Find detailed solutions for Chapter 11 “Hydrocarbons” from the 9th Class Chemistry syllabus. Designed for Punjab Boards and Lahore Board, this guide covers alkanes, alkenes, alkynes, and their applications with step-by-step explanations.
Q1: Which other atom is almost always present along with carbon atoms in all organic compounds?
Options:
(a) Oxygen
(b) Nitrogen
(c) Hydrogen
(d) Halogen
Correct Answer: (c) Hydrogen
Explanation:
Hydrogen is almost always bonded to carbon atoms in organic compounds because it helps satisfy the valency of carbon, forming stable molecules.
Tip: Remember that carbon-hydrogen bonds are the backbone of organic chemistry.
Q2: Which other metal can be used to reduce alkyl halides?
Options:
(a) Al
(b) Mg
(c) Ni
(d) Co
Correct Answer: (b) Mg
Explanation:
Grignard reagents are formed when alkyl halides react with magnesium in dry ether, which is a reduction process.
Tip: Magnesium is crucial for the preparation of organometallic compounds like Grignard reagents.
Q3: If n-heptane undergoes a combustion reaction, what products do you expect to form?
Options:
(a) Alkanes
(b) Alkenes
(c) CO₂ and H₂O
(d) Both alkanes and alkenes
Correct Answer: (c) CO₂ and H₂O
Explanation:
Combustion of hydrocarbons like n-heptane always produces carbon dioxide and water in the presence of excess oxygen.
Tip: Hydrocarbon + Oxygen → CO₂ + H₂O (Exothermic Reaction).
Q4: Why does a mixture of zinc and hydrochloric acid act as a reducing agent?
Options:
(a) Because zinc acts as a reducing agent.
(b) Because atomic hydrogen is produced with Zn/HCl which acts as a reducing agent.
(c) Because molecular hydrogen is produced with Zn/HCl which acts as a reducing agent.
(d) Because chloride ions are produced with Zn/HCl which act as a reducing agent.
Correct Answer: (b) Because atomic hydrogen is produced with Zn/HCl which acts as a reducing agent.
Explanation:
Zinc reacts with HCl to produce hydrogen gas, which in its atomic form is a strong reducing agent.
Tip: Think about how atomic hydrogen is more reactive than molecular hydrogen.
Q5: Which alkane will evolve the most amount of heat when it is burnt with oxygen?
Options:
(a) Ethane
(b) Propane
(c) Butane
(d) Iso-butane
Correct Answer: (c) Butane
Explanation:
The heat evolved in combustion depends on the number of carbon and hydrogen atoms in the alkane. Butane has more C-H bonds, leading to higher energy release.
Tip: Longer chain alkanes release more energy upon combustion.
Q6: Indicate the most reactive hydrocarbon.
Options:
(a) CH≡CH
(b) CH₂=CH₂
(c) CH₃-CH₃
(d) CH₃-CH₂-CH₃
Correct Answer: (a) CH≡CH
Explanation:
Acetylene (CH≡CH) has a triple bond, which is more reactive compared to single and double bonds because of the high energy stored in the triple bond.
Tip: Reactivity order: Triple bond > Double bond > Single bond.
Q7: Which hydrocarbon is responsible for explosions in coal mines?
Options:
(a) Butane
(b) Pentane
(c) Methane
(d) Ethene
Correct Answer: (c) Methane
Explanation:
Methane (CH₄), also called “firedamp,” is a primary cause of explosions in coal mines due to its flammable nature and the formation of explosive mixtures with air.
Tip: Methane-air mixtures are dangerous; always associate methane with safety risks in mining.
Q8: Which product will be formed when ethyl bromide (C₂H₅Br) is treated with Zn/HCl?
Options:
(a) CH₄
(b) C₂H₆
(c) CH₃-CH₂-CH₂-CH₃
(d) CH₃-CH₃
Correct Answer: (b) C₂H₆
Explanation:
Ethyl bromide undergoes reduction with Zn/HCl to produce ethane (C₂H₆).
Tip: Alkyl halides reduce to the corresponding alkane when treated with Zn/HCl.
Multiple-Choice Questions (MCQs)
- Which of the following is not a process of halogenation of alkanes?
- (a) Cracking
- (b) Chlorination
- (c) Bromination
- (d) Iodination
- How many moles of oxygen will be required to completely burn propane?
- (a) 4 moles
- (b) 5 moles
- (c) 3 moles
- (d) 6 moles
Questions for Short Answers
- Differentiate between an organic and an inorganic compound.
