10th Class Chemistry Notes 2026 — Chapter 22
Class 10 Chemistry — Chapter 22: Hydrocarbons
Quran Education Point
Board: Punjab Board
A. Multiple Choice Questions (MCQs)
22.1 Which of the following compounds is expected to give an addition reaction?
(a) CH₃−CH₃
(b) CH₂=CH₂
(c) CH₄
(d) CH₃−CH₂−CH₃
Correct Answer: (b) CH₂=CH₂
22.2 Which of the following is not used as a fuel?
(a) LPG
(b) CNG
(c) Diesel
(d) Asphalt
Correct Answer: (d) Asphalt
Asphalt or bitumen is mainly used for road surfacing and roofing rather than as a conventional fuel.
22.3 Identify the type of reaction.
(a) Substitution
(b) Oxidation
(c) Reduction
(d) Addition
Correct Answer: (b) Oxidation
22.4 Which species acts as a reducing agent in the reaction between ethene and KMnO₄?
(a) H₂O
(b) CH₂=CH₂
(c) KMnO₄
(d) NaOH
Correct Answer: (b) CH₂=CH₂
Ethene acts as a reducing agent because it is oxidized during the reaction.
22.5 Which product is formed when propene reacts with a bromine molecule?
(a) 1,2-Dibromopropane
(b) 1,1-Dibromopropane
(c) 2,3-Dibromopropane
(d) 1,3-Dibromopropane
Correct Answer: (a) 1,2-Dibromopropane
22.6 Which petroleum fraction is used to generate electricity in power plants?
(a) Bitumen
(b) Lubricating oil
(c) Kerosene oil
(d) Fuel oil
Correct Answer: (d) Fuel oil
22.7 Which molecule is eliminated in the following reaction?
(a) Br₂
(b) H₂
(c) HBr
(d) H₂O
Correct Answer: (d) H₂O
In this elimination reaction, hydrogen and bromine are removed from adjacent carbon atoms, and the products include water and potassium bromide.
22.8 Fractional distillation of petroleum is based on:
(a) Viscosity
(b) Boiling point range
(c) Flammability
(d) Melting point range
Correct Answer: (b) Boiling point range
B. Short Answer Questions
22.1 Give two examples of addition reactions.
1. Hydrogenation of ethene
Hydrogen adds across the double bond of ethene to form ethane.
2. Halogenation of ethene
Bromine adds across the double bond of ethene to form 1,2-dibromoethane.
22.2 Give the names of compounds obtained from coal tar and petroleum.
From coal tar:
Benzene
Toluene
Phenol
Naphthalene
Anthracene
From petroleum:
Gasoline (petrol)
Kerosene
Diesel oil
Fuel oil
Lubricating oils
Paraffin wax
22.3 How is ethene oxidized by a cold aqueous solution of potassium permanganate?
Ethene reacts with a cold, dilute alkaline solution of potassium permanganate, known as Baeyer’s reagent. Ethene is oxidized to ethane-1,2-diol (ethylene glycol), while the purple permanganate solution produces a brown precipitate of manganese dioxide.
22.4 Differentiate between addition and elimination reactions.
Addition reaction: Two reactants combine to form a single product. It commonly occurs in unsaturated hydrocarbons when atoms or groups add across a double or triple bond.
Example:
Elimination reaction: Atoms or groups are removed from adjacent carbon atoms, resulting in the formation of a multiple bond.
Example:
22.5 Name the starting materials used to generate an oxyacetylene flame.
Calcium carbide (CaC₂) reacts with water to produce ethyne (acetylene). The ethyne is then burned in pure oxygen to produce the high-temperature oxyacetylene flame.
22.6 How can an alkene be identified?
An alkene can be identified using the following tests:
1. Bromine water test: An alkene decolourizes reddish-brown bromine water because bromine adds across the carbon–carbon double bond.
2. Baeyer’s test: An alkene decolourizes cold, dilute alkaline potassium permanganate solution, producing a brown precipitate of manganese dioxide.
