Class 9 Chemistry Notes – Chapter 11

Hydrocarbons

Textbook Exercise Solutions

1. Tick (✓) the Correct Answer

(i) Which other atom is almost always present along with carbon atom in all organic compounds?

Answer: (c) Hydrogen

(ii) Which other metal can be used to reduce alkyl halides?

Answer: (b) Mg

(iii) If naphtha undergoes a combustion reaction, what products do you expect it to form?

Answer: (c) CO₂ and H₂O

(iv) Why does a mixture of zinc and hydrochloric acid act as a reducing agent?

Answer: (b) Because atomic hydrogen is produced with Zn/HCl, which acts as a reducing agent.

(v) Which alkane will evolve the most amount of heat when it is burnt with oxygen?

Answer: (c) n-Butane (a higher carbon chain length releases more heat per mole).

(vi) Which reaction is not given by alkanes?

Answer: (c) Addition (alkanes are saturated and generally undergo substitution rather than addition reactions).

(vii) Which hydrocarbon is responsible for explosions in coal mines?

(viii) Which product will be formed when ethyl bromide (CH₃–CH₂–Br) is treated with Zn/HCl?

Answer: (b) CH₃–CH₃

(ix) Which of the following is not a process of halogenation of alkanes?

Answer: (d) Iodination

(x) How many moles of oxygen will be required to completely burn propane?

Answer: (b) 5 moles

Equation:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

2. Questions for Short Answers

(i) Differentiate between an organic and an inorganic compound.

Answer: Organic compounds are primarily composed of carbon and hydrogen, along with their derivatives, and generally contain covalent bonds. Many have relatively low melting and boiling points and do not conduct electricity in aqueous solution. Inorganic compounds include many substances derived from minerals and may contain ionic or covalent bonds. Their physical properties and electrical conductivity vary according to their structures and bonding.

(ii) Why are organic compounds found in large numbers?

Answer: Organic compounds exist in millions due to catenation, which is carbon’s ability to form long chains and rings through carbon–carbon bonds, and isomerism, which allows compounds to have different structural arrangements despite having the same molecular formula.

(iii) Name the products which are obtained when natural gas is oxidized under controlled conditions.

Answer: Controlled oxidation of methane, the main component of natural gas, can yield methanol, formaldehyde (methanal), or other products, depending on the reaction conditions and catalysts used.

(iv) How is the naphtha fraction decomposed to give lower hydrocarbons?

Answer: Naphtha undergoes thermal or catalytic cracking. In this process, larger hydrocarbon molecules are heated at high temperatures, often in the presence of a catalyst, and broken down into smaller hydrocarbons, such as lower alkanes and alkenes.

(v) Write down the molecular formula, structural formula, and condensed structural formula for iso-butane.

Answer:

Molecular Formula: C₄H₁₀

Structural Formula:

    CH₃
     |
CH₃—CH—CH₃

Condensed Structural Formula: (CH₃)₃CH

(vi) How are organic compounds useful for us?

Answer: Organic compounds form the basis of life and modern industry. They are used as fuels, such as petroleum and natural gas; food nutrients, such as carbohydrates, proteins, and fats; materials for fabrics, such as cotton and nylon; medicines, such as antibiotics and painkillers; and products such as plastics.

(vii) Write down the names of five organic compounds or substances that exist naturally.

Answer:

1. Cellulose

2. Glucose

3. Starch

4. Natural rubber

5. Methane

(viii) Give the IUPAC name of the following compound:

CH₃–CH(CH₃)–CH₂–CH₃

Answer: 2-Methylbutane

(ix) How do the melting and boiling points of alkanes change when we move from lower members to higher members?

Answer: The boiling points of alkanes generally increase as we move from lower to higher members because increasing molecular size and surface area strengthen London dispersion forces between molecules. Melting points generally show an overall increasing trend, although they do not increase uniformly because molecular symmetry and crystal packing also affect them.

3. Constructed Response Questions

(i) Why do alkanes show little reactivity towards other reagents?

Answer: Alkanes are often called paraffins, meaning “little affinity,” because they are saturated hydrocarbons containing strong, nearly non-polar carbon–carbon (C–C) and carbon–hydrogen (C–H) sigma bonds. These bonds require considerable activation energy to break, making alkanes relatively unreactive under ordinary conditions.

(ii) Why does a mixture of natural gas and air explode?

Answer: Natural gas, which mainly consists of methane, forms a combustible mixture when mixed with air in suitable proportions. When ignited, it undergoes rapid, highly exothermic combustion, releasing heat and expanding gases quickly. In an enclosed space, this rapid expansion can cause an explosion.

(iii) How do you compare the melting and boiling points of inorganic and organic compounds?

Answer: Many ionic inorganic compounds have high melting and boiling points because strong electrostatic forces hold their ions together. Many molecular organic compounds have lower melting and boiling points because the intermolecular forces between their molecules are relatively weak. However, these properties depend on the type of bonding, molecular structure, and size of the compounds.

(iv) Reactions of alkanes with chlorine take place in the presence of sunlight. What is the role of sunlight in the reaction?

