Chemistry for Grade 9

Chapter 11: Hydrocarbons Exercise- 9th Class (Punjab & Lahore Board)

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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)

  1. Which of the following is not a process of halogenation of alkanes?
    • (a) Cracking
    • (b) Chlorination
    • (c) Bromination
    • (d) Iodination
    Answer: (a) Cracking
  2. How many moles of oxygen will be required to completely burn propane?
    • (a) 4 moles
    • (b) 5 moles
    • (c) 3 moles
    • (d) 6 moles
    Answer: (b) 5 moles

Questions for Short Answers

  1. 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.
  2. 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.
  3. Name the products which are obtained when natural gas is oxidized under controlled conditions.
    Answer: Carbon dioxide, water, and heat energy are produced.
  4. 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.
  5. Write the molecular formula, structural formula, and condensed structural formula for iso-butane.
    Answer:
    • Molecular formula: C4H10
structural formula of isobutane
  1. How are organic compounds useful for us?
    Answer: They are used as fuels, medicines, plastics, and in many industrial and household applications.
  2. Write down the names of five organic compounds that exist naturally.
    Answer: Methane, glucose, ethanol, acetic acid, and citric acid.
  3. Give the IUPAC name of the compound:
    Structure: CH3-CH(CH3)-CH2-CH3
    Answer: 2-Methylbutane
  4. 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

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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.
  6. 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

  1. 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.
  2. 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.
  3. 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.
  4. 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.
  5. 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

  1. 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.
  2. 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.
  3. 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.

Chapter 10: Environmental Chemistry – 9th Class New Syllabus (Punjab Boards & Lahore Board)

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Explore Chapter 10 “Environmental Chemistry” from the 9th Class new syllabus for Punjab Boards and Lahore Board. Understand air pollution, greenhouse effect, acid rain, and strategies to protect the environment with easy-to-learn explanations.

Multiple Choice Questions

(i) Which gases are responsible for the greenhouse effect?
Options:
(a) SO₂, NO
(b) CO₂, CO
(c) CO₂, CH₄
(d) O₂, O₃
Correct Answer: (c) CO₂, CH₄
Explanation: Carbon dioxide (CO₂) and methane (CH₄) are major greenhouse gases that trap heat in the Earth’s atmosphere, leading to the greenhouse effect.
Tip: Always associate greenhouse gases with their ability to trap heat (e.g., CO₂, CH₄, N₂O). Gases like O₂ and O₃ don’t significantly contribute to this effect.

(ii) Indicate the source of sulfur which is responsible for the presence of oxides of sulfur in the atmosphere.
Options:
(a) Decomposition of vegetation
(b) Waste gases from digestion of animals
(c) Photochemical smog
(d) Combustion of fossil fuels
Correct Answer: (d) Combustion of fossil fuels
Explanation: Burning fossil fuels (coal, oil, etc.) releases sulfur dioxide (SO₂) and other sulfur oxides into the atmosphere.
Tip: Think of industrial processes and power plants as key contributors to sulfur pollution.

(iii) Concentration of which gas in the atmosphere is decreased by photosynthesis in plants?
Options:
(a) Oxygen
(b) Nitrogen
(c) Carbon dioxide
(d) Water vapor
Correct Answer: (c) Carbon dioxide
Explanation: Plants absorb CO₂ during photosynthesis to produce oxygen and glucose.
Tip: Photosynthesis reduces CO₂, not oxygen or water vapor, in the environment.

(iv) Which substance remains unaffected in the catalytic converter fixed in the exhaust of automobiles?
Options:
(a) CO₂
(b) CO
(c) NO
(d) NO₂
Correct Answer: (a) CO₂
Explanation: Catalytic converters reduce pollutants like CO, NO, and hydrocarbons but do not affect CO₂, which is not harmful in small amounts.
Tip: Remember that catalytic converters aim to reduce harmful emissions but allow CO₂ to pass through.

(v) People of which age groups are the most affected by air pollution?
Options:
(a) Young adults
(b) Cancer patients
(c) Children
(d) Both children and aged people
Correct Answer: (d) Both children and aged people
Explanation: Children and the elderly have weaker immune and respiratory systems, making them more vulnerable to air pollution.
Tip: When asked about susceptibility, think of the most physically vulnerable groups (young and old).

