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Alkenes — KCSE Chemistry

KCSE Chemistry · 83 practice questions · 4 syllabus objectives · 4 revision lessons

19 easy32 medium32 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 write structural formulae of alkenes and describe their preparation from alcohols by dehydration

Describe the addition reactions of alkenes with H₂, Br₂, HBr, H₂O and state conditions required

Describe addition polymerisation of alkenes and give examples of common polymers and their uses

Alkenes

Revision Notes

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

Alkenes: Naming and Preparation

Alkenes are hydrocarbons containing at least one carbon-carbon double bond (C=C). They are unsaturated compounds and can be named using the IUPAC nomenclature system. The general formula for alkenes is C_nH_(2n).

Naming Alkenes:

  1. Identify the longest carbon chain containing the double bond.
  2. Number the chain from the end nearest the double bond.
  3. Use the suffix '-ene' for alkenes.

Structural Formulae:

  • Ethene: C₂H₄
    • Structural formula: H₂C=CH₂
  • Propene: C₃H₆
    • Structural formula: H₂C=CH-CH₃

Preparation from Alcohols by Dehydration:
Alkenes can be prepared from alcohols through dehydration, which involves the removal of a water molecule. This reaction typically requires an acid catalyst, such as sulfuric acid. The general reaction can be represented as:

Alcohol → Alkene + Water

For example, when ethanol (C₂H₅OH) is dehydrated, it forms ethene (C₂H₄) and water (H₂O).

Key points to remember

  • Alkenes contain at least one C=C double bond.
  • General formula for alkenes is C_nH_(2n).
  • Dehydration of alcohols produces alkenes.
  • Use acid catalyst for dehydration reactions.
  • Naming follows IUPAC rules with '-ene' suffix.

Worked example

Name the alkene formed from the dehydration of butanol.

  • The alkene is butene.
  • Structural formula: H₂C=CH-CH₂-CH₃.

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Lesson 2: Addition Reactions of Alkenes

Objective: Describe the addition reactions of alkenes with H₂, Br₂, HBr, H₂O and state conditions required

Alkenes undergo addition reactions due to the presence of a double bond (C=C). Here are the key addition reactions:

  • Hydrogenation (with H₂): Alkenes react with hydrogen in the presence of a catalyst (e.g., Pt, Pd, or Ni) to form alkanes.

    • Condition: Catalyst and high temperature.

  • Bromination (with Br₂): Alkenes react with bromine to form vicinal dibromides. This reaction occurs in the absence of light.

    • Condition: Room temperature.

  • Hydrobromination (with HBr): Alkenes react with HBr to form bromoalkanes. This reaction follows Markovnikov's rule.

    • Condition: No specific conditions required; can occur at room temperature.

  • Hydration (with H₂O): Alkenes react with water in the presence of an acid catalyst (e.g., H₂SO₄) to form alcohols.

    • Condition: Acid catalyst and heat.

Understanding these reactions is crucial for predicting the products formed during chemical reactions involving alkenes.

  • Alkenes react with H₂ to form alkanes under catalytic conditions.
  • Bromination with Br₂ yields vicinal dibromides at room temperature.
  • Hydrobromination with HBr follows Markovnikov's rule.
  • Hydration with H₂O requires an acid catalyst and heat.
  • Addition reactions are key in organic synthesis.

Q: Describe the addition reaction of ethene with H₂. A: Ethene reacts with H₂ in the presence of a catalyst (Pt) to form ethane.

Lesson 3: Addition Polymerisation of Alkenes

Objective: Describe addition polymerisation of alkenes and give examples of common polymers and their uses

Addition polymerisation is a chemical process where alkenes, which contain carbon-carbon double bonds, react to form long-chain polymers. This reaction involves the breaking of the double bond in alkenes, allowing them to join together to create a polymer. The general equation for this process can be represented as:

nC_nH_{2n} → (C_nH_{2n})_n

where 'n' represents the number of monomer units.

