Combustion Flashcards
Forensic Chemistry (47 cards)
1
Q
What is a combustion reaction?
A
- Fuel reacting with oxygen and light energy.
- Water and carbon dioxide are produced.
- Three types of fuels and they are gas, solid and liquid.
2
Q
Which type of explosives do not contain oxygen?
A
- Azide groups, as they naturally do not contain oxygen.
3
Q
What is required and produced in a combustion reaction?
A
- Required= oxygen , fuel, heat energy.
- Produced= carbon dioxide, water vapour, light energy.
4
Q
Is combustion an endothermic or exothermic reaction?
A
- Exothermic.
- Heat being released to the surroundings.
5
Q
How is a combustion reaction initiated?
A
- 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.
6
Q
When will a combustion reaction finish?
A
- When all of the fuel is consumed first.
7
Q
When will an explosive reaction finish?
A
- When all of the oxygen is consumed first.
8
Q
What is detonation combustion?
A
- 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.
9
Q
What is deflagration combustion?
A
- The speed of the combustion reaction is slower than the speed of sound.
- A subsonic reaction.
- A wave on flame front is produced.
10
Q
What will happen if there is not enough oxygen in a combustion reaction?
A
- Combustion will not take place.
11
Q
What is a lean mixture?
A
- When there is too much oxygen compared to fuel in a reaction.
12
Q
What is a rich mixture?
A
- when there is too much fuel compared to oxygen in a combustion reaction.
13
Q
What is a pyrolyzable solid and give an example.
A
- 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.
14
Q
What is a non-pyrolyzable solid and give an example.
A
- 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.
15
Q
What is the thermodynamics equation and what does each part of the equation stand for?
A
^G = ^H- T . ^S
^G= Gibbs free energy
^H= Change in enthalpy
^S= Change in entropy
T= temperature in k
16
Q
Will Delta G have a positive or negative value in a combustion reaction?
A
- 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.
17
Q
What does detonate mean?
A
- Explode or cause to explode.
18
Q
What is a primer in a gun barrel?
A
- it is an explosive.
19
Q
Describe a bullet ejection from a gun barrel.
A
- 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.
20
Q
Put in order all the types of energies in a bullet ejection in a gun barrel.
A
- Mechanical
- Heat
- Chemical
- Kinetic
21
Q
What can a candle flame be known as?
A
A Laminar flame
22
Q
What is a candle made out of?
A
- Hydrocarbons
23
Q
How big are the carbon particles in a candle?
A
- 20 nanometres.
24
Q
What part of a candle has the lowest temperature and why?
A
- The wick as it contains large alkanes.
25
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.
26
What does UFL and UEL mean?
UFL= upper flammability limit.
UEL= upper explosive limit.
27
What does LFL and LEL mean?
LFL= low flammability limit.
LEL= low explosive limit
28
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.
29
What two types of conventional fires are there?
- Flashover
-Backdraft
30
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.
31
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.
32
What is a shockwave?
- They are formed by pressurised air.
- They can be devastating.
33
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.
34
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.
35
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.
36
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.
37
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.
38
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.
39
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
40
Describe propellents.
- They ignite.
-Used to accelerate a projectile.
-Smokeless powders are used.
41
Name the three smokeless powders.
1. Single base- nitrocellulose.
2. Double base- nitrocellulose and nitroglycerine.
3. Triple base- nitrocellulose, nitroglycerine, nitroguanidine.
42
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.
43
What does GSR stand for?
- Gun shot residue.
44
What is GSR made up of?
- Antimony, lead and barium.
- For a positive result all three need to be present.
45
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.
46
What factors can affect the rate of a bullet?
- Burnt rate
- Rate of production gas
- Burn pattern
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