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Alkynes — KCSE Chemistry

KCSE Chemistry · 62 practice questions · 3 syllabus objectives · 3 revision lessons

20 easy21 medium21 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Name and draw structural formulae of alkynes up to C5

Describe the preparation and chemical reactions of ethyne

Compare the properties and reactions of alkanes, alkenes and alkynes

Revision Notes

Concise lesson notes for Alkynes, written to the KCSE Chemistry marking standard. Read the first lesson free below.

Naming and Drawing Alkynes

Alkynes are hydrocarbons that contain at least one triple bond between carbon atoms. They follow the general formula CₙH₂ₙ₋₂. Here are the first five alkynes:

  1. Ethyne (C₂H₂): The simplest alkyne, also known as acetylene. Its structural formula is:

    H-C≡C-H

  2. Propyne (C₃H₄): The next alkyne with three carbon atoms. Its structural formula is:

    H-C≡C-CH₃

  3. Butyne (C₄H₆): This alkyne can exist in two isomeric forms, 1-butyne and 2-butyne. Here’s the structural formula for 1-butyne:

    H-C≡C-CH₂-CH₃

  4. Pentyne (C₅H₈): Similar to butyne, pentyne also has isomers. The structural formula for 1-pentyne is:

    H-C≡C-CH₂-CH₂-CH₃

To effectively name and draw alkynes, remember to identify the longest carbon chain containing the triple bond and use the appropriate prefix for the number of carbon atoms.

Key points to remember

  • Alkynes contain a triple bond between carbon atoms.
  • General formula for alkynes is CₙH₂ₙ₋₂.
  • First five alkynes are ethyne, propyne, butyne, and pentyne.
  • Structural formulas show arrangement of atoms and bonds.
  • Isomers exist for butyne and pentyne.

Worked example

Name the alkyne with the formula C₄H₆ and draw its structural formula.

  • The alkyne is butyne.
  • Structural formula for 1-butyne is H-C≡C-CH₂-CH₃.

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Lesson 2: Preparation and Reactions of Ethyne

Objective: Describe the preparation and chemical reactions of ethyne

Ethyne, also known as acetylene, is the simplest alkyne with the formula C₂H₂. Preparation methods include:

  • Dehydration of ethanol: Ethanol reacts with strong acids like sulfuric acid to form ethyne.
  • Calcium carbide reaction: When water reacts with calcium carbide (CaC₂), ethyne is produced.

Chemical reactions of ethyne include:

  • Combustion: Ethyne burns in oxygen to produce carbon dioxide and water.
    • Example: 2 C₂H₂ + 5 O₂ → 4 CO₂ + 2 H₂O
  • Addition reactions: Ethyne undergoes hydrogenation to form ethane or halogenation to form dihaloalkanes.
    • Example: C₂H₂ + H₂ → C₂H₆ (in the presence of a catalyst)

Understanding these reactions is crucial, as they demonstrate the reactivity of alkynes and their applications in organic synthesis and industrial processes.

  • Ethyne is prepared by dehydration of ethanol and calcium carbide reaction.
  • Ethyne undergoes combustion to produce carbon dioxide and water.
  • Ethyne reacts with hydrogen to form ethane in hydrogenation.
  • Ethyne can react with halogens to form dihaloalkanes.

Describe the preparation of ethyne using calcium carbide.

  • Ethyne is produced by reacting calcium carbide (CaC₂) with water (H₂O).
  • The reaction is: CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂.
Lesson 3: Comparing Properties of Hydrocarbons

Objective: Compare the properties and reactions of alkanes, alkenes and alkynes

Alkanes, alkenes, and alkynes are three classes of hydrocarbons with distinct properties and reactions. Alkanes are saturated hydrocarbons with single bonds (C-C) and are relatively unreactive. They undergo combustion and substitution reactions. Alkenes, on the other hand, are unsaturated hydrocarbons with at least one double bond (C=C). They are more reactive than alkanes and undergo addition reactions, such as hydrogenation and halogenation. Alkynes contain at least one triple bond (C≡C) and are even more reactive than alkenes, participating in similar addition reactions.

Key Properties:

  • Alkanes: Non-polar, low reactivity, higher boiling points than alkenes.
  • Alkenes: More reactive than alkanes, lower boiling points than alkanes, can form polymers.
  • Alkynes: Most reactive, can form multiple products in addition reactions.

Understanding these differences helps predict the behavior of these compounds in chemical reactions. For example, alkenes can react with bromine to form dibromoalkanes, while alkynes can react with the same reagent to form tetrabromoalkanes.

In summary, the key differences in bonding lead to variations in reactivity and properties among these hydrocarbons.

  • Alkanes are saturated with single C-C bonds.
  • Alkenes have double bonds, making them more reactive.
  • Alkynes contain triple bonds, showing highest reactivity.
  • Alkenes and alkynes undergo addition reactions.
  • Boiling points vary with saturation level and molecular weight.

Compare the reactivity of alkenes and alkynes.

  • Alkenes react by addition, forming alkanes.
  • Alkynes react similarly but are more reactive due to triple bonds.

Sample Questions

Read 3 questions and answers free. Sign up to access all 62 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

Define the term 'alkyne' and explain how alkynes differ from alkenes and alkanes in terms of their bonding and saturation. (4 marks)

Answer & marking scheme

Part (a) — 1 mark
Alkynes are hydrocarbons that contain at least one carbon-carbon triple bond. (1 mk)
Part (b) — 1 mark
Alkynes have a triple bond (C≡C), while alkenes have a double bond (C=C). (1 mk)
Part (c) — 1 mark
Alkynes are unsaturated due to the presence of the triple bond, whereas alkanes are fully saturated with only single bonds. (1 mk)
Part (d) — 1 mark
The triple bond in alkynes makes them more reactive than alkenes and alkanes due to the presence of multiple electrons available for reactions. (1 mk)
2
easySHORT ANSWER3 marks

Define the differences in the reactivity of butyne compared to butane and butene when subjected to hydrogenation. (3 marks)

Answer & marking scheme

Part (a) — 1 mark
Butane is the expected product of the hydrogenation of butyne. (1 mk)
Part (b) — 1 mark
Butyne contains a triple bond which is more reactive than the single bonds in butane. (1 mk)
Part (c) — 1 mark
Presence of a catalyst such as nickel, platinum, or palladium. (1 mk)
3
easySHORT ANSWER2 marks

Define the main chemical reaction that ethyne undergoes when it reacts with hydrogen in the presence of a catalyst. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
Ethyne reacts with hydrogen to form ethane. (1 mk)
The reaction can be represented by the equation: C₂H₂ + H₂ → C₂H₆ in the presence of a catalyst (e.g., palladium). (1 mk)
4

Define how ethyne can be prepared from the reaction of calcium carbide with water, including the balanced equation. (3 marks)

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Frequently asked questions

What does the KCSE Chemistry topic "Alkynes" cover?

Nomenclature, structural formulae, preparation and reactions of alkynes; comparison with alkanes and alkenes

How many practice questions are available for Alkynes?

HighMarks has 62 Alkynes practice questions for KCSE Chemistry, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Chemistry syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Alkynes for the KCSE exam?

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