Answer: Organic compounds contain carbon and hydrogen, while inorganic compounds typically do not contain both carbon and hydrogen. - Why are organic compounds found in large numbers?
Answer: Carbon atoms can form stable chains and rings, allowing a vast variety of structures and compounds. - Name the products which are obtained when natural gas is oxidized under controlled conditions.
Answer: Carbon dioxide, water, and heat energy are produced. - How is the naphtha fraction decomposed to give lower hydrocarbons?
Answer: By cracking, where larger hydrocarbon molecules are broken down into smaller ones using heat or a catalyst. - Write the molecular formula, structural formula, and condensed structural formula for iso-butane.
Answer:- Molecular formula: C4H10
- How are organic compounds useful for us?
Answer: They are used as fuels, medicines, plastics, and in many industrial and household applications. - Write down the names of five organic compounds that exist naturally.
Answer: Methane, glucose, ethanol, acetic acid, and citric acid. - Give the IUPAC name of the compound:
Structure: CH3-CH(CH3)-CH2-CH3
Answer: 2-Methylbutane - How do the melting and boiling points of alkanes change when we move from lower members to higher members?
Answer: The melting and boiling points increase as the molecular weight increases due to stronger van der Waals forces.
Constructed Response Questions
- Why do alkanes show little reactivity towards other reagents?
Answer: Alkanes have strong C-C and C-H bonds and are non-polar, making them chemically stable and less reactive. - Why does a mixture of natural gas and air explode?
Answer: The mixture forms a highly combustible combination. When ignited, it releases energy rapidly, causing an explosion. - How do you compare the melting and boiling points of inorganic and organic compounds?
Answer: Organic compounds generally have lower melting and boiling points compared to inorganic compounds due to weaker intermolecular forces. - Reactions of alkanes with chlorine take place in the presence of sunlight. What is the role of sunlight in the reaction?
Answer: Sunlight provides energy to break the chlorine molecule (Cl2) into reactive chlorine radicals, initiating the reaction. - How do you compare the boiling point of n-butane with that of iso-butane?
Answer: n-Butane has a higher boiling point than iso-butane due to its linear structure, which allows stronger van der Waals forces compared to the branched structure of iso-butane. - Why are organic compounds not generally soluble in water?
Answer: Organic compounds are non-polar, while water is polar. Due to the “like dissolves like” principle, non-polar substances do not dissolve well in polar solvents like water.
Descriptive Questions
- Describe the importance of organic compounds in daily life.
Answer: Organic compounds are essential in our lives. They are found in food, clothing (cotton, wool, polyester), fuels (petrol, diesel, natural gas), medicines, plastics, cosmetics, and more. They form the basis of modern life. - Why is carbon so important as an element that the whole branch of chemistry is based on it?
Answer: Carbon is unique due to its ability to form stable covalent bonds with other carbon atoms, leading to long chains and complex structures. This versatility makes it the foundation of organic chemistry. - A carbon-carbon single bond (C-C) does not behave as a functional group but a carbon-carbon double bond (C=C) does. Explain.
Answer: A single C-C bond is non-reactive and does not impart specific chemical properties, while a double C=C bond is reactive and contributes to the reactivity of molecules, making it a functional group. - Explain the IUPAC system of nomenclature for alkanes.
Answer: The IUPAC naming system involves:- Identifying the longest continuous carbon chain as the parent hydrocarbon.
- Numbering the chain to give substituents the lowest possible numbers.
- Naming and arranging substituents alphabetically.
- Using prefixes (e.g., di-, tri-) for multiple identical substituents.
- How is the combustion reaction of alkanes useful for us?
Answer: Combustion of alkanes releases energy in the form of heat and light. It is used for generating power, cooking, heating, and running vehicles.
Investigative Questions
- When the natural gas valve is kept open in the kitchen, the gas spreads through the whole kitchen. This may cause an explosion. What is the reason for this explosion, and how can you avoid it?
Answer:- Reason: Natural gas is highly flammable. When mixed with air, it forms an explosive mixture that ignites upon encountering a spark or flame.
- Prevention: Always ensure the gas valve is turned off after use. Regularly check for leaks and ensure proper ventilation in the kitchen.
- Neem is a common tree grown throughout our country. Comment on the medicinal benefits of this tree.
Answer: Neem has antibacterial, antifungal, and anti-inflammatory properties. It is used in traditional medicine to treat skin conditions, boost immunity, and act as a natural pesticide. - Name a few popular medicines which are, in fact, organic compounds.
Answer: Aspirin, paracetamol, ibuprofen, penicillin, and vitamins like vitamin C are all organic compounds used in medicine.
