Topic: Organic Chemistry – Alkenes
Lesson 1: Structure and Properties of Alkenes
| Section | Details |
|---|---|
| Objective | – State that alkenes have a double carbon–carbon covalent bond.- Define alkenes as unsaturated hydrocarbons. |
| Activities | – Starter (5 mins): Show molecular models of ethene and propane and ask: “What is the difference in bonding between these molecules?”- Main (25 mins): 1. Define alkenes as hydrocarbons containing a double carbon–carbon bond (C=C). 2. Explain the term “unsaturated hydrocarbon” (presence of C=C bonds). 3. Provide examples: ethene (C2H4C_2H_4), propene (C3H6C_3H_6), but-1-ene (C4H8C_4H_8). 4. Students practice drawing structural and displayed formulae for alkenes.- Plenary (10 mins): Students complete a worksheet identifying alkenes and explaining why they are unsaturated hydrocarbons. |
| Resources | Molecular models, periodic table, worksheets. |
| Time | 40 minutes total: 5 mins (Starter) + 25 mins (Main) + 10 mins (Plenary). |
| Homework | Draw the structural and displayed formulae for the first three alkenes. |
| Assessment | Worksheet: Identify alkenes based on structural or displayed formulae. |
| Past Paper Practice | IGCSE Chemistry 0620/41/M/J/20 Q7(a) – Structure and bonding in alkenes. |
Lesson 2: Manufacture of Alkenes by Cracking
| Section | Details |
|---|---|
| Objective | – Describe the manufacture of alkenes and hydrogen by cracking of larger alkanes.- Explain the reasons for cracking. |
| Activities | – Starter (5 mins): Show a long-chain alkane (e.g., kerosene) and ask: “Why do we need to break this molecule into smaller pieces?”- Main (25 mins): 1. Define cracking as breaking down large alkanes into smaller alkanes, alkenes, and hydrogen. 2. Explain the conditions for cracking: – High temperature. – Catalyst (e.g., silica or alumina). 3. Discuss reasons for cracking: – To meet the demand for smaller hydrocarbons (e.g., petrol, ethene). – To produce alkenes for polymers and chemicals. 4. Write and balance equations for cracking reactions, e.g., C10H22→C2H4+C8H18C_{10}H_{22} → C_2H_4 + C_8H_{18}.- Plenary (10 mins): Students label a diagram of a cracking setup and write equations for cracking reactions. |
| Resources | Cracking diagrams, reaction equations, worksheets. |
| Time | 40 minutes total: 5 mins (Starter) + 25 mins (Main) + 10 mins (Plenary). |
| Homework | Write an explanation of why cracking is important for the production of fuels and alkenes. |
| Assessment | Worksheet: Describe cracking and its importance in industry. |
| Past Paper Practice | IGCSE Chemistry 0620/41/O/N/21 Q7(b) – Manufacture of alkenes by cracking. |
Lesson 3: Test for Unsaturation and Addition Reactions
| Section | Details |
|---|---|
| Objective | – Describe the test for unsaturation using bromine water.- Explain addition reactions of alkenes with bromine, hydrogen, and steam. |
| Activities | – Starter (5 mins): Show a bottle of bromine water and ask: “What happens when bromine water reacts with a double bond?”- Main (25 mins): 1. Demonstrate the bromine water test: – Unsaturated compounds (alkenes) decolorize bromine water. – Saturated compounds (alkanes) do not react. 2. Explain addition reactions of alkenes: – With bromine: C2H4+Br2→C2H4Br2C_2H_4 + Br_2 → C_2H_4Br_2. – With hydrogen (nickel catalyst): C2H4+H2→C2H6C_2H_4 + H_2 → C_2H_6. – With steam (acid catalyst): C2H4+H2O→C2H5OHC_2H_4 + H_2O → C_2H_5OH. 3. Practice drawing structural and displayed formulae for the products of these reactions.- Plenary (10 mins): Students complete a worksheet identifying saturated and unsaturated compounds and predicting addition reaction products. |
| Resources | Bromine water, molecular models, reaction diagrams, worksheets. |
| Time | 40 minutes total: 5 mins (Starter) + 25 mins (Main) + 10 mins (Plenary). |
| Homework | Draw the structural formula for the products of ethene reacting with bromine, hydrogen, and steam. |
| Assessment | Worksheet: Write equations and draw products of addition reactions with alkenes. |
| Past Paper Practice | IGCSE Chemistry 0620/41/M/J/19 Q6(b) – Addition reactions of alkenes. |
Key Notes for Students:
- Structure of Alkenes:
- Alkenes contain at least one C=C bond.
- They are unsaturated hydrocarbons.
- Cracking:
- Breaks down large alkanes into smaller hydrocarbons (alkanes, alkenes, and hydrogen).
- Conditions: High temperature and a catalyst.
- Test for Unsaturation:
- Bromine water decolorizes in the presence of alkenes (unsaturated compounds).
- Addition Reactions:
- With bromine: Forms dibromo compounds (e.g., C2H4Br2C_2H_4Br_2).
- With hydrogen (nickel catalyst): Forms alkanes (e.g., C2H6C_2H_6).
- With steam (acid catalyst): Forms alcohols (e.g., C2H5OHC_2H_5OH).
Notes for Teachers:
- Use hands-on experiments to demonstrate the bromine water test.
- Provide clear visuals and molecular models to explain addition reactions.
- Reinforce the industrial importance of cracking and alkenes.
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