(vi) In which area there is a greater possibility of acid rain?
Options:
(a) Around villages
(b) Around big cities
(c) Around industrial areas
(d) Around water bodies
Correct Answer: (c) Around industrial areas
Explanation: Industrial areas release large amounts of sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), which cause acid rain when mixed with water in the atmosphere.
Tip: Acid rain is linked to industrial emissions, so look for industrial-related options.

(vii) Why is smog not felt in summer?
Options:
(a) Because fog is not present in summer
(b) Because due to heat of the earth the smoke rises up
(c) Because in summer smoke and fog cannot mix with each other
(d) Because less fossil fuels are burnt in summer
Correct Answer: (b) Because due to heat of the earth the smoke rises up
Explanation: In summer, higher temperatures cause pollutants to rise higher into the atmosphere, reducing the formation of smog near the ground.
Tip: Smog is more common in winter due to temperature inversion.

(viii) Which catalyst is used in the catalytic converter fixed in the exhaust systems of automobiles?
Options:
(a) Ni
(b) Cu
(c) Pt, Pd and Rh
(d) CaO
Correct Answer: (c) Pt, Pd and Rh
Explanation: Platinum (Pt), Palladium (Pd), and Rhodium (Rh) are used in catalytic converters to reduce harmful vehicle emissions.
Tip: Remember the precious metals (Pt, Pd, Rh) for catalytic converters.

(ix) Which components are essential for the formation of photochemical smog?
Options:
(a) CO, NO₂, CO₂
(b) NO₂, volatile organic compounds, sunlight
(c) CO₂, NO₂, sunlight
(d) Volatile organic compounds, NO₂, CO
Correct Answer: (b) NO₂, volatile organic compounds, sunlight
Explanation: Photochemical smog forms when sunlight reacts with nitrogen oxides (NOₓ) and volatile organic compounds (VOCs).
Tip: Photochemical smog always involves sunlight, NO₂, and VOCs.

(x) Which air pollutant can cause respiratory problems?
Options:
(a) CO₂
(b) Ozone
(c) Smog
(d) CO
Correct Answer: (c) Smog
Explanation: Smog contains harmful pollutants like particulate matter and ground-level ozone, which irritate the respiratory system.
Tip: Respiratory issues are typically associated with smog or particulate pollutants.

Short Questions

1. What is the main objective of environmental education?
To make people aware of environmental problems and teach them how to protect nature and use resources responsibly.

2. How is particulate matter released in the atmosphere?
Particulate matter is released through vehicle emissions, industrial activities, burning of fossil fuels, and construction activities.

3. Which gas is more poisonous, CO₂ or CO?
Carbon monoxide (CO) is more poisonous because it binds to hemoglobin in blood, reducing oxygen supply to the body.

4. How does acid rain affect forests?
Acid rain damages tree leaves, weakens roots, and makes soil acidic, harming plant growth.

5. In what way is sulfur present in fossil fuels dangerous?
When fossil fuels are burned, sulfur is released as sulfur dioxide (SO₂), which can cause acid rain and respiratory problems.

6. Name any three major sources responsible for the greenhouse effect.
The major sources are:

  • Burning of fossil fuels (CO₂)
  • Agriculture (CH₄ from livestock)
  • Deforestation (less CO₂ absorption).

7. How is geothermal energy useful for us?
Geothermal energy is used to generate electricity and provide heating by using heat from inside the Earth.

Constructed Response Questions

i. How is the excessive use of insecticides and pesticides harmful for birds?
Insecticides and pesticides contain harmful chemicals that can enter the food chain. When birds eat insects or seeds contaminated with these chemicals, they can get poisoned. Pesticides can weaken birds’ immune systems, affect their reproduction by thinning eggshells, and even lead to their death. Overuse of such chemicals also reduces the availability of insects that birds feed on, disrupting their natural diet.

ii. Percentage of CO₂ in air is only 0.03%. Then how does it become harmful for the ecosystem?
Even though CO₂ is present in small amounts, it plays a significant role in trapping heat in the Earth’s atmosphere. Excess CO₂ from activities like burning fossil fuels causes global warming, leading to climate change. This disrupts weather patterns, melts glaciers, raises sea levels, and affects ecosystems, harming plants, animals, and humans. It also makes oceans acidic, threatening marine life.

iii. Why only some pollutant gases present in the atmosphere cause greenhouse effect while others do not?
Gases like CO₂, CH₄ (methane), and N₂O absorb heat and trap it in the atmosphere, causing the greenhouse effect. These gases are called greenhouse gases because they allow sunlight to enter the Earth but prevent heat from escaping. Other gases, like nitrogen (N₂) and oxygen (O₂), do not have this property, so they don’t contribute to the greenhouse effect.

iv. How can you reduce the emission of CO present in the gases emitted by the burning of fuel in automobile engines?
To reduce CO emissions:

  1. Use catalytic converters in vehicles to convert CO into less harmful gases.
  2. Use cleaner fuels like compressed natural gas (CNG) or electric vehicles.
  3. Maintain vehicles regularly to ensure efficient fuel burning.
  4. Carpooling and public transport reduce the number of vehicles on the road, lowering emissions.

v. Mention three different ways in which solar energy can be useful for us.

