Frank Brothers Solutions for ICSE Class 10 Chemistry Chapter 11b Alkanes

Alkanes

Solution 1:
Answer: Methane is indicated by marsh gas and fire damp.
In simple words: Methane is found naturally in marshy areas and coal mines, where it can be dangerous if it catches fire.

📝 Teacher's Note: Connect this to students' experience with gas stoves at home - that's purified methane. Emphasize the safety aspect in coal mines to make it memorable.

🎯 Exam Tip: Remember the two common names for methane: "marsh gas" (from swamps) and "fire damp" (from coal mines) - both indicate natural occurrence.

Solution 2:
(a) Mixture of sodium ethanoate and soda lime is heated in a hard glass tube.

\( CH_3COONa + NaOH \xrightarrow{CaO} CH_4 + Na_2CO_3 \)

(b) Gas is collected by the downward displacement of water, since it is only slightly soluble in water and lighter than air.

(c) Soda lime is used as it is not deliquescent and does not attack glass.
Answer: This is the laboratory preparation of methane using sodium ethanoate and soda lime, where the gas is collected over water due to its low solubility and lighter density compared to air.
In simple words: We heat a mixture of chemicals to make methane gas, then collect it by pushing water down because methane doesn't dissolve much in water and floats on air.

📝 Teacher's Note: Demonstrate why we can't use simple NaOH - it attacks glass. Show students the setup diagram and explain why collection over water works for gases lighter than air.

🎯 Exam Tip: Always mention "soda lime" not just NaOH, and explain the collection method - examiners look for understanding of practical details.

Solution 3:
(a) When methyl bromide or methyl iodide and sodium are heated in presence of dry ether, ethane is formed.

\( CH_3I + 2Na + ICH_3 \xrightarrow{Dry ether} CH_3 - CH_3 + 2NaI \)

(b) By reduction of Ethyl iodide using Zn +Cu couple in alcohol, ethane is formed.

\( CH_3CH_2I + 2[H] \rightarrow C_2H_6 + HI \)
Answer: Ethane can be prepared by (a) Wurtz reaction using methyl halides with sodium in dry ether, or (b) reduction of ethyl iodide using zinc-copper couple in alcohol.
In simple words: We can make ethane by joining two methyl groups together using sodium, or by removing iodine from ethyl iodide using special metals.

📝 Teacher's Note: Emphasize that Wurtz reaction joins two identical alkyl groups. Use molecular models to show how two CH₃ groups combine to form ethane.

🎯 Exam Tip: For Wurtz reaction, always write "dry ether" as the solvent and show the mechanism with sodium atoms. For reduction, mention the specific reducing agent.

Solution 4:
(a) The products obtained when methane reacts with chlorine in diffused sunlight are Chloromethane, Dichloromethane, Trichloromethane and Tetrachloromethane.

(b) \( CH_4 + Cl_2 \xrightarrow{UV light or \triangle} CH_3Cl + HCl \)
\( CH_3Cl + Cl_2 \rightarrow CH_2Cl_2 + HCl \)
\( CH_2Cl_2 + Cl_2 \rightarrow CHCl_3 + HCl \)
\( CHCl_3 + Cl_2 \rightarrow CCl_4 + HCl \)

(c) The above reaction is substitution reaction. Here the substitution of alkanes with chlorine takes place hence it is called chlorination.

The reactions in which the hydrogen of the alkane molecule is replaced by another atom or group of atoms resulting in the formation of the derivative of that hydrocarbon are called substitution reactions.

Substitution by halogen atom is called halogenations.
Answer: Methane undergoes successive chlorination reactions to form chloromethane, dichloromethane, trichloromethane, and tetrachloromethane. This is a substitution reaction where hydrogen atoms are progressively replaced by chlorine atoms.
In simple words: When methane meets chlorine in sunlight, chlorine atoms slowly replace hydrogen atoms one by one, creating different chlorinated compounds.

📝 Teacher's Note: Use a step-by-step diagram showing how each hydrogen is replaced. Relate to real-world applications like chloroform (CHCl₃) as an anesthetic.

🎯 Exam Tip: Write all four successive reactions and clearly state "substitution reaction" and "halogenation." Mention UV light or heat as conditions.

Solution 5:
(i) Uses of Methane
• As a domestic fuel in the form of natural gas
• In the manufacture of methanol and hydrogen.

