Carbon and its Compounds
CBSE Class 10 · Science · Notes, formulas and practice questions
CBSE Class 10 Science revision notes on Carbon and its Compounds. Learn covalent bonding, catenation, allotropes, saturated and unsaturated hydrocarbons, naming and homologous series, functional groups, and the reactions and uses of ethanol and ethanoic acid.
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Carbon is placed in group 14 and has four valence electrons, so it never forms an ionic bond by losing or gaining four electrons. Instead it shares electrons, producing covalent bonds. Carbon can link to itself through catenation and can bond to hydrogen, oxygen, nitrogen and halogens, creating millions of stable compounds. This chapter introduces carbon's structure, the strength of its bonds, and why this single element lies at the heart of organic chemistry. The geometric arrangement of bonds – straight, branched or cyclic – decides the physical and chemical behaviour of the resulting compounds.
The element carbon exists in several allotropic forms. Diamond and graphite are the two most familiar allotropes: both contain only carbon atoms but differ in how those atoms are joined. Diamond has a rigid tetrahedral network making it the hardest natural substance, while graphite has flat hexagonal layers that slide over one another, giving lubricant properties and electrical conductivity. Fullerenes such as C₆₀ are cage-like molecules. The chapter uses these examples to show that different structures of the same element can have drastically different properties.
To tackle the count of carbon compounds, chemists classify them into saturated and unsaturated hydrocarbons. Saturated hydrocarbons (alkanes) contain only single bonds, while unsaturated hydrocarbons (alkenes and alkynes) contain a double or triple bond. The carbon skeleton can be straight, branched, or closed into rings. Each family follows a general formula and forms a homologous series where consecutive members differ by a CH₂ group and have similar chemical properties. Naming follows systematic IUPAC rules: the suffix -ane, -ene or -yne indicates the type of bond, while prefixes indicate branches and functional groups.
The chemical properties of carbon compounds are dominated by the presence of functional groups. Alcohols (-OH), carboxylic acids (-COOH) and the multiple bonds in alkenes determine whether a compound undergoes combustion, oxidation, addition or substitution. Ethanol and ethanoic acid are studied in detail: ethanol is oxidised to ethanoic acid, and the two react together to form an ester in an esterification reaction. Students learn their physical properties, their reactions with common reagents, and everyday uses such as ethanoic acid in vinegar and ethanol as an antiseptic and solvent.
Key terms
- Covalent bond
- A chemical bond formed by mutual sharing of one or more pairs of electrons between two atoms. Carbon forms four covalent bonds because it has four valence electrons and can achieve a stable octet by sharing rather than transferring electrons. Example: a molecule of methane, CH₄.
- Catenation
- The self-linking of atoms of the same element into chains, rings or branched structures. Carbon exhibits catenation to the greatest extent because the C–C bond is strong and stable, leading to an immense number of organic compounds.
- Allotropes of carbon
- Different physical forms of the same pure element. Diamond (tetrahedral, hard, non-conducting), graphite (layered, soft, conducts electricity) and fullerenes (cage molecules like C₆₀) are allotropes of carbon. Their different atomic arrangements give different properties.
- Saturated hydrocarbon
- A hydrocarbon whose carbon atoms are connected only by single bonds. It contains the maximum possible number of hydrogen atoms. Alkanes are saturated hydrocarbons with general formula CₙH₂ₙ₊₂, such as CH₄ and C₂H₆.
- Unsaturated hydrocarbon
- A hydrocarbon that has at least one carbon-carbon double or triple bond, hence fewer hydrogen atoms than the saturated counterpart. Alkenes have a C=C bond and alkynes have a C≡C bond. They are more reactive and undergo addition reactions.
- Homologous series
- A family of compounds with the same general formula, similar chemical properties, and a successive difference of a CH₂ unit between adjacent members. For example, alkanes CH₄, C₂H₆, C₃H₈ form a homologous series.
- Functional group
- An atom or group of atoms attached to a hydrocarbon that gives the compound its characteristic chemical properties. Examples include –OH (hydroxyl) in alcohols and –COOH (carboxyl) in carboxylic acids. The same functional group behaves similarly in different molecules.
- Addition reaction
- A reaction in which atoms or groups attach to a molecule across a double or triple bond without losing any other atom. Hydrogenation of ethene, C₂H₄ + H₂ → C₂H₆, is an example. It is typical of unsaturated compounds.
