The actual CBSE Class 10 Science 2024 board exam theory questions from Chapter 4 – Carbon and Its Compounds (3 marks & 5 marks). Attempt each question on paper, then click to reveal the board-exam style model answer.
Based on Ethanol, Esterification & Dehydration Reaction
Write the name and structural formula of an organic compound X having two carbon atoms in its molecule, whose name has the suffix "-ol".
The compound X is Ethanol (C₂H₅OH), with structural formula CH₃–CH₂–OH. The suffix "-ol" indicates the presence of the hydroxyl (–OH) functional group, which is characteristic of alcohols. With exactly two carbon atoms, the compound is named ethanol.
What happens when X (ethanol) is heated with excess concentrated sulphuric acid at 443 K? Write the chemical equation for the reaction, stating the conditions. Also state the role played by concentrated sulphuric acid in the reaction.
When ethanol is heated with excess concentrated sulphuric acid at 443 K, it undergoes dehydration to form ethene (an unsaturated hydrocarbon).
Conditions: Excess concentrated H₂SO₄, temperature of 443 K.
Role of concentrated H₂SO₄: It acts as a dehydrating agent — it removes a water molecule from ethanol.
Name the hydrocarbon produced in the above reaction (Q.A2) and draw its electron dot structure.
The hydrocarbon produced is Ethene (C₂H₄).
Electron dot structure: Each carbon atom has 4 valence electrons. In ethene, the two carbon atoms share two pairs of electrons between them (forming a double bond, C=C), and each carbon additionally forms a single covalent bond with two hydrogen atoms.
Total: 1 C=C double bond (2 shared electron pairs) + 4 C–H single bonds (4 shared electron pairs).
Based on Homologous Series & Functional Groups
Define a homologous series of carbon compounds.
A homologous series is a series of organic compounds having the same functional group, the same general formula, and similar chemical properties, in which each successive member differs from the previous one by a –CH₂– unit (a difference of 14 u in molecular mass).
Example: Alkanes — CH₄, C₂H₆, C₃H₈, C₄H₁₀ ... (general formula CₙH₂ₙ₊₂)
Why is the melting and boiling point of C₄H₈ higher than that of C₃H₆ or C₂H₄?
C₄H₈, C₃H₆, and C₂H₄ belong to the same homologous series (alkenes). As we move up a homologous series, the molecular mass increases. A higher molecular mass means a larger molecule with a greater number of electrons, which increases the strength of the intermolecular (van der Waals) forces of attraction between molecules.
Since C₄H₈ has the highest molecular mass among the three, it has the strongest intermolecular forces, and therefore requires more energy (higher temperature) to melt or boil — giving it the highest melting and boiling point.
Why do we not see any gradation in the chemical properties of homologous series compounds?
Chemical properties of an organic compound are mainly decided by its functional group. Since all members of a homologous series contain the same functional group, they show similar chemical properties — there is no gradual change (gradation) in chemical behaviour as we move up the series, unlike physical properties (melting point, boiling point) which do show a gradual increase with molecular mass.
Write the name and structure of an aldehyde and a ketone, both having the molecular formula C₃H₆O.
Both compounds have the same molecular formula C₃H₆O but belong to different homologous series — this is an example of functional isomerism.
Aldehyde — Propanal:
Ketone — Propanone (Acetone):
The aldehyde group (–CHO) is always at the end of the carbon chain, while the ketone group (>C=O) occurs within the chain.
Based on Bonding, Homologous Series of Aldehydes & Cyclic Structures
Give reason: Carbon can neither form a C⁴⁺ cation nor a C⁴⁻ anion, but forms covalent compounds instead.
Carbon has 4 electrons in its outermost shell (valence shell) and needs 4 more to complete its octet.
Therefore, carbon achieves a stable noble gas configuration by sharing its valence electrons with other atoms, forming covalent bonds instead of ionic bonds.
What is a homologous series of carbon compounds? Write the molecular formula of any two consecutive members of the homologous series of aldehydes.
Homologous series definition: A series of carbon compounds having the same functional group and general formula, where successive members differ by a –CH₂– unit (14 u), is called a homologous series.
Two consecutive members of the aldehyde series (general formula CₙH₂ₙO, functional group –CHO):
The difference between them is exactly one –CH₂– unit (14 u in molecular mass).
Draw the structure of the molecule cyclohexane, C₆H₁₂.
Cyclohexane (C₆H₁₂) is a saturated cyclic (ring) hydrocarbon formed due to carbon's property of catenation.
Since it is a closed ring with only single bonds, all 6 carbon atoms together carry exactly 12 hydrogen atoms, matching the formula C₆H₁₂.
Based on Ethanol — Reactions with Na, H₂SO₄, Ethanoic Acid & KMnO₄
Name a commercially important carbon compound having the functional group –OH, and write its molecular formula.
