Hydrocarbons are important organic compounds made up of only carbon and hydrogen atoms. They form the backbone of organic chemistry and are widely used in fuels, plastics, and chemical industries. Hydrocarbons are classified into three main types: Alkanes, which are saturated hydrocarbons with single bonds and are stable and less reactive; Alkenes, which are unsaturated hydrocarbons with at least one double bond and are more reactive; and Alkynes, which are unsaturated hydrocarbons with at least one triple bond and are highly reactive. Understanding hydrocarbons is essential for Class 10 students as it helps in solving MCQs, short questions, and long questions in exams. This chapter covers structures, properties, examples, and uses of Alkanes, Alkenes, and Alkynes, making it easier for students to learn and prepare effectively.
Important MCQs – Chapter 12 Hydrocarbons
1. Which of the following is a saturated hydrocarbon?
(a) Ethene
(b) Methane ✅
(c) Ethyne
(d) Propyne
2.What is the general formula of alkanes?
(a) CₙH₂ₙ
(b) CₙH₂ₙ₊₂ ✅
(c) CₙH₂ₙ–₂
(d) CₙHₙ
3. Which of the following is a saturated hydrocarbon?
(a) Ethene
(b) Methane ✅
(c) Ethyne
(d) Propyne
4. The main type of reaction shown by alkynes is:
(a) Substitution
(b) Addition ✅
(c) Neutralization
(d) Hydrolysis
Short Questions – Chapter 12 Hydrocarbons
1. Difference Between Saturated and Unsaturated Hydrocarbons.
Saturated Hydrocarbons (Alkanes)
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Contain only single bonds between carbon atoms.
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General formula: CnH2n+2
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Examples:
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Methane: CH4
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Ethane: C2H6
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Less reactive; mainly undergo substitution reactions.
Unsaturated Hydrocarbons
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Contain one or more double or triple bonds between carbon atoms.
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More reactive; mainly undergo addition reactions.
Types:
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Alkenes (Double bond C=C)
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General formula: CnH2n
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Example: Ethene: C2H4
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Alkynes (Triple bond C≡C)
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General formula: CnH2n-2
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Example: Ethyne: C2H2
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2. Why are the alkenes called ‘olefins’?
Answer:
Alkenes are called olefins (from Latin: oleum = oil + facere = to make) because:
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The first member of alkenes, ethene, forms an oily compound when it reacts with chlorine.
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Historically, alkenes were known as oil-formers, so they were named “olefins”.
3. Both alkenes and alkynes are unsaturated hydrocarbons. State the one most significant difference between them.
Answer:
The most significant difference is:
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Alkenes contain one double bond (C=C).
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Alkynes contain one triple bond (C≡C).
This difference affects their reactivity and chemical properties.
4. Give the physical properties of alkanes.
Answer:
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Alkanes are non-polar compounds.
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They are insoluble in water but soluble in organic solvents.
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Low molecular mass alkanes are gases (e.g., methane, ethane).
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Medium molecular mass alkanes are liquids (e.g., pentane, hexane).
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High molecular mass alkanes are waxy solids.
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Alkanes are colorless, odorless, and less dense than water.
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They have low boiling and melting points, which increase with molecular size.
Long Questions – Chapter 12 Hydrocarbons
Q1: What type of reactions are given by alkanes? Explain with reference to halogenation of alkanes.
Answer:
Introduction:
Alkanes are saturated hydrocarbons, which means they contain only single covalent bonds (C–C and C–H). Because of these single bonds, alkanes are very stable and less reactive compared to alkenes or alkynes. However, they can react in a special type of reaction called a substitution reaction.
Substitution Reaction:
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In a substitution reaction, one or more hydrogen atoms in an alkane are replaced by atoms of another element, usually a halogen like chlorine (Cl) or bromine (Br).
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These reactions typically require sunlight or UV light to start the process.
Example: Halogenation of Methane (CH4)
Step 1 – First Substitution:
CH4 + Cl2 → CH3Cl + HCl (with sunlight)
Explanation:
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One hydrogen atom of methane is replaced by one chlorine atom.
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This forms methyl chloride (CH3Cl) and hydrogen chloride (HCl).
Step 2 – Further Substitution (if chlorine is in excess):
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CH3Cl + Cl2 → CH2Cl2 + HCl (forms dichloromethane)
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CH2Cl2 + Cl2 → CHCl3 + HCl (forms chloroform)
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CHCl3 + Cl2 → CCl4 + HCl (forms carbon tetrachloride)
Key Points:
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The reaction happens in steps, replacing one hydrogen atom at a time.
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Excess halogen ensures further substitution to produce compounds like CCl4.
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Sunlight or UV light provides the energy needed to break the Cl–Cl bond and start the reaction.
Summary of Conditions Required:
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Presence of sunlight or UV light – provides energy to start the reaction.
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Excess halogen – to allow multiple substitutions.
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Stepwise reaction – one hydrogen atom is replaced at a time.
Why alkanes undergo substitution instead of addition?
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Alkanes are saturated, so they cannot add more atoms without breaking their stable single bonds.
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Instead, they replace hydrogen atoms with halogens.
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