Key Learning Objectives
- Recall general formulas and structures for alkanes, alkenes, and alcohols.
- Explain the bromine water test for saturation (orange to colourless with alkenes).
- Describe thermal and catalytic cracking of long-chain hydrocarbons into useful shorter alkanes and alkenes.
Topic Summary & Core Notes
Explore homologous series, general formulas (CnH2n+2, CnH2n, CnH2n+1OH), fractional distillation, cracking, and bromine water saturation tests.
Key Rules, Formulas & Definitions
Alkane Formula
CnH2n+2 (saturated, single C-C bonds)
Alkene Formula
CnH2n (unsaturated, contains C=C double bond)
Alcohol Formula
CnH2n+1OH (contains -OH hydroxyl functional group)
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Testing for Alkenes
Question: Describe the chemical test to distinguish between ethane (alkane) and ethene (alkene).
Step-by-Step Solution:
- Step 1: Reagent: Add orange bromine water (Br2(aq)) to both hydrocarbons.
- Step 2: Ethane (alkane): Saturated hydrocarbon; no reaction; bromine water remains orange.
- Step 3: Ethene (alkene): Unsaturated C=C opens up in electrophilic addition; bromine water turns colourless (decolourises).
Final Answer: Add bromine water. With ethene it turns from orange to colourless; with ethane it remains orange.
Senior Examiner Insight: Write 'colourless', NEVER 'clear'.
Worked Example 2
Worked Example 2: Cracking Reaction Equation
Question: Decane (C10H22) is cracked to produce ethene (C2H4) and one other alkane. Write the balanced equation.
Step-by-Step Solution:
- Step 1: Carbon balance: 10 Carbons on left. Ethene has 2 Carbons. 10 - 2 = 8 Carbons for the remaining alkane.
- Step 2: Hydrogen balance: 22 Hydrogens on left. Ethene has 4. 22 - 4 = 18 Hydrogens for the remaining alkane.
- Step 3: Deduce formula: C8H18 (octane). Equation: C10H22 → C2H4 + C8H18.
Final Answer: C10H22 → C2H4 + C8H18 (Octane)
Senior Examiner Insight: Check total carbons and hydrogens on both sides of cracking equations.
Common Pitfalls & Examiner Warnings
Common Mistake: Writing 'turns clear' for bromine water
Bromine water turns COLOURLESS; 'clear' loses the mark in GCSE and A-Level mark schemes.
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What colour change is observed when an alkene is shaken with bromine water?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Orange to Colourless
Detailed Explanation: Alkenes add bromine across their C=C double bond, turning orange bromine water colourless.
Mark Scheme & Scoring Points: 1 mark for Orange to Colourless.
Question 2 of 3
What is the molecular formula of an alkane containing 6 carbon atoms?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) C6H14
Detailed Explanation: Alkanes follow CnH2n+2: (2 × 6) + 2 = 14, giving C6H14 (hexane).
Mark Scheme & Scoring Points: 1 mark for C6H14.
Question 3 of 3
Which catalyst and temperature condition is typically used in industrial catalytic cracking?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) Hot ceramic zeolite catalyst at ~550 °C
Detailed Explanation: Catalytic cracking uses high temperatures (~550 °C) and a zeolite catalyst.
Mark Scheme & Scoring Points: 1 mark for hot zeolite catalyst.
Created by Fiaraz Iqbal
Former Headteacher & Senior Science / Maths Examiner
Fiaraz produces structured video walkthroughs, Tier 3 literacy packs, and exam mark scheme breakdowns to help secondary students achieve top grades.
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