The functional group –OH (hydroxyl) belongs to the alcohol family. The most commercially important alcohol is Ethanol, molecular formula C₂H₅OH (or CH₃CH₂OH).
It is commonly known as "alcohol" and is used as a solvent, in the preparation of medicines, and as a fuel (mixed with petrol).
Write chemical equations to show the reaction of ethanol with: (a) sodium metal, (b) excess concentrated sulphuric acid, (c) ethanoic acid in the presence of an acid catalyst, (d) acidified potassium dichromate. Also write the name of the product formed in each case.
(a) Reaction with sodium metal:
(b) Reaction with excess conc. H₂SO₄ at 443 K (dehydration):
(c) Reaction with ethanoic acid + acid catalyst (Esterification):
(d) Reaction with acidified potassium dichromate (Oxidation):
Versatile Nature of Carbon, Functional Groups & Isomerism
Passage: More than 3 million carbon compounds have been discovered so far. The diversity of these compounds arises from carbon's capacity for bonding with other carbon atoms as well as with atoms of other elements. Most carbon compounds are poor conductors of electricity and have low melting and boiling points.
Write the molecular formula of the first two members of the homologous series of haloalkanes having the functional group –Br.
The functional group –Br belongs to the bromoalkane (haloalkane) series.
The difference between them is one –CH₂– unit, consistent with homologous series rules.
Given below are the formulas of some functional groups: –OH, >C=O, –CHO. Write the name of these functional groups.
Note: Both aldehyde and ketone contain the carbonyl group (C=O), but they differ in position — aldehyde is terminal, ketone is internal.
What would be observed on adding 5% alkaline potassium permanganate (KMnO₄) solution drop by drop to some warm ethanol taken in a test tube? State the role of KMnO₄ in the reaction and write the chemical equation for the reaction involved.
On adding alkaline KMnO₄ drop by drop to warm ethanol, the pink colour of KMnO₄ fades/disappears as ethanol gets oxidised to ethanoic acid.
Role of KMnO₄: It acts as an oxidising agent — it provides oxygen [O] needed to convert the alcohol (–OH group) into a carboxylic acid (–COOH group).
Write the name of the compound formed when ethanol is heated at 443 K with excess concentrated H₂SO₄. What is the role of concentrated H₂SO₄ in the reaction? Write the chemical equation for the reaction involved.
This repeats the reaction from question A2 — a frequently repeated CBSE board question.
Compound formed: Ethene (C₂H₄)
Role of conc. H₂SO₄: Acts as a dehydrating agent — removes a water molecule from ethanol.
Passage: Carbon is a versatile element that forms the basis of all living organisms and many of the things we use. A large variety of compounds is formed because of its tetravalency. Compounds of carbon are formed with oxygen, hydrogen, sulphur, chlorine, and many other elements.
What are hydrocarbons?
Hydrocarbons are organic compounds made up of only carbon and hydrogen atoms. They are classified as saturated (alkanes) and unsaturated (alkenes and alkynes), and further into open-chain and cyclic (ring) hydrocarbons.
Examples: Methane (CH₄), Ethene (C₂H₄), Ethyne (C₂H₂), Cyclohexane (C₆H₁₂).
List two properties of carbon by virtue of which it can form a large number of compounds.
1. Catenation: Carbon has the unique property of forming bonds with other carbon atoms to form long chains, branched chains, or rings. C–C bonds are very strong and stable.
2. Tetravalency: Carbon has a valency of 4, allowing it to bond with 4 other atoms of carbon, hydrogen, oxygen, nitrogen, sulphur, chlorine, etc., forming a vast variety of compounds.
Write the formula of the functional group present in (1) aldehydes and (2) ketones. Write the chemical equation for the reaction that occurs between ethanoic acid and ethanol in the presence of a catalyst.
Functional groups:
Reaction between ethanoic acid and ethanol (Esterification):
This reaction is called esterification. The reverse reaction (ester + NaOH → soap + alcohol) is called saponification.
What are structural isomers? Write the structures of two isomers of butane, C₄H₁₀.
Structural isomers are compounds that have the same molecular formula but different structural arrangements of atoms, resulting in different physical and sometimes chemical properties.
Two isomers of Butane (C₄H₁₀):
Both have the formula C₄H₁₀ but different arrangements of carbon atoms — n-butane is unbranched, isobutane is branched.
Write the formula and the molecular mass of the third member of the homologous series of alcohols. State how the boiling point of an alcohol changes as one moves from lower to higher members of the homologous series.
The alcohol homologous series (general formula CₙH₂ₙ₊₁OH) begins as: Methanol (1st), Ethanol (2nd), Propanol (3rd).
Boiling point trend: As we move from lower to higher members of the homologous series of alcohols, the molecular mass increases, which increases the strength of intermolecular forces. Therefore, the boiling point increases as we move up the homologous series.