Chemistry of Fire

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Chemistry of Fire
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What is fire?

• Answer Fire is heat and light from rapid
  combination of oxygen and other materials.
• For fire to exist, a combustible substance must
  be present, the temperature must be high enough
  to cause combustion, and enough oxygen must be
  present to sustain rapid combustion.

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Chemistry of Fire

• Oxygen combines with other substances to produce
  new products oxidation
• CH4 2O2 ? CO2 2H2O
• Stored chemical energy converted to energy in the
  form of heat and light.
• Energy in chemical reaction comes from the
  breaking and formation of bonds.

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• Heat of combustion exothermic
• Activation energy needed to start the combustion
  ignition temperature
• Ignition Temperature Minimum temperature at
  which a fuel will spontaneously ignite.
• Fire continues to burn until the supply of oxygen
  is exhausted or fuel is gone

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• Activation energy the minimum energy necessary
  for a specific chemical reaction to occur i.e.
  matches, electrical discharges, sparks, and
  chemicals.
• Fuel needs to be in gaseous state to ignite
  temperature must be high enough to vaporize the
  fuel.

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• Spontaneous Combustion is a type of combustion
  which occurs without an external ignition source.
  
• Heat builds up to the point of ignition
• Use chemicals to supply oxygen oxidizing agents

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Requirements for Combustion

• A fuel must be present
• Oxygen must be available in sufficient quantity
• Heat must be applied to initiate combustion, and
  sufficient heat must be generated to sustain the
  reaction.
• Rate of oxidation (reaction) Rate of oxidation
  of a fuel must be capable of sustaining a
  flaming fire.

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• Flash Point Minimum temperature at which a
  liquid fuel will produce enough vapor to burn.
• Flammable Range The (range of) composition of
  fuel-air mixture required for combustion.
• Flammable range of gasoline is 1.3- 6.

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Detonation

• The speed at which explosives decompose
  (detonation) varies greatly and permits the
  classification of explosives as high and low
  explosives.
• Low Explosives Low explosives have velocities
  of detonation less than 1,000 m/s.
• Black powder (KNO3CarbonSulfur751510)
  Merely burns when unconfined, used as safety fuse
  because of its low rate of detonation (rate of
  deflagration)

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• High Explosives classify high explosives as
  primary and secondary explosives .
• Primary explosives are very sensitive to heat and
  shock and will detonate violently (not burn).
• Lead azide and diazodinitrophenol are examples of
  Primary explosives.
• TNT is an example of a secondary explosive.

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Difficult to Analyze Arson

• These crimes are carried out at the convenience
  of the perpetrator and are often "well-planned"
  to hide crucial evidences
• Inability to collect crucial/useful evidence due
  to the accompanied destruction of the crime scene
  
• (iii) volatile evidences are hard to collect and
  preserve.

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Searching the Fire Scence

• Must be done soon after the suspected fire.
• Determine fire's origin (use of streamers to
  ignite multiple points at the same time)
• Normally, a fire has a tendency to move in an
  upward direction. The origin of the fire may be
  closest to the lowest point that show the most
  intense characteristic of burning. (weather
  condition will alter the normal progression of
  fire)
• GC-MS is the analytical method of choice