C. Constructed Response Questions
22.1 What is the difference between dehydration and dehydrohalogenation?
Dehydration: The elimination of a water molecule (H₂O) from an alcohol, usually by heating it with a concentrated acid such as sulphuric acid.
Example:
Dehydrohalogenation: The elimination of a hydrogen halide (HX) from an alkyl halide, usually by heating it with an alcoholic solution of a base such as KOH.
Example:
22.2 You have two test tubes, A and B, containing a saturated compound and an unsaturated compound (alkene). How will you identify them?
Add a few drops of reddish-brown bromine water to each test tube.
The test tube containing the alkene will rapidly lose the reddish-brown colour because bromine adds across the double bond.
The test tube containing the saturated hydrocarbon will show no visible colour change under ordinary dark conditions.
Therefore, the test tube that decolourizes bromine water contains the unsaturated compound.
22.3 Why is the oxyacetylene flame so hot?
Ethyne (acetylene) burns in pure oxygen and releases a large amount of heat. The combustion produces a very high temperature, reaching approximately 3,300°C under suitable conditions. This makes the oxyacetylene flame useful for welding and cutting metals.
22.4 Do you expect ethyne to decolourize a cold solution of KMnO₄?
Yes. Ethyne is an unsaturated hydrocarbon containing a carbon–carbon triple bond. It reacts with cold, dilute alkaline potassium permanganate, causing the purple colour to disappear and forming a brown precipitate of manganese dioxide.
22.5 How do chlorine and hydrogen chloride add to ethene?
1. Addition of chlorine
Chlorine adds across the double bond of ethene to form 1,2-dichloroethane.
2. Addition of hydrogen chloride
Hydrogen chloride adds across the double bond of ethene to form chloroethane.
D. Descriptive Questions
22.1 Describe two methods of preparation of ethene.
1. Dehydration of ethanol
Ethene is prepared by heating ethanol with concentrated sulphuric acid at approximately 170°C. A water molecule is eliminated from ethanol.
2. Dehydrohalogenation of ethyl bromide
Ethyl bromide is heated with alcoholic potassium hydroxide. Hydrogen and bromine are eliminated from adjacent carbon atoms, producing ethene.
22.2 Describe the fractional distillation of petroleum.
Fractional distillation separates crude petroleum into useful fractions according to their boiling point ranges.
Crude petroleum is heated in a furnace, and the resulting mixture of vapours and liquids enters a fractionating column.
The column is hot at the bottom and progressively cooler towards the top.
Hydrocarbons with higher boiling points condense in the lower part of the column.
Hydrocarbons with lower boiling points rise higher before condensing.
The different fractions are collected separately at different levels of the column.
Examples of petroleum fractions include refinery gases, gasoline, kerosene, diesel, fuel oil, lubricating oil, and bitumen.
22.3 Which properties of petroleum fractions change with their boiling points?
As the boiling point of petroleum fractions generally increases:
Carbon-chain length: Increases.
Viscosity: Increases; the fractions become thicker.
Volatility: Decreases; the fractions evaporate less readily.
Flammability: Generally decreases; heavier fractions are less easily ignited than lighter fractions.
22.4 Explain the cracking of alkanes with examples.
Cracking is the process in which large hydrocarbon molecules are broken down into smaller, more useful hydrocarbons by heating, often in the presence of a catalyst.
Cracking produces smaller alkanes and alkenes that are useful as fuels and as raw materials for the chemical industry.
Example: Cracking of dodecane into heptane and pentene.
22.5 Give the uses of acetylene (ethyne).
It is used in oxyacetylene torches for welding and cutting metals.
It has been used as a starting material in the manufacture of various organic chemicals.
It can be used to prepare certain industrial chemicals and polymers through appropriate chemical processes.
Calcium carbide reacting with water produces acetylene, which has also been used in carbide lamps.
Notes by Sir Dilawar
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