9th class chemistry solutions

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.

9th Class Chemistry Chapter 5: Energetics Solved Exercises New syllabus

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Looking for complete solved exercises of Chapter 5 new syllabus 2025 Punjab boards: Energetics from the 9th class Chemistry book? Find detailed, step-by-step solutions designed to help students understand core concepts. Download the free PDF and ace your exams with ease.

Question 1:

Tick (✓) the correct answer.

(i) The following reaction is an exothermic reaction:

H2+Cl2→(Sunlight) 2HCl
From where does the energy come to break the bond of H2and Cl2?
Options:
(a) By collisions between the molecules
(b) From sunlight
(c) From the surroundings
(d) By collisions of the molecules with the walls of the container

Correct Answer:
(b) From sunlight

Explanation:
The reaction requires sunlight to break the bonds of H2 and Cl2, as the photons provide the energy needed to overcome the bond energy.

(ii) Which of the following reactions has the least value of activation energy?

Options:
(a) H2(g)+12O2(g)→H2O(g)
(b) C(s)+O2(g)→CO2(g)
(c) NaCl(s)+AgNO3(aq)→AgCl(s)+NaNO3(aq)
(d) H2(g)+I2(g)→2HI(g)

Correct Answer:
(c) NaCl(s)+AgNO3(aq)→AgCl(s)+NaNO3(aq)

Explanation:
The reaction between NaCl and AgNO3 involves ionic compounds, which react readily in aqueous solutions with negligible activation energy, unlike other reactions that require bond-breaking or bond formation.

(iii) Formation of which hydrogen halide from the elements is an endothermic reaction?

Options:
(a) HCl
(b) HF
(c) HBr
(d) HI

Correct Answer:
(d) HI

Explanation:
The formation of HI is endothermic because the bond dissociation energy of iodine is high, requiring more energy to form the product compared to the energy released.

(iv) What are the products of anaerobic respiration?

Options:
(a) ATP + CO2 + H2O
(b) CO2 + H2O
(c) ATP + Ethanol + H2O
(d) Ethanol + H2O

Correct Answer:
(c) ATP + Ethanol + H2O

Explanation:
Anaerobic respiration in organisms such as yeast produces ethanol, water, and ATP, as it occurs in the absence of oxygen.

(v) Which reaction do you expect to be a reversible reaction?

Options:
(a) (Graph showing energy decrease after reaction)
(b) (Graph showing energy increase after reaction)

Correct Answer:
(b)

Explanation:
The graph labeled (b) shows a smaller energy difference between the reactants and products, indicating that the reaction is more likely to reach equilibrium and be reversible.

(vi) What does it show when a chemical reaction is exothermic?

Options:
(a) It shows the bonds which break are weaker than those formed.
(b) It shows the bonds which break are stronger than those formed.
(c) Exothermic nature of the reaction is not concerned with bond formation or bond breakage.
(d) It shows that the reactants are more stable than the products.

Correct Answer:
(a) It shows the bonds which break are weaker than those formed.

Explanation:
In an exothermic reaction, the energy released during bond formation is greater than the energy required to break the bonds. Hence, the bonds formed are stronger than the bonds broken.

(vii) When NaOH and HCl are mixed, the temperature increases. The reaction is:

Options:
(a) Endothermic with a positive enthalpy change.
(b) Endothermic with a negative enthalpy change.
(c) Exothermic with a positive enthalpy change.
(d) Exothermic with a negative enthalpy change.

Correct Answer:
(d) Exothermic with a negative enthalpy change.

Explanation:
The reaction between NaOH and HCl releases heat, causing an increase in temperature. This indicates an exothermic reaction with a negative enthalpy change.

(viii) The average bond dissociation energy for the C-H bond is 412 kJ mol−1. Which of the following processes will have an enthalpy change close to 412 kJ mol−1?