Answer: Sunlight, particularly ultraviolet radiation, provides the energy needed to split chlorine molecules (Cl₂) homolytically into highly reactive chlorine free radicals (Cl•). These radicals initiate the free-radical substitution chain reaction between chlorine and an alkane.

Cl₂ → 2Cl•

(v) How do you compare the boiling point of n-butane with that of iso-butane?

Answer: n-Butane has a higher boiling point, approximately −0.5°C, than iso-butane, approximately −11.7°C. The straight-chain structure of n-butane provides greater surface contact between molecules, resulting in stronger London dispersion forces than those between the more compact, branched iso-butane molecules.

(vi) Why are organic compounds not generally soluble in water?

Answer: Many organic compounds are non-polar, whereas water is a polar solvent. According to the principle “like dissolves like,” non-polar compounds generally do not dissolve readily in water because they cannot form sufficiently strong interactions with water molecules. However, some organic compounds, such as alcohols and sugars, are water-soluble because they contain polar groups.

4. Descriptive Questions

(i) Describe the importance of organic compounds in daily life.

Answer: Organic compounds are essential in almost every aspect of daily life. Foods contain organic substances such as sugars, lipids, and proteins. Clothing is made from materials such as cotton, wool, and polyester. Medicines include aspirin and penicillin. Fuels such as gasoline and diesel are mixtures of hydrocarbons. Plastics, adhesives, cosmetics, detergents, and many other everyday products are also made from organic compounds.

(ii) Why is carbon so important as an element that a whole branch of chemistry is based on it?

Answer: Carbon is important because of its tetravalency and its unique ability to undergo catenation. It can form four covalent bonds and bond with itself and other elements, including hydrogen, oxygen, nitrogen, and halogens. This enables carbon to form millions of stable compounds with linear, branched, and cyclic structures. The study of these compounds forms the basis of organic chemistry.

(iii) A carbon–carbon single bond (C–C) in alkanes does not behave as a functional group, but a carbon–carbon double bond (C=C) in alkenes does. Explain.

Answer: A carbon–carbon single bond consists of one strong sigma (σ) bond, which is relatively stable and generally less reactive. A carbon–carbon double bond consists of one sigma bond and one pi (π) bond. The pi bond is more exposed and more easily broken during chemical reactions. Therefore, the double bond is the reactive site in alkenes and is responsible for their characteristic addition reactions.

(iv) Explain the IUPAC system of nomenclature for alkanes.

Answer: The IUPAC system for naming alkanes follows these steps:

1. Find the longest continuous carbon chain to determine the parent alkane name.

2. Number the chain from the end nearest to the first substituent to give the substituents the lowest possible numbers.

3. Identify the substituents, such as methyl and ethyl groups, and note their positions.

4. If identical substituents occur more than once, use prefixes such as di-, tri-, and tetra-.

5. Arrange different substituent names alphabetically, ignoring multiplicative prefixes when alphabetizing.

6. Write the complete name using commas between numbers and hyphens between numbers and words.

Example: CH₃–CH(CH₃)–CH₂–CH₃ is named 2-methylbutane.

(v) How is the combustion reaction of alkanes useful to us?

Answer: The complete combustion of alkanes is highly exothermic and releases a large amount of heat energy. This energy is used to power internal combustion engines, heat homes, generate electricity in power plants, and operate cooking stoves.

Example:

CH₄ + 2O₂ → CO₂ + 2H₂O + Heat

5. Investigative Questions

(i) When a natural gas valve is left open in the kitchen, the gas spreads throughout the kitchen. This may cause an explosion. What is the reason for this explosion, and how can you avoid it?

Answer: Natural gas, which mainly contains methane, mixes with oxygen in the air. If the gas concentration reaches a combustible range, even a small spark can ignite the mixture and cause rapid combustion. In an enclosed space, the sudden expansion of hot gases can produce an explosion.

Preventive Measures:

1. Immediately turn off the gas supply if it is safe to do so.

2. Do not operate electrical switches, appliances, or lighters.

3. Avoid creating sparks or flames.

4. Leave the area and alert others.

5. From a safe location, contact the gas emergency service or relevant authorities.

6. Have the area checked by a qualified professional before using the gas supply again.

(ii) Neem is a common tree grown throughout our country. Comment on the medicinal benefits of this tree.

Answer: Neem (Azadirachta indica) contains various bioactive organic compounds, including azadirachtin. Neem extracts have been investigated for antibacterial, antifungal, anti-inflammatory, and insecticidal properties. Neem products are traditionally used in skin care, dental hygiene, and pest control. However, neem preparations are not a substitute for proven medical treatment, and some products, particularly neem oil when swallowed, can be toxic.

(iii) Name a few popular medicines which are, in fact, organic compounds.

Answer:

1. Aspirin (acetylsalicylic acid): Used for pain relief and reducing inflammation.

2. Paracetamol (acetaminophen): Used to relieve pain and reduce fever.

3. Amoxicillin: An antibiotic used to treat certain bacterial infections.

4. Ibuprofen: Used to relieve pain, reduce fever, and decrease inflammation.

Notes by Sir Dilawar

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