Ch6: Fossil Fuels and Combustion

Fossil Fuels & Their Physical States

  • Fossil fuels = energy-rich substances derived from long-buried, decaying plant matter.
    • Solid ➜ coal
    • Liquid ➜ gasoline
    • Gas ➜ natural gas
  • Extracted by a variety of mining / drilling techniques (details postponed until later sections).
  • Purpose: serve as fuels that can be combusted to release usable energy.

Photosynthesis: The Prototype “Fuel-Making” Reaction

  • Overall reaction (simplified):
    6CO2 + 6H2O + 2{,}800\,\text{kJ}\; (\text{sunlight}) \;\longrightarrow\; C6H{12}O6 + 6O2
  • Key ideas
    • 2{,}800\,\text{kJ} appears on the reactant side ⇒ the process absorbs (stores) energy.
    • Energy is not visible on the product side because it is locked as chemical (bond) energy inside C6H{12}O_6 (glucose).
  • Microscopic view
    • Solar (radiant) energy → raises potential energy of specific C–C and C–H bonds.
    • When those bonds are later broken (e.g., during respiration or combustion), stored energy is released.

First Law of Thermodynamics (Energy Conservation)

  • Statement: Energy in the universe is conserved—it can change form but cannot be created or destroyed.
  • Photosynthesis ↔ respiration/combustion demonstrate reversible energy interconversion:
    • Solar → chemical potential (photosynthesis)
    • Chemical potential → heat / work (combustion, metabolism)

Hydrocarbons: Definition & Basic Structure

  • Simplest class of organic compounds composed only of C and H atoms.
  • Example: butane
    • Molecular formula: C4H{10}
    • Several common formula styles:
    • Molecular: shows only totals (no bonding detail).
    • Condensed structural: $$CH3CH2CH_2