Common examples of polymers formed through addition polymerisation include:

  • Polyethylene: Used in plastic bags and bottles.
  • Polypropylene: Found in containers and automotive parts.
  • Polystyrene: Used in insulation and disposable cutlery.

These polymers have diverse applications in everyday life, making them essential materials in various industries. Addition polymerisation is crucial because it transforms simple alkenes into complex, useful products that are integral to modern living.

  • Addition polymerisation involves alkenes reacting to form polymers.
  • The carbon-carbon double bond is broken during the reaction.
  • Common polymers include polyethylene, polypropylene, and polystyrene.
  • Polymers have various applications in packaging and manufacturing.
  • The process is essential for creating useful materials from simple molecules.

Describe addition polymerisation and give an example.

  • Addition polymerisation is the process where alkenes react to form polymers.
  • An example is polyethylene, used in plastic bags.
Lesson 4: Understanding Alkenes and Their Properties

Objective: Alkenes

Alkenes are unsaturated hydrocarbons characterized by at least one carbon-carbon double bond (C=C). They follow the general formula C_nH_(2n), where 'n' is the number of carbon atoms. Alkenes are important in organic chemistry due to their reactivity and role in various chemical reactions, such as addition reactions.

Key Properties of Alkenes:

  • Reactivity: Alkenes react readily with halogens, hydrogen, and water.
  • Isomerism: They can exhibit geometric isomerism due to the restricted rotation around the double bond.
  • Combustion: Alkenes burn in oxygen to produce carbon dioxide and water, releasing energy.

Examples of Alkenes:

  • Ethene (C₂H₄)
  • Propene (C₃H₆)

In summary, alkenes are crucial in both laboratory and industrial applications, including the production of plastics and other materials.

  • Alkenes have at least one carbon-carbon double bond.
  • General formula for alkenes is C_nH_(2n).
  • Alkenes undergo addition reactions with various reagents.
  • They can exhibit geometric isomerism.
  • Alkenes are used in the production of plastics.

Question: Write the molecular formula for butene and explain its structure. Answer: Butene has the molecular formula C₄H₈. It contains four carbon atoms with one double bond between two of the carbons.

Sample Questions

Read 3 questions and answers free. Sign up to access all 83 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

Name two common polymers produced from alkenes and state one use for each. (4 marks)

Answer & marking scheme

Part (a) — 2 marks
Polyethylene (1 mk)
Polypropylene (1 mk)
Part (b) — 2 marks
Used in plastic bags, bottles, and containers for polyethylene (1 mk)
Used in packaging, textiles, and automotive parts for polypropylene (1 mk)
2
easySHORT ANSWER2 marks

Identify the process by which alkenes are converted into polymers through repeated addition reactions. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
Addition polymerisation (1 mk)
Involves the joining of many alkene monomers to form a polymer (1 mk)
3
easySHORT ANSWER3 marks

Identify the type of reaction that occurs when ethene reacts with water in the presence of sulfuric acid and state the condition needed for this reaction. (3 marks)

Answer & marking scheme

Part (a) — 2 marks
Electrophilic addition (2 mks)
Part (b) — 1 mark
Presence of sulfuric acid as a catalyst (1 mk)
4

Identify the product formed when butene reacts with bromine and state the condition required for this reaction. (3 marks)

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Frequently asked questions

What does the KCSE Chemistry topic "Alkenes" cover?

Alkenes covers Name and write structural formulae of alkenes and describe their preparation from alcohols by dehydration; Describe the addition reactions of alkenes with H₂, Br₂, HBr, H₂O and state conditions required; Describe addition polymerisation of alkenes and give examples of common polymers and their uses, and more, all aligned to the official KNEC KCSE Chemistry syllabus.

How many practice questions are available for Alkenes?

HighMarks has 83 Alkenes 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 Alkenes for the KCSE exam?

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