Crude Oil

What a fuel is

• A fuel is a substance that releases heat when burned
• All combustion reactions are exothermic, and combustion is the most common way of releasing the energy stored in hydrocarbons
• All hydrocarbon combustion gives carbon-containing oxide products plus water; which oxides form depends on how much oxygen is available

Complete combustion

• Complete combustion happens when there is plenty of oxygen around the burning fuel
• The only products are carbon dioxide and water
• General word equation:

hydrocarbon + oxygen → carbon dioxide + water

• Worked equation for butane (C₄H₁₀), the fuel in a camping stove:

2 C₄H₁₀(g) + 13 O₂(g) → 8 CO₂(g) + 10 H₂O(l)

• Complete combustion releases the maximum energy per gram of fuel because every carbon atom finishes as CO₂

Incomplete combustion

• Incomplete combustion happens whenever the oxygen supply is restricted — a blocked vent, a clogged burner, or a poorly tuned engine all cause it
• The products may include any of: water, carbon monoxide (CO), unburnt carbon (soot) and unburnt hydrocarbons
• A characteristic clue: a smoky yellow flame instead of the clean blue flame of complete combustion
• Worked equation for propane with restricted oxygen:

2 C₃H₈(g) + 7 O₂(g) → 6 CO(g) + 8 H₂O(l)

• Less energy is released per gram of fuel because the carbon has not been fully oxidised
• Soot (unburnt carbon) blackens the underside of saucepans and the inside of chimneys

Why carbon monoxide is so dangerous

• Carbon monoxide is a particularly hazardous product of incomplete combustion because it is:
  • Colourless
  • Odourless
  • Tasteless
• A person breathing CO has no warning until the symptoms (headache, dizziness, nausea, drowsiness) begin
• Once in the bloodstream, CO binds tightly and almost irreversibly to the oxygen-carrying molecules in the red blood cells
• This reduces the capacity of the blood to carry oxygen to the body's tissues
• Severe CO poisoning starves vital organs of oxygen and can cause unconsciousness and death
• This is why gas heaters and boilers must be ventilated and serviced — a faulty burner can produce CO without any visible smoke