(ii) Uses of Ethane
• As a fuel, it has high calorific value than methane. Liquefied ethane is also used as a fuel.
• In the preparation of ethanol, acetaldehyde and acetic acid which find use in paints, varnishes, adhesive, plastic etc.
Answer: Methane is used as domestic fuel (natural gas) and for manufacturing methanol and hydrogen. Ethane serves as a high-calorific fuel and raw material for producing ethanol, acetaldehyde, and acetic acid used in paints, varnishes, and plastics.
In simple words: Methane is the gas we cook with at home, while ethane is used to make many chemicals that go into paints and plastics we use daily.

📝 Teacher's Note: Connect to students' daily life - cooking gas, plastic bottles, paint in their homes. Explain why ethane has higher calorific value (more carbon atoms).

🎯 Exam Tip: Mention specific applications and distinguish between the two alkanes clearly. Include both fuel and chemical industry uses.

Solution 6:
\( C_2H_6 + Cl_2 \xrightarrow{UV light} C_2H_5Cl + HCl \)
\( C_2H_5Cl + Cl_2 \rightarrow C_2H_4Cl_2 + HCl \)
\( C_2H_4Cl_2 + Cl_2 \rightarrow C_2H_3Cl_3 + HCl \)
\( C_2H_3Cl_3 + Cl_2 \rightarrow C_2H_2Cl_4 + HCl \)
\( C_2H_2Cl_4 + Cl_2 \rightarrow C_2HCl_5 + HCl \)
\( C_2HCl_5 + Cl_2 \rightarrow C_2Cl_6 + HCl \)
Answer: Ethane undergoes progressive chlorination similar to methane, producing chloroethane, dichloroethane, trichloroethane, tetrachloroethane, pentachloroethane, and hexachloroethane in successive steps.
In simple words: Ethane behaves like methane with chlorine, but since it has more hydrogen atoms, it can form more chlorinated products step by step.

📝 Teacher's Note: Compare with methane chlorination to show the pattern. Emphasize that ethane has 6 hydrogen atoms, so 6 substitution steps are possible.

🎯 Exam Tip: Write all six sequential reactions showing progressive substitution. Each step removes one H and adds one Cl.

Solution 7:
(a) Methane to methanol:
\( 2CH_4 + O_2 \xrightarrow{Cu, 200°C} 2CH_3OH \)

(b) Ethane to ethanal(acetaldehyde):
\( 2C_2H_6 + O_2 \xrightarrow{Cu, 200°C} 2C_2H_5OH \)
\( C_2H_5OH \xrightarrow{K_2Cr_2O_7, acidic} CH_3CHO \)

(c) Methane to methanoic acid:
\( 2CH_4 + O_2 \xrightarrow{Cu, 200°C} 2CH_3OH \)
\( CH_3OH \xrightarrow{K_2Cr_2O_7, acidic} HCHO \xrightarrow{K_2Cr_2O_7, acidic} HCOOH \)
Answer: These conversions involve controlled oxidation of alkanes to alcohols using copper catalyst, followed by further oxidation using potassium dichromate to form aldehydes and acids.
In simple words: We can change alkanes into useful chemicals by carefully adding oxygen in steps - first to alcohols, then to aldehydes, and finally to acids.

📝 Teacher's Note: Explain the concept of controlled vs. complete oxidation. Show how temperature and catalyst control the extent of reaction.

🎯 Exam Tip: Always mention specific conditions (Cu, 200°C for first step; K₂Cr₂O₇, acidic for oxidation). Show the stepwise nature clearly.

Solution 8:
Structural Formulae of isomers of butane:
[The image shows two structural formulas - n-butane (straight chain) and methylpropane/isobutane (branched chain)]
Answer: Butane has two structural isomers: n-butane (straight chain) with formula H₃C-CH₂-CH₂-CH₃, and methylpropane or isobutane (branched chain) with a methyl group attached to the middle carbon.
In simple words: Butane can be arranged in two ways - like a straight line of four carbons, or with three carbons in a row and one carbon branching off from the middle.

📝 Teacher's Note: Use molecular model kits to show how the same atoms can be arranged differently. Explain how branching affects boiling point and other properties.

🎯 Exam Tip: Draw both structures clearly showing all bonds. Label them as n-butane and isobutane (or methylpropane). Remember C₄H₁₀ formula for both.

Solution 9:
(a) Ethane is mixed with oxygen and is passed through hot copper tube; it gets oxidized to ethyl alcohol.