Options:
(a) CH4(g)→CH3(g)+H2(g)
(b) CH4(g)→C2H2(g)+H2(g)
(c) CH4(g)→C5(g)+4H2(g)
(d) CH4(g)→CH3(g)+H(g)

Correct Answer:
(d) CH4(g)→CH3(g)+H(g)
The process involves breaking one C-H bond, with the enthalpy change equal to the bond dissociation energy (412 kJ mol−1.

(ix) The average bond energies for O-O and O=O are 146 kJ mol−1and 496 kJ mol−1, respectively. Find the enthalpy in kJ for the following reaction:

H−O−O-H →H−O-H +1/2O2(g)
Options:
(a) -102 kJ
(b) +102 kJ
(c) +350 kJ
(d) +394 kJ

Correct Answer:
(a) -102 kJ

Explanation:
The reaction involves breaking one O-O bond (146 kJ) and forming a half O=O bond (496/2 = 248 kJ).
Enthalpy = Energy of bonds broken – Energy of bonds formed
= 146 – 248
= -102 kJ.

Question 1 (x):

Why does the following exothermic reaction not occur?
C (Diamond)→C (Graphite)ΔH=−3 kJ mol−1
Options:
(a) Structure of diamond is more stable than that of graphite.
(b) Diamond has strong covalent bonds than does the graphite.
(c) The change from diamond to graphite has high activation energy.
(d) Density of graphite is less than that of diamond.

Correct Answer:
(c) The change from diamond to graphite has high activation energy.

Explanation:
Even though the reaction is exothermic, it does not occur easily because breaking the strong bonds in diamond requires a lot of energy, which makes the activation energy very high.

2. Questions for Short Answers:

i. What is the difference between enthalpy and enthalpy change?

  • Enthalpy (H): It is the total heat content of a system.
  • Enthalpy Change (ΔH): It is the difference in enthalpy between the products and reactants during a reaction.

ii. Why is breaking of a bond an endothermic process?

  • Breaking a bond requires energy to overcome the attractive forces holding the atoms together. This energy input makes it an endothermic process.

iii. Depict the transition state for the following reaction: H2+Cl2→2HCl

  • The transition state is the highest energy point during the reaction. For this reaction, it is when the bonds in H2 and Cl2 are partially broken, and new H-Cl bonds are partially formed.

iv. Draw the reaction profiles for two exothermic reactions, one of which moves faster than the other.

  • A faster reaction will have a lower activation energy, and both will release energy (negative ΔH).

v. What is the role of glycogen in our body?

  • Glycogen is the storage form of glucose in the body. It provides energy by breaking down into glucose when required.

3. Constructed Response Questions:

i. Physical changes which usually occur around us are given in the table. Write down whether they are exothermic or endothermic.

Physical ChangeExothermic or Endothermic
Conversion of hydrated salt into anhydrous saltEndothermic
Burning paperExothermic
Vaporizing liquid nitrogenEndothermic
Evaporation of dry iceEndothermic
Conduction of electricity by metalsNeither (No heat change)
Dissolving ammonium chloride in waterEndothermic
Formation of rain from cloudsExothermic
Dissolving sodium carbonate in waterExothermic

Q1 (i): Explain why the reaction between atmospheric gases oxygen and nitrogen does not take place under normal conditions. But in the presence of lightning, these gases react to give NO. The reaction stops as soon as the lightning stops.

Answer: Under normal conditions, oxygen (O₂) and nitrogen (N₂) in the atmosphere do not react because of the high activation energy required to break their strong bonds (triple bond in N₂ and double bond in O₂). However, during lightning, the intense heat and energy provide sufficient activation energy to break these bonds, enabling the formation of nitrogen monoxide (NO) through the reaction: N2+O2→2NO

As soon as the lightning stops, the energy source is no longer available, and the reaction ceases because the required activation energy cannot be sustained under normal atmospheric conditions.

Q1 (ii): Explain why a reaction between natural gas (CH₄) and atmospheric oxygen does not take place when you mix them. As soon as you show a burning matchstick, the reaction starts immediately and continues until one or both of the reactants are used up.

