Forensic Chemistry- Chemistry of Fire

Chemistry of Fire- Arson & Explosives

Chemical Reaction: Oxidation

  • Chemically, fire is a type of oxidation
    • Oxidation: the combination of oxygen with other substances to produce new substances
    • Fire→ oxidation produces heat and light (flame)
  • From Gr 11 CHM
    • Combustion reactions= oxygen combining with another substance and releasing energy (heat and light)
    • One of the products will be an oxide

Fire!

  • Ignition Temperature: the min. the temperature needed to ignite fuel (start a fire)
  • Common Ignitors- match, electrical discharges, sparks, chemicals \n The heat of Combustion: The heat evolved when a substance burns (how much energy is given off)
    • Exothermic- chemical rxn that gives off heat
  • An additional factor= is the rate or speed at which the oxidation reaction takes place

Rate of the Reaction

  • The speed at which a reaction occurs:

  1. State

  2. Temperature

    \n

SOLID: solid→gas, temp. must be REALLY high

  • pyrolysis: solid decomposes into gas (products combine with O2  to make fire

    \n

LIQUID: liquid→gas, only burns if it reaches “flash point” temp., then ignited by an outside source (match, sparks, chemicals, etc.)

  • ignition temp. always higher than flash point temp.

  • Eg gasoline FP = -50ºF vs IT = 495ºF

    \n GAS: will produce a flame only if enough oxygen

The Chemistry of Fire

  • Fuel can burn without a flame…
    • Glowing combustion (smouldering): burning at the fuel-air interface, such as a cigarette

Spontaneous combustion: the result of a natural heat-producing process in poorly ventilated areas (rare)

  • Combustion will only occur when the fuel O2 mix is “just right”
    • Flammable Range: gas fuel-air mix (the range that fuel

concentrations in air are capable of burning)

  • ex-gasoline 1.3-6.0%

    \n

  • Lean-  low fuel concentration vs. high air conc.

  • Rich- high fuel concentration vs. low air conc.

    \n Fire is a “chain reaction”:

fuel + air + energy 🡪 heat 🡪 burn 🡪 heat

( cont. until oxygen or fuel runs out) \n

  • Increasing T = Increasing rate of reaction = fires spread QUICKLY!

Air is NOT the only source of O2

  • Oxidizing Agents: chemicals that supply oxygen for detonation (explosions)
    • Ex- black powder (75%KNO3, 15%C, 10%S)

Nitroglycerin (O2 and fuel in the same molecule)

Summary

  • Three Requirements of Combustion:
  1. Fuel
  2. Sufficient oxygen

(oxidizing agent)

  1. Initiation heat, heat to sustain rxn