lecture 8 - Chemistry of combustion Flashcards
what is the general formula of a combustion reaction
Fuel + Oxygen → Gases (CO2 and water vapour) + Heat + Light
whats the difference between a fire and an explosive in regards to oxygen
fire gains oxygen from the air, whereas explosives produce their own
what 3 things do all fires require to initiate
fuel, oxygen and heat
name 3 factors that affect the rate of combustion
- collision theory - the faster molecules are moving, the higher the collision rate
- the physical state of the fuel - fires burn faster when the fuel is a gas
- temperature - converts flammable liquids to vapour
what is the definition of detonation
the speed of the combustion reaction is faster than the speed of sound, its sonic, and produces a devastating shock wave. It destroys anything in its path
what is the definition of deflagration
the speed of your combustion reaction is slower than the speed of sound, it is subsonic, and it produces a wave of flame front
what does a lean mixture consist of
too much oxygen and not enough fuel
what does a rich mixture consist of
too much fuel and not enough oxygen
what is the flammable range for flammable liquids
the temperature required for the top layer of the liquid to become vapour
give an example of a pyrolysable solid
wood
what type of combustion do pyrolysable solids undergo and if they undergo further combustion which kind?
flaming combustion - wood decomposes by pyrolysis producing pyrolysable gaseous products that when mixed with oxygen will ignite. This will continue until all that’s left is a carbon based residue that will undergo glowing combustion/smouldering
give two examples of non-pyrolysable solids
charcoal and cigarettes
what type of combustion does non-pyrolysable solids undergo
glowing combustion - oxidation at every surface
what must you do to stop a fire
eliminate one of the fires three main elements
what is the Gibbs free energy equation and what do all the letters mean
ΔG = ΔH - TΔS
▪ ΔH = the change in enthalpy - determines if the reaction is endo/exothermic
▪ ΔS = the change in the order of your system - solid is ordered, gas is disordered
▪ ΔG = changes in free energy, and also measures the amount of work undertaken by a system
what happens when you fire a gun?
- The firing pin strikes the primer (a primary explosive), causing it to detonate and produce flames
- The flames ignite the propellent in the gun barrel, which then produces gases which cause the bullet to travel down the barrel of the gun
- This is a low explosive train as the first combustion reaction causes the secondary explosive to ignite
what is an example of deflagration and why?
A candle - the temperature is not high enough so the molecules are moving slower, therefore, there are less collisions and you only see a flame
give an example of detonation and why?
An explosive - the temperature is high enough and will therefore cause a devastating shockwave
give an example of a self-sustaining combustion reaction and how it sustains itself?
A candle - The wick burns, creating heat to melt the wax beneath it, this produces vapour molecules which will combust in the presence of oxygen, creating a flame. The flame radiates heat outwards and back down towards the wax, so that more wax melts in order to maintain the flame
explain the 3 different colours/areas of a laminar flame using a Candle as an example
White/yellow flame - Carbon particles around 20nm are heated to incandescence
Close to the wick - lowest temperature as it has very large alkanes in it that are doing nothing
Blue flame - hydrocarbon cracking region, produces blue light due to the long chain alkanes being broken down carbon, by carbon, systematically absorbing heat and emitting light
what are the 3 main parts of fire dynamics general model
- Ignition and growth
- Steady state
- Decay
what happens during a conventional fire
gases move upwards and outwards from the seat of the fire/origin, to produce a V-shaped burn pattern
what is a flashover
where a layer of smoke will be constantly gaining more pyrolysable vapours until at around 600 degrees all vapours will ignite and the whole room will combust
what is backdraft
where a tightly closed room is starved of oxygen, allowing for the build up of fuel vapours, but then is reintroduced to oxygen causing the whole room to combust into flaming fire
explain the changes in energy stores during a bullet ejection from a gun
mechanical to heat, heat to chemical and then kinetic
what is a shockwave and how is it created?
A ring of high pressurised air - a spark causes a combustion reaction, which creates a wave front. The heat produced then causes a second combustion reaction, producing a second wave front. This continues until one wave front is faster than them all (the shock wave)
what detonation speeds based on?
the explosives density - higher = faster
name 3 examples of molecular explosives
Nitro-glycerine
Mercury fulminate
Nitrocellulose
what are molecular explosives
the purest form of explosives that; detonate easily, are highly unstable and difficult to handle safely
what is a low explosive
A deflagration explosive which is decomposed by a flame front which moves sub-sonically through the explosive material to produce a propelling/throwing action. They will only detonate if the explosive is confined
Name three low explosives
Nitrocellulose
Black powder - gun powder
Propellants
Flares
what is a high explosive
a detonating explosive that is decomposed by high pressure shock waves which move sonically through the explosive material. Split into primary and secondary
give 3 examples of primary explosives
Lead azide
Lead styphnate
Mercury fulminate
what properties does a primary explosive have
- ultra sensitive to shock, impact, sparks and friction
produce very strong shock waves that detonate secondary explosives and ignite propellants - can be one, two or three step - second step requires a blasting cap and the last step is called main charge
What is a secondary explosive and how does it work
the main explosive - its insensitive to shock and therefore, the only way to detonate it is via the primary explosives’ shockwave
name 3 examples of secondary explosives
- 2,4,6-Trinitroluene (TNT)
- Nitroglycerin (dynamite)
- 1,3,5-Trinitro (RDX)
- 1,3,5,7-Tetranitro (HMX)
- ANFO - homemade bombs
name a type of propellent and what its used for
smokeless powders - to accelerate a projectile to a high speed
do propellants detonate?
NO - they ignite
explain the difference between a single, double and triple propellant
Single - just nitrocellulose
Double - Nitrocellulose and Nitro-glycerine
Triple - Nitrocellulose + Nitro-glycerine + Nitroguanidine
what can we use to analyse explosives
chromatography and spectroscopy
what is the shockwave called that is created by the nose of the bullet
Mach cone
What is the shockwave called that is created from the lower end of the bullet (not the back)
Cannelure
what is the shockwave called created from the back of a bullet
Wake
what are the 3 factors that affect the acceleration of a bullet
- Rate of production of gas
- Burn rate
- Burn pattern
how does the rate of production of gas affect the acceleration of a bullet
To fast - the bullet will slow down as it leaves the barrel
To slow - the bullet will leave the barrel before all the propellant is used up
how does the burn rate affect the acceleration of a bullet
it ensures maximum kinetic energy is imparted to the bullet and can be controlled by propellant particle shape and whether the particles are coated or not.
how does burn pattern affect the acceleration of a bullet
Progressive - when the bullet is coated in a deterrent - burns from one end to the other
Degressive - burns from the outside in
Neutral - it will burn, produce pores and increase its surface area
what are the 3 components found in gunshot residue
- Lead (Pb)
- Antimony (Sb)
- Barium (Ba)
what does definitive identification of GSR require
all three elements to be present
what is used to detect GSR
Scanning electron microscopy-energy dispersive X-ray (SEMEDX)
what is used to analyse GSR and how do you use it
Solid phase micro extraction using GC-MS:
-Insert the fibre into the corner of the nylon bag
-Then open up the fibre to reveal the stationary phase
-The explosives/accelerants are adsorbed onto this stationary phase
-Allow for time - use standard operating procedure
-Move the fibre out of the bags, and place directly into a GC-MS - NO NEED FOR SOLVENTS
