Key Learning Objectives
- Define hydrocarbons as compounds containing carbon and hydrogen ONLY.
- Compare saturated alkanes (CₙH₂ₙ₊₂) with unsaturated alkenes (CₙH₂ₙ).
- Explain the bromine water test used to distinguish between alkanes and alkenes.
Topic Summary & Core Notes
Introduction to organic chemistry and hydrocarbons. Compare saturated alkanes (single C-C bonds) with unsaturated alkenes (double C=C bonds) and the bromine water test.
Key Rules, Formulas & Definitions
Alkane General Formula
CₙH₂ₙ₊₂ (e.g. Methane CH₄, Ethane C₂H₆, Propane C₃H₈)
Alkene General Formula
CₙH₂ₙ (e.g. Ethene C₂H₄, Propene C₃H₆, Butene C₄H₈)
Bromine Water Test
Alkanes = Remains Orange/Brown | Alkenes = Decolourises to Colourless
2 Step-by-Step Worked Examples
Worked Example 1
Worked Example 1: Deducing Alkane Molecular Formulas
Question: A saturated alkane molecule contains 5 carbon atoms. Determine its molecular formula and name.
Step-by-Step Solution:
- Step 1: Apply general formula: For alkanes, formula is CₙH₂ₙ₊₂.
- Step 2: Substitute n = 5: Number of Hydrogens = (2 × 5) + 2 = 10 + 2 = 12.
- Step 3: Combine & name: Formula is C₅H₁₂ (Pentane).
Final Answer: C₅H₁₂, Pentane
Senior Examiner Insight: Prefixes: 1=Meth, 2=Eth, 3=Prop, 4=But, 5=Pent.
Worked Example 2
Worked Example 2: Interpreting the Bromine Water Test
Question: A student shakes an unknown clear hydrocarbon with orange bromine water. The solution turns colourless. Identify whether the hydrocarbon is an alkane or alkene and explain why.
Step-by-Step Solution:
- Step 1: Recall reaction principle: Alkenes contain a reactive C=C double bond that undergoes addition reactions.
- Step 2: Bromine addition: Bromine adds across the double bond to form a colourless dibromoalkane.
- Step 3: Conclusion: Saturated alkanes do not react with bromine water in the dark.
Final Answer: The hydrocarbon is an alkene because its C=C double bond reacts with bromine water, turning it colourless.
Senior Examiner Insight: Always write 'turns colourless', NEVER write 'turns clear' (clear is not a colour).
Common Pitfalls & Examiner Warnings
Common Mistake: Writing 'turns clear' instead of 'turns colourless'
Examiners reject 'clear' (orange juice is not clear, but orange water can be clear). The correct observation is 'colourless'.
Common Mistake: Saying hydrocarbons contain oxygen
Hydrocarbons contain ONLY hydrogen and carbon atoms.
Common Mistake: Mixing up Alkane and Alkene general formulas
Alkanes have +2 at the end (CₙH₂ₙ₊₂); Alkenes do not (CₙH₂ₙ).
Interactive Self-Check Quiz (3 Questions)
Question 1 of 3
What is the molecular formula of an alkene containing 4 carbon atoms?
Click to Reveal Answer & Mark Scheme
Correct Answer: A) C₄H₈
Detailed Explanation: Alkenes follow CₙH₂ₙ, so for 4 carbons: 2 × 4 = 8 Hydrogens (Butene, C₄H₈).
Mark Scheme & Scoring Points: 1 mark for C₄H₈.
Question 2 of 3
What colour change is observed when an alkene is shaken with bromine water?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) Orange to colourless
Detailed Explanation: The unsaturated C=C double bond breaks, bonding with bromine to decolourise the orange solution.
Mark Scheme & Scoring Points: 1 mark for Orange to colourless.
Question 3 of 3
Why are alkanes described as 'saturated' hydrocarbons?
Click to Reveal Answer & Mark Scheme
Correct Answer: B) All carbon-carbon bonds are single covalent bonds, holding the maximum number of hydrogen atoms.
Detailed Explanation: Saturated means no more hydrogen atoms can be added because all C-C bonds are single.
Mark Scheme & Scoring Points: 1 mark for only single C-C bonds / holding maximum hydrogens.
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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