- Substitution reaction
- A reaction in which one atom in a molecule is replaced by another atom or group of atoms. For example, methane reacts with chlorine in sunlight: CH₄ + Cl₂ → CH₃Cl + HCl. This reaction is characteristic of saturated hydrocarbons.
Formula sheet
| What | Formula | Notes |
|---|---|---|
| Alkane general formula | CₙH₂ₙ₊₂ | n is the number of carbon atoms in a saturated acyclic hydrocarbon (single bonds only). For n = 1 you get CH₄, n = 2 C₂H₆, and so on. |
| Alkene general formula | CₙH₂ₙ | n is the number of carbon atoms and the molecule has exactly one C=C double bond. Must have n ≥ 2, e.g., ethene C₂H₄. |
| Alkyne general formula | CₙH₂ₙ₋₂ | n is the number of carbon atoms and the molecule has exactly one C≡C triple bond. Must have n ≥ 2, e.g., ethyne C₂H₂. |
Practice questions with answers
1. Define a covalent bond. Give one example of a carbon compound formed by it.
A covalent bond is formed when two atoms share one or more pairs of electrons. Carbon forms four such bonds by sharing its valence electrons. Example: methane (CH₄), where the carbon shares one electron with each of four hydrogen atoms.
2. Why does carbon form a very large number of compounds?
Carbon has the property of catenation, so its atoms readily join together in long straight, branched, or cyclic chains. It also forms strong covalent bonds with other elements such as hydrogen, oxygen, nitrogen and halogens, producing an enormous variety of stable organic compounds.
3. What is the functional group in ethanoic acid? Write its molecular formula and indicate the functional group.
Ethanoic acid has the formula CH₃COOH. Its functional group is the carboxyl group, –COOH, which consists of a carbon atom double-bonded to one oxygen, single-bonded to a hydroxyl group, and bonded to the rest of the molecule. The –COOH is the characteristic group of carboxylic acids.
4. Ethene reacts with hydrogen in the presence of nickel catalyst. What type of reaction is this? Write the equation.
This is an addition reaction, specifically hydrogenation. C₂H₄ + H₂ → C₂H₆. The double bond of ethene opens and two hydrogen atoms add across it, changing ethene to ethane, a saturated hydrocarbon.
5. A saturated hydrocarbon has 8 carbon atoms. Determine its molecular formula.
Saturated hydrocarbons are alkanes with the formula CₙH₂ₙ₊₂. For n = 8, the number of hydrogen atoms is 2×8 + 2 = 18. Hence the molecular formula is C₈H₁₈ (octane).
6. Calculate the molar mass of ethanol, C₂H₅OH. (Atomic masses: C = 12 u, H = 1 u, O = 16 u)
Ethanol contains 2 C, 6 H and 1 O atoms. Molar mass = (2 × 12 u) + (6 × 1 u) + (1 × 16 u) = 24 + 6 + 16 = 46 u. Thus the molar mass is 46 g/mol.
7. Give one point of difference between addition and substitution reactions, with an example of each.
An addition reaction occurs when atoms add across a double or triple bond without eliminating any atoms, as in C₂H₄ + H₂ → C₂H₆. A substitution reaction replaces one atom in a molecule with another atom, as in CH₄ + Cl₂ → CH₃Cl + HCl in sunlight. Alkenes undergo addition; alkanes undergo substitution.
8. Explain why graphite conducts electricity but diamond does not.
In graphite, each carbon atom is bonded to only three neighbouring atoms in layers, leaving the fourth valence electron free as a delocalised electron, which can move and carry charge. In diamond, each carbon is bonded tetrahedrally to four others using all four valence electrons, so there are no free electrons to conduct electricity.
9. Name the two chain isomers of butane, C₄H₁₀, and give their structural formulas.
The straight-chain isomer is n-butane: CH₃–CH₂–CH₂–CH₃. The branched-chain isomer is 2-methylpropane (isobutane): (CH₃)₃CH, i.e., a central CH group attached to three methyl groups. Both have the molecular formula C₄H₁₀.
10. What happens when ethanoic acid is added to sodium carbonate? Write the balanced chemical equation and the observation.
It produces sodium acetate, water and carbon dioxide gas. The balanced equation is 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂. Effervescence is seen because CO₂ is released, and the gas turns lime water milky.
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