Frank Brothers Solutions for ICSE Class 10 Chemistry Chapter 11d Alkynes

Alkynes

Solution 1:
Answer: First four members of the homologous series of alkynes are:

• Ethyne
• Propyne
• Butyne
• Pentyne

These are hydrocarbons with a triple bond between carbon atoms, following the general formula \( C_nH_{2n-2} \).

📝 Teacher's Note: Start by showing the structural formulas alongside the names to help students visualize the triple bond pattern. Emphasize how each successive alkyne adds a CH₂ unit to the chain.

🎯 Exam Tip: Remember the formula \( C_nH_{2n-2} \) and practice writing both molecular and structural formulas for full marks in naming questions.

Solution 2:
Answer: Ethyne is prepared by the reaction of calcium carbide with water:
\( CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2 \)

The diagram shows the laboratory setup with calcium carbide (CaC₂) reacting with water to produce acetylene gas, which is collected by downward displacement of water. An acidified solution of CuSO₄ is used to purify the gas by removing impurities like hydrogen sulfide and phosphine.

📝 Teacher's Note: Demonstrate this reaction carefully as it's vigorous and produces heat. Explain why water displacement is used - acetylene is only slightly soluble in water.

🎯 Exam Tip: Always write the balanced chemical equation and mention the purification step with acidified CuSO₄ for complete answers.

Solution 3:
Answer: Addition Reactions: The reactions in which molecules of the attacking reagent add across the double or triple bond of an unsaturated compound to yield saturated compound.

In case of ethene the addition occurs due to presence of double bond hence one molecule of the compound is added whereas in case of alkynes there is presence of triple bond hence two molecules of the compound is added. This means alkynes can undergo two successive addition reactions because they have a triple bond, while alkenes undergo only one addition reaction due to their double bond.

📝 Teacher's Note: Use molecular models to show how the triple bond "opens up" in two steps during addition reactions. This visual aid helps students understand the stepwise mechanism.

🎯 Exam Tip: Always mention that alkynes undergo two successive additions and compare with alkenes' single addition for complete conceptual understanding marks.

Solution 4:
Answer: Equations:
(i) Bromine water:
\( CH \equiv CH + Br - Br \xrightarrow{CCl_4} CHBr = CHBr \)
\( CHBr = CHBr + Br - Br \xrightarrow{CCl_4} CHBr_2 - CHBr_2 \)

(ii) Excess of Hydrochloric acid:
\( CH \equiv CH \xrightarrow{HCl} CH_2 = CHCl \)
\( CH_2 = CHCl \xrightarrow{HCl} CH_3 - CHCl_2 \)

These reactions show the stepwise addition across the triple bond of ethyne.

📝 Teacher's Note: Emphasize that these are two-step reactions where the first step forms an alkene intermediate, then the second step gives the final saturated product.

🎯 Exam Tip: Write both steps clearly with proper arrow notation and mention the solvent CCl₄ for bromine reactions to score full marks.

Solution 5:
Answer:
(i) Ethyne to ethane:
\( CH \equiv CH + H_2 \xrightarrow{Ni,300°C} CH_2 = CH_2 \)
\( CH_2 = CH_2 + H_2 \xrightarrow{Ni} CH_3 - CH_3 \)

(ii) Ethyne to acetaldehyde:
\( CH \equiv CH + H_2O + [O] \xrightarrow{HgSO_4/H_2SO_4} CH_3CHO \)

The first reaction is hydrogenation using nickel catalyst, while the second is hydration followed by oxidation using mercury sulfate catalyst.

📝 Teacher's Note: Explain that hydrogenation can be controlled to stop at the alkene stage by using poisoned catalysts like Lindlar's catalyst if needed.

🎯 Exam Tip: Always mention the catalyst and conditions for each reaction - this shows complete understanding of the reaction mechanism.

Solution 6:
Answer: Two chemical tests to distinguish between ethane and ethyne are:
(i) Bromine water test
(ii) Baeyers test

Ethyne will decolorize both bromine water and Baeyer's reagent (alkaline KMnO₄) due to its unsaturated nature, while ethane will not react with either reagent as it is saturated.

📝 Teacher's Note: Demonstrate these tests practically - the color changes are dramatic and help students remember the difference between saturated and unsaturated hydrocarbons.

🎯 Exam Tip: Mention the observation clearly - "decolorizes" for positive test and "no reaction" for negative test to get full marks.

Solution 7:
Answer:

📝 Teacher's Note: Use examples like methane (saturated) vs ethene (unsaturated) to make these differences concrete for students.

🎯 Exam Tip: Focus on bonding differences and reactivity patterns - these are the key distinguishing features examiners look for.

Solution 8:
Answer: Structural formulae and IUPAC name of Second Homologues:

Propyne
\( H-C \equiv C-H \) with H attached to first carbon

Structural formulae and IUPAC name of third homologues:

Butyne
Structure shows H-C≡C-C-C-H with appropriate hydrogen atoms attached.

These represent the second and third members of the alkyne homologous series respectively.

📝 Teacher's Note: Draw the structures step by step, showing how to count carbon atoms for IUPAC naming and where to place the triple bond.

🎯 Exam Tip: Always draw complete structural formulas showing all bonds and atoms, not just condensed formulas, for maximum marks.

Solution 9:
Answer:
(a) Hydrogen:
\( CH \equiv CH + H_2 \xrightarrow{Ni,300°C} CH_2 = CH_2 \)
\( CH_2 = CH_2 + H_2 \xrightarrow{Ni} CH_3 - CH_3 \)

(b) Water:
\( CH \equiv CH + H_2O + [O] \xrightarrow{HgSO_4/H_2SO_4} CH_3CHO \)

(c) Bromine:
\( CH \equiv CH + Br - Br \xrightarrow{CCl_4} CHBr = CHBr \)
\( CHBr = CHBr + Br - Br \xrightarrow{CCl_4} CHBr_2 - CHBr_2 \)

These show the typical addition reactions of ethyne with different reagents.

📝 Teacher's Note: Emphasize that all these are addition reactions where the triple bond gets converted to single bonds through intermediate double bond formation.

🎯 Exam Tip: Include proper catalysts and conditions for each reaction type - this demonstrates thorough knowledge of reaction mechanisms.

Solution 10:
Answer:
(i) Ethene and ethyne burns with a sooty luminous flame as all the carbon atoms do not get oxidized.
(ii) Ethane does not undergo addition reaction due to absence of double and triple bond.
(iii) Ethyne is used for welding and cutting metals as ethylene produce high temperature.

These properties distinguish alkynes from alkanes based on their unsaturated nature and combustion characteristics.

📝 Teacher's Note: Demonstrate the difference in flame characteristics if possible - the sooty flame of ethyne vs clean flame of ethane is very noticeable.

🎯 Exam Tip: Connect the sooty flame to incomplete combustion and relate ethyne's high temperature production to its practical welding applications.

Solution 11:
Answer: Uses of ethyne:

• (i) For welding and cutting metals
• (ii) For artificial ripening of fruits
• (iii) As a general anaesthetic under the name Narcylene

These applications utilize ethyne's ability to produce high temperatures when burned and its chemical properties for industrial and medical purposes.

📝 Teacher's Note: Discuss safety precautions when handling ethyne, especially in welding applications, as it forms explosive mixtures with air.

🎯 Exam Tip: Remember the alternative name "Narcylene" for the anaesthetic use - this specific terminology often appears in exams.