Combustion Flashcards
Forensic Chemistry
What is a combustion reaction?
- Fuel reacting with oxygen and light energy.
- Water and carbon dioxide are produced.
- Three types of fuels and they are gas, solid and liquid.
Which type of explosives do not contain oxygen?
- Azide groups, as they naturally do not contain oxygen.
What is required and produced in a combustion reaction?
- Required= oxygen , fuel, heat energy.
- Produced= carbon dioxide, water vapour, light energy.
Is combustion an endothermic or exothermic reaction?
- Exothermic.
- Heat being released to the surroundings.
How is a combustion reaction initiated?
- When the fuel and oxygen are in the correct ratio.
- There is enough heat to break the bonds of the reactants and for the products to form.
- It is a chain reaction, as the energy that has been released will further break bonds in the reactants.
When will a combustion reaction finish?
- When all of the fuel is consumed first.
When will an explosive reaction finish?
- When all of the oxygen is consumed first.
What is detonation combustion?
- The speed of combustion reaction is faster than the speed of sound.
- A sonic reaction.
- Shockwaves have the ability to destroy anything in their path.
What is deflagration combustion?
- The speed of the combustion reaction is slower than the speed of sound.
- A subsonic reaction.
- A wave on flame front is produced.
What will happen if there is not enough oxygen in a combustion reaction?
- Combustion will not take place.
What is a lean mixture?
- When there is too much oxygen compared to fuel in a reaction.
What is a rich mixture?
- when there is too much fuel compared to oxygen in a combustion reaction.
What is a pyrolyzable solid and give an example.
- An example is wood and will decompose by pyrolysis when there is oxygen and high temperatures.
- Pyrolyzable gases will be produced and they will react with oxygen to produce heat and fire.
- When all the wood has been consumed, a carbon based residue will remain and it undergoes glowing combustion.
What is a non-pyrolyzable solid and give an example.
- An example is charcoal or cigarettes.
- Glowing combustion occurs, which is oxidation at every surface.
- Oxidation occurs at the outer surface and then it works on the next surface until there is nothing left.
What is the thermodynamics equation and what does each part of the equation stand for?
^G = ^H- T . ^S
^G= Gibbs free energy
^H= Change in enthalpy
^S= Change in entropy
T= temperature in k
Will Delta G have a positive or negative value in a combustion reaction?
- Delta G will have a high negative energy of free energy.
- Delta S will be positive as the gaseous products are cladding with one another.
- Delta H will have a negative value.
What does detonate mean?
- Explode or cause to explode.
What is a primer in a gun barrel?
- it is an explosive.
Describe a bullet ejection from a gun barrel.
- The firing pin in a gun strikes the primer, which is an explosive.
- The primer detonates and it produces a flame.
- The flame ignites the propellent which produces gases.
- The gases travel down the gun.
- Mechanical energy is converted into heat energy.
- Heat energy is then converted into chemical energy, which is then converted into kinetic.
Put in order all the types of energies in a bullet ejection in a gun barrel.
- Mechanical
- Heat
- Chemical
- Kinetic
What can a candle flame be known as?
A Laminar flame
What is a candle made out of?
- Hydrocarbons
How big are the carbon particles in a candle?
- 20 nanometres.
What part of a candle has the lowest temperature and why?
- The wick as it contains large alkanes.
What is the blue flame known as in a candle and why?
- Hydrocarbon cracking region.
- Hydrocarbons are being broken down.
- The aborption and emission of light.
What does UFL and UEL mean?
UFL= upper flammability limit.
UEL= upper explosive limit.
What does LFL and LEL mean?
LFL= low flammability limit.
LEL= low explosive limit
What is a conventional fire and what pattern shape is produced?
- Gases move upwards and outwards from the origin of the fire.
- A V shape pattern is produced.
What two types of conventional fires are there?
- Flashover
-Backdraft
Describe a flashover fire.
- The point during a fire where all the flammable vapours ignite.
- A layer of hot smoke is produced which is across the ceiling, heat is radiated downwards.
- Anything which is flammable will decompose and produce vapours.
- Temperature will exceed 600 and flashover will occur.
- Flashover will destroy everything.
Describe a backdraft fire.
- When the fire is contained in a room and it is starved of oxygen.
- The temperature is high enough for any flammables to combust and to produce vapours.
- If a door or window is opened and oxygen enters the room, it will combust into a flaming fire.
What is a shockwave?
- They are formed by pressurised air.
- They can be devastating.
What is the relationship between the density of an explosive and its speed.
- The higher the density of an explosive the higher the speed of detonation.
- The explosive that has the highest density and speed is HMX and the lowest explosive is TNT.
What are explosives made up of?
- They have a carbon hydrogen back bone.
- Carbon and nitrogen is contained in it.
- For azide groups, oxygen needs to be added so they can undergo combustion.
- When the explosive detonates, water vapour, carbon dioxide and nitrogen gas.
What are molecular explosives and give an example.
- They are difficult to handle and unstable.
- Explosive in their purest form.
- An example is mercury fulminate.
Describe low explosives and give an example.
- Deflagation explosive.
- Will detonate in a confined space.
- A shockwave will be produced at 100m/s and will produce a propelling or throwing action.
- An example of an explosive is black powder and flares.
Describe high explosives.
- Can be split into primary and secondary explosives.
- Decomposed by high pressure.
- Detonation explosive, shockwave moving faster than the speed of sound.
What is a primary explosives and provide an example.
- They are ultra sensitive.
- Will produce a powerful shockwave that can detonate a secondary explosive.
- An example is lead azide.
What is a secondary explosive and provide three examples.
- They are classed as main explosives.
- Will burn as they are sensitive to shock and friction.
- shock wave will only detonate from a primary explosive.
Three examples are:
1. Dynamite
2. ANFO
3. Semtex
Describe propellents.
- They ignite.
-Used to accelerate a projectile.
-Smokeless powders are used.
Name the three smokeless powders.
- Single base- nitrocellulose.
- Double base- nitrocellulose and nitroglycerine.
- Triple base- nitrocellulose, nitroglycerine, nitroguanidine.
How many bullet shockwaves are there and what are they known as?
1st shockwave= known as the mach cone and comes from the bullet nose.
2nd shockwave= known as the cannelure and comes from the middle of the bullet.
3rd shockwave= known as the wake and comes from the base of the bullet.
What does GSR stand for?
- Gun shot residue.
What is GSR made up of?
- Antimony, lead and barium.
- For a positive result all three need to be present.
How can GSR be analysed?
-Analysed by using Scanning electron microscopy-energy dispersive X ray (SEM-EDX).
- Can determine the elemental composition of the particles.
- Can produce images of the particles.
What factors can affect the rate of a bullet?
- Burnt rate
- Rate of production gas
- Burn pattern