  1. Electricity Generation: Solar panels convert sunlight into electricity for homes, schools, and industries.
  2. Heating: Solar energy can be used to heat water and buildings, reducing the use of gas or electricity.
  3. Cooking: Solar cookers use sunlight to prepare food without using gas or electricity, making them eco-friendly.

Descriptive Questions

i. Describe the harmful effects of the major pollutants present in the air.

  • Carbon monoxide (CO): Reduces oxygen supply to the body, causing dizziness, fatigue, and even death.
  • Sulfur dioxide (SO₂): Causes acid rain, damages plants, and irritates the respiratory system.
  • Nitrogen oxides (NOₓ): Contribute to smog and acid rain and affect lung function.
  • Particulate matter (PM): Tiny particles can enter the lungs, causing breathing issues, asthma, and heart problems.
  • Ozone (O₃): At ground level, it causes respiratory problems and harms crops and materials.

ii. Explain greenhouse effect. How is global warming dangerous for us?
The greenhouse effect is the process by which certain gases (like CO₂, CH₄, and N₂O) trap heat in the Earth’s atmosphere, keeping the planet warm. This is essential for life, but excessive greenhouse gases lead to global warming.
Global warming is dangerous because it:

  • Causes extreme weather events like hurricanes and floods.
  • Melts glaciers, raising sea levels and flooding coastal areas.
  • Disrupts ecosystems, endangering animals and plants.
  • Makes some regions too hot or dry for farming, leading to food shortages.

iii. What is air quality index? What information does it convey?
The Air Quality Index (AQI) measures the quality of air in a specific area and tells us how clean or polluted the air is. It assigns a score or color based on pollutants like CO, NO₂, PM, and O₃.

  • Good AQI: Air is clean and safe to breathe.
  • Poor AQI: Air is polluted and can harm health, especially for sensitive groups like children and the elderly.

iv. Who are at high risk groups and why is pollution more dangerous for them?
High-risk groups include:

  • Children: Their lungs are still developing, and they breathe faster than adults, so they inhale more polluted air.
  • Elderly people: They often have weaker immune systems and pre-existing health problems like asthma or heart disease.
  • Pregnant women: Pollution can harm the developing baby.
  • People with respiratory issues: They are more sensitive to pollutants, which can worsen their conditions.

v. Describe three strategies to address environmental issues.

  1. Reduce Pollution: Use renewable energy sources (solar, wind) and improve waste management to reduce air, water, and land pollution.
  2. Conservation of Resources: Save water, use energy-efficient appliances, and plant more trees to conserve natural resources.
  3. Raise Awareness: Educate people about the importance of protecting the environment through schools, campaigns, and community programs.

Investigative Questions

i. Major Pakistani cities experience a very high AQI in winter. Point out some of the major causes of high AQI in these cities.

  • Vehicle Emissions: Increased use of vehicles releases pollutants like CO, NOₓ, and particulate matter.
  • Industrial Emissions: Factories release smoke containing SO₂ and other harmful gases.
  • Burning of Crop Residue: Farmers burn crop stubble, which adds to air pollution.
  • Temperature Inversion: In winter, cooler air traps pollutants near the ground, worsening air quality.

ii. Why does AQI not rise in Pakistan in hot days of summer?
In summer, higher temperatures and winds disperse pollutants, preventing them from accumulating near the ground. Unlike winter, there is no temperature inversion to trap pollutants.

iii. How has climate change affected Pakistan during the last five years?

  • Increased Floods: Heavy rains and melting glaciers have caused severe flooding, displacing millions of people.
  • Heatwaves: High temperatures have caused heat-related illnesses and deaths.
  • Droughts: Some regions face water shortages due to irregular rainfall.
  • Crop Damage: Changing weather patterns have affected agricultural production, leading to food insecurity.

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