Answer: Natural gas (CH₄) and oxygen (O₂) require activation energy to overcome the energy barrier for the combustion reaction. Mixing these gases does not provide sufficient energy for the reaction to start. A burning matchstick acts as an external source of activation energy, initiating the reaction. Once started, the reaction becomes self-sustaining due to the release of heat energy during combustion, which provides the necessary energy to continue until one or both reactants are consumed.

The reaction is: CH4+2O2→CO2+2H2O+energy (heat)

Q4 (i): Find out the enthalpy change of the reaction: N2+O2→2NO

Given data:

  • Bond dissociation energy of N2: 958.38 kJ/mol
  • Bond dissociation energy of O2: 498 kJ/mol
  • Bond formation energy of NO: -626 kJ/mol

Solution: ΔH=Total energy required to break bonds−Total energy released in forming bonds

  1. Energy to break bonds:

Breaking 1 mol of N2:958.38 kJ/mol

Breaking 1 mol of O2:498 kJ/mol

Total energy required = 958.38+498=1456.38 kJ

  1. Energy released in forming bonds:

Forming 2 mol of NO:2×(−626)=−1252 kJ

  1. Enthalpy change:

ΔH=1456.38−1252=204.38 kJ

Answer: The enthalpy change of the reaction is +204.38 kJ (endothermic process).

Q4 (ii): Explain the difference between the terms heat and enthalpy.

Answer:

  • Heat: Heat is the transfer of thermal energy between a system and its surroundings due to a temperature difference. It is a path-dependent quantity and depends on the conditions of the transfer.
  • Enthalpy: Enthalpy (HH) is a thermodynamic property of a system that represents the total heat content. It is defined as:

H=U+PV

where UU is the internal energy, PP is pressure, and VV is volume. Enthalpy changes (ΔH) occur during chemical reactions and are measured under constant pressure.

Q4 (iii): Explain why the formation of a bond is always an exothermic process.

Answer: The formation of a chemical bond releases energy because atoms move to a lower-energy, more stable state when bonded. The energy released during bond formation is the same as the energy required to break the bond. This release of energy is what makes bond formation an exothermic process.

Q4 (iv): Explain the role of lipids in our body.

Answer: Lipids play several crucial roles in the body:

  1. Energy Storage: Lipids are a dense source of energy, providing more than twice the energy per gram compared to carbohydrates or proteins.
  2. Insulation and Protection: They act as thermal insulators and cushion organs against physical shocks.
  3. Cell Membranes: Lipids, particularly phospholipids, form the structural components of cell membranes.
  4. Hormone Production: Lipids are precursors for hormones like steroids.
  5. Absorption of Vitamins: They help in the absorption of fat-soluble vitamins (A, D, E, K).

Q4 (v): Explain the following terms.

  1. Activation Energy: The minimum energy required to start a chemical reaction by breaking bonds in the reactants.
  2. Transition State: A high-energy, unstable state during a reaction in which reactants are partially converted into products.
  3. Aerobic Respiration: A metabolic process in which cells use oxygen to convert glucose into energy (ATP), producing carbon dioxide and water as byproducts.

Q5 (i): Why is it essential to cook some food items while others can be eaten without cooking?

Answer: Cooking is essential for certain food items because it:

  1. Breaks Down Tough Structures: Cooking softens cellulose in plant cells and denatures proteins, making food easier to digest.
  2. Destroys Harmful Microorganisms: Cooking kills bacteria and pathogens, ensuring food safety.
  3. Improves Taste and Texture: It enhances the flavor and makes food more palatable. Some foods (like fruits and vegetables) can be eaten raw because they are naturally safe and digestible without cooking.

Q5 (ii): Why do fireworks look spectacular? What type of chemical compounds undergo reactions during this activity?

Answer: Fireworks look spectacular due to the bright colors and light effects caused by chemical reactions. Metal salts, like:

  • Strontium compounds (red)
  • Barium compounds (green)
  • Sodium compounds (yellow)

These compounds release energy as light of specific wavelengths when they are heated. The combustion of black powder (a mixture of potassium nitrate, sulfur, and charcoal) produces the heat required to excite these metal ions, creating the vibrant displays.

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