AQA Alkanes Revision Notes

Jun 10, 2024

AQA Alkanes - Revision Notes

Introduction

  • Presenter: Chris Harrison from Alawis Tutors
  • Topic: Quick overview of alkanes for AQA specification
  • Resources: Past papers, PowerPoint slides for purchase (link in description)

Alkanes

  • General formula: CnH2n+2
    • n = number of carbons
  • Characteristics: Saturated hydrocarbons
    • Contain only hydrogen and carbon
    • No double bonds
  • Examples: Methane (CH4), Ethane (C2H6), Propane (C3H8)
  • Cycloalkanes: Same general formula as alkenes (CnH2n), but are saturated hydrocarbons (no double bonds)
    • Functional group isomers

Fractional Distillation

  • Alkanes are found in crude oil (mixture of hydrocarbons of different lengths)
  • Process: Crude oil vaporization
    • Heated to ~350°C
    • Vaporized oil enters the distillation column
    • Longest hydrocarbons do not vaporize, collect at the bottom
    • Shorter hydrocarbons rise and are separated based on boiling points
  • Temperature Gradient: Cooler at the top, warmer at the bottom
    • Shorter chains condense at the top (lower boiling point)
    • Longer chains condense at the bottom (higher boiling point)
  • Uses of Fractions:
    • Gas (LPG), Petrol (cars), Kerosene (jet fuel), Diesel oil, Fuel oil (ships), Bitumen (tarmac)

Cracking

  • Purpose: Convert heavier fractions to more valuable lighter fractions
  • Types: Thermal Cracking and Catalytic Cracking
    • Thermal Cracking:
      • High temperature (~1000°C) and pressure (~70 atm)
      • Produces mainly alkenes (used to make plastics)
    • Catalytic Cracking:
      • Uses zeolite catalyst (honeycomb structure)
      • Lower temperature and pressure
      • Produces aromatic hydrocarbons (fuels)

Combustion of Alkanes

  • Complete Combustion:
    • Alkanes + Oxygen → Carbon dioxide + Water
    • Energy release used for vehicles and power generation
  • Incomplete Combustion:
    • Limited oxygen supply
    • Produces carbon monoxide (poisonous) and soot (carbon)
    • Harmful effects: respiratory problems, environmental pollution

Environmental Impact

  • Global Warming:
    • Greenhouse gases (CO2) absorb infrared radiation
    • Leads to increase in global temperatures
  • Photochemical Smog:
    • Combination of sunlight, hydrocarbons, and nitrogen dioxide
    • Causes respiratory issues, environmental haze
  • Acid Rain:
    • Produced by sulfur dioxide and oxides of nitrogen
    • Damages vegetation, aquatic life, and infrastructure
    • Mitigation: Wet scrubbing (neutralizes acidic gases)

Free Radical Reactions

  • Mechanism Involves:
    • Initiation: Radicals produced by UV light (photochemical reactions)
    • Propagation: Radicals react with non-radicals forming new radicals
    • Termination: Radicals combine to form stable non-radical molecules
  • Example: Chlorination of Methane
    • Initiation: Cl2 → 2Cl• (by UV light)
    • Propagation: CH4 + Cl• → CH3• + HCl, then CH3• + Cl2 → CH3Cl + Cl•
    • Termination: Cl• + Cl• → Cl2 or CH3• + Cl• → CH3Cl

Closing Remarks

  • Quick overview of alkanes, including mechanisms, environmental impact, and industrial applications
  • Resources available for purchase for detailed revision