Acid
4.1 Ethanol (Ethyl Alcohol - C₂H₅OH)
Physical Properties of Ethanol:
Colourless liquid with a pleasant characteristic odor and burning taste.
Soluble in water in all proportions due to hydrogen bonding.
Volatile liquid with a low boiling point of 351 K (78°C).
Neutral compound (does not affect red or blue litmus paper).
Chemical Properties of Ethanol:
(i) Reaction with Sodium Metal: Ethanol reacts with sodium to evolve Hydrogen gas which burns with a characteristic 'pop' sound: 2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa (Sodium ethoxide) + H₂↑
(ii) Dehydration Reaction: Heating ethanol at 443 K with excess concentrated sulfuric acid (H₂SO₄) dehydrates it to form ethene: CH₃CH₂OH ──[ Conc. H₂SO₄, 443 K ]──>CH₂=CH₂ + H₂O
4.2 Ethanoic Acid (Acetic Acid - CH₃COOH)
Physical Properties of Ethanoic Acid:
Colourless liquid with a sour taste and pungent smell of vinegar.
Boiling point is 391 K (118°C).
Pure ethanoic acid freezes at 290 K (16.6°C) to form ice-like crystals, hence known as Glacial Acetic Acid.
5-8% solution of acetic acid in water is called Vinegar.
Chemical Properties of Ethanoic Acid:
(i) Esterification Reaction: Ethanoic acid reacts with absolute ethanol in the presence of an acid catalyst to form a sweet-smelling ester (Ethyl ethanoate):
CH₃COOH + C₂H₅OH ──[ Acid Catalyst ]──>CH₃COOC₂H₅ (Ethyl ethanoate) + H₂O
• Saponification: Esters react with sodium hydroxide (base) to yield sodium salt of carboxylic acid (soap) and alcohol.
(ii) Reaction with Base: NaOH + CH₃COOH → CH₃COONa (Sodium acetate) + H₂O
(iii) Reaction with Carbonates & Hydrogen Carbonates: Ethanoic acid reacts with Na₂CO₃ and NaHCO₃ to produce sodium acetate, water, and brisk effervescence of CO₂ gas:
• 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂↑
• CH₃COOH + NaHCO₃ → CH₃COONa + H₂O + CO₂↑
5. Soaps and Detergents
Cleansing Action of Soap and Micelle Formation
A soap molecule has two distinct ends:
(i) Ionic / Head Part (-COO⁻Na⁺): Hydrophilic (water-loving) part that dissolves in water.
(ii) Long Hydrocarbon Tail: Hydrophobic (water-fearing / oil-loving) part that dissolves in dirt, oil, and grease.
When soap is dissolved in water, the hydrophobic tails attach themselves to oily dirt while the hydrophilic ionic heads project outward into the water. This forms a spherical radial aggregate called a Micelle. The micelle entrap grease in its center and suspends it in water, allowing dirt to be washed away upon rinsing.
Figure 5.1: Structure of a Soap Micelle Entrapping Oily Dirt particle in Water
Hard Water Behavior and Scum Formation:
Hard water contains dissolved Calcium (Ca²⁺) and Magnesium (Mg²⁺) salts. When soap is added to hard water, it reacts with these ions to form a sticky, insoluble white precipitate called Scum (e.g., Calcium stearate). Scum wastes soap and adheres to clothes. In contrast, synthetic detergents do not form scum because their charged ends do not form insoluble salts with Ca²⁺ and Mg²⁺ ions.
No comments:
Post a Comment