\( 2C_2H_6 + O_2 \xrightarrow{Cu, 200°C} 2C_2H_5OH \)

(b) Ethane is first converted into ethyl alcohol by passing over hot copper tube, then further oxidation with acidified potassium dichromate yield aldehyde and carboxylic acids.

\( 2C_2H_6 + O_2 \xrightarrow{Cu, 200°C} 2C_2H_5OH \)
\( C_2H_5OH \xrightarrow{K_2Cr_2O_7, acidic} CH_3CHO \xrightarrow{K_2Cr_2O_7, acidic} CH_3COOH \)
Answer: Ethane undergoes controlled oxidation to form ethanol, which can be further oxidized to acetaldehyde and acetic acid using acidified potassium dichromate.
In simple words: We can convert ethane into alcohol by controlled heating with oxygen, then change that alcohol into vinegar (acetic acid) through further oxidation.

📝 Teacher's Note: Connect to everyday examples - ethanol in sanitizers, acetic acid in vinegar. Show how controlled conditions prevent complete combustion to CO₂.

🎯 Exam Tip: Emphasize the stepwise oxidation process and specific reagents for each step. Mention temperature and catalyst conditions clearly.

Solution 10:
Answer: The main sources of alkanes are natural gas and petroleum. Alkanes are known as saturated hydrocarbons because the carbon atoms in their molecules are bonded to each other by single covalent bond. Each carbon atom is again bonded to hydrogen atom.
In simple words: Alkanes come from underground gas and oil deposits. They're called "saturated" because every carbon atom is holding as many hydrogen atoms as it possibly can, like a sponge that's completely full of water.

📝 Teacher's Note: Use the analogy of saturation like a saturated salt solution. Explain how single bonds mean no more atoms can be added without breaking the molecule.

🎯 Exam Tip: Always mention both natural gas and petroleum as sources. Define "saturated" in terms of single bonds and maximum hydrogen content.

Solution 11:
Answer: When methane and chlorine are exposed to direct sunlight they give carbon and HCl.

\( CH_4 + 2Cl_2 \xrightarrow{direct sunlight} C + 4HCl \)
In simple words: In very strong sunlight, methane and chlorine react so violently that they break apart completely, leaving behind just carbon powder and hydrogen chloride gas.

📝 Teacher's Note: Distinguish between diffused sunlight (substitution) and direct sunlight (complete breakdown). This shows how reaction conditions affect products.

🎯 Exam Tip: Clearly state "direct sunlight" as the condition and show the complete breakdown to carbon, not substitution products.

Solution 12:
(a) Ethane reacts in excess of air to give carbon dioxide and water.
\( 2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O + Heat \)

(b) Ethane reacts in absence of air to give carbon black.
\( C_2H_6 + O_2 \rightarrow 2C + 3H_2O \)

(c) Ethane reacts in presence of copper and 120atm pressure to give ethanol
\( 2C_2H_6 + O_2 \xrightarrow{Cu, 200°C} 2C_2H_5OH \)

(d) Ethane reacts with molybdenum oxide at 100 atm pressure to give ethanal.
\( C_2H_6 + O_2 \xrightarrow{MoO} CH_3CHO + H_2O \)
Answer: Ethane shows different oxidation reactions depending on conditions: complete combustion in excess air gives CO₂ and water, limited oxygen gives carbon black, controlled conditions with copper catalyst gives ethanol, and molybdenum oxide catalyst gives acetaldehyde.
In simple words: Ethane reacts with oxygen differently based on how much air is present and what helper chemicals we use - it can burn completely, form soot, or create useful chemicals like alcohol.

📝 Teacher's Note: Demonstrate how the amount of oxygen and type of catalyst determines the product. This shows the importance of reaction conditions in chemistry.

🎯 Exam Tip: Write balanced equations for each condition. Emphasize the role of oxygen supply and specific catalysts in determining products.

Solution 13:
Answer: Seven covalent bonds are present in Ethane.
In simple words: Ethane has 7 bonds holding its atoms together - 1 bond between the two carbon atoms and 6 bonds connecting carbon atoms to hydrogen atoms.

📝 Teacher's Note: Draw the structural formula and count bonds with students. Explain that each line represents a covalent bond between atoms.

🎯 Exam Tip: Draw the structure of ethane and count: 1 C-C bond + 6 C-H bonds = 7 total covalent bonds. Show your counting clearly.