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