Alkynes — KCSE Chemistry

KCSE Chemistry · 42 practice questions · 3 syllabus objectives

15 easy15 medium12 hard

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

Sample Questions

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easySHORT ANSWER4 marks

Explain how the reactivity of alkynes compares to that of alkenes and alkanes in terms of hydrogenation. (4 marks)

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Part (a) — 2 marks

Alkynes react readily with hydrogen in the presence of a catalyst to form alkenes or alkanes. (1 mk)

The reaction occurs due to the presence of the triple bond, which is more reactive than a double or single bond. (1 mk)

Part (b) — 2 marks

Alkenes can also undergo hydrogenation but are less reactive than alkynes due to the presence of a double bond. (1 mk)

Alkanes do not undergo hydrogenation under normal conditions because they are already saturated and have single bonds. (1 mk)

easySHORT ANSWER4 marks

Name two types of reactions that alkynes can undergo, and provide a brief description of each. (4 marks)

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Part (a) — 2 marks

Addition reactions (1 mk)

Combustion reactions (1 mk)

Part (b) — 2 marks

Addition reactions involve the addition of atoms or groups across the triple bond, converting it to a double or single bond. (1 mk)

Combustion reactions involve the reaction of alkynes with oxygen to produce carbon dioxide and water, releasing energy. (1 mk)

easySHORT ANSWER3 marks

List three physical properties that differentiate alkynes from alkenes and alkanes. (3 marks)

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Part (a) — 3 marks

Alkynes generally have higher boiling points than alkenes and alkanes of similar molecular weight. (1 mk)

Alkynes are typically less dense than water, similar to alkenes and alkanes. (1 mk)

Alkynes have linear geometry around the triple bond, while alkenes have planar geometry around the double bond and alkanes have tetrahedral geometry. (1 mk)

Identify the type of reaction that occurs when ethyne is mixed with hydrogen in the presence of a catalyst, and write a balanced equation for this reaction. (3 marks)

Alkynes — KCSE Chemistry

What You'll Learn

Sample Questions

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