lecture 8 - Chemistry of combustion

Question 1

what is the general formula of a combustion reaction

Answer 1

Fuel + Oxygen → Gases (CO2 and water vapour) + Heat + Light

Question 2

whats the difference between a fire and an explosive in regards to oxygen

Answer 2

fire gains oxygen from the air, whereas explosives produce their own

Question 3

what 3 things do all fires require to initiate

Answer 3

fuel, oxygen and heat

Question 4

name 3 factors that affect the rate of combustion

Answer 4

1. collision theory - the faster molecules are moving, the higher the collision rate
2. the physical state of the fuel - fires burn faster when the fuel is a gas
3. temperature - converts flammable liquids to vapour

Question 5

what is the definition of detonation

Answer 5

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

Question 6

what is the definition of deflagration

Answer 6

the speed of your combustion reaction is slower than the speed of sound, it is subsonic, and it produces a wave of flame front

Question 7

what does a lean mixture consist of

Answer 7

too much oxygen and not enough fuel

Question 8

what does a rich mixture consist of

Answer 8

too much fuel and not enough oxygen

Question 9

what is the flammable range for flammable liquids

Answer 9

the temperature required for the top layer of the liquid to become vapour

Question 10

give an example of a pyrolysable solid

Answer 10

wood

Question 11

what type of combustion do pyrolysable solids undergo and if they undergo further combustion which kind?

Answer 11

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

Question 12

give two examples of non-pyrolysable solids

Answer 12

charcoal and cigarettes

Question 13

what type of combustion does non-pyrolysable solids undergo

Answer 13

glowing combustion - oxidation at every surface

Question 14

what must you do to stop a fire

Answer 14

eliminate one of the fires three main elements

Question 15

what is the Gibbs free energy equation and what do all the letters mean

Answer 15

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

Question 16

what happens when you fire a gun?

Answer 16

1. The firing pin strikes the primer (a primary explosive), causing it to detonate and produce flames
2. 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
3. This is a low explosive train as the first combustion reaction causes the secondary explosive to ignite

Question 17

what is an example of deflagration and why?

Answer 17

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

Question 18

give an example of detonation and why?

Answer 18

An explosive - the temperature is high enough and will therefore cause a devastating shockwave

Question 19

give an example of a self-sustaining combustion reaction and how it sustains itself?

Answer 19

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

Question 20

explain the 3 different colours/areas of a laminar flame using a Candle as an example

Answer 20

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

Question 21

what are the 3 main parts of fire dynamics general model

Answer 21

1. Ignition and growth
2. Steady state
3. Decay

Question 22

what happens during a conventional fire

Answer 22

gases move upwards and outwards from the seat of the fire/origin, to produce a V-shaped burn pattern

Question 23

what is a flashover

Answer 23

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

Question 24

what is backdraft

Answer 24

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

Question 25

explain the changes in energy stores during a bullet ejection from a gun

Answer 25

mechanical to heat, heat to chemical and then kinetic

Question 26

what is a shockwave and how is it created?

Answer 26

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)

Question 27

what detonation speeds based on?

Answer 27

the explosives density - higher = faster

Question 28

name 3 examples of molecular explosives

Answer 28

Nitro-glycerine Mercury fulminate Nitrocellulose

Question 29

what are molecular explosives

Answer 29

the purest form of explosives that; detonate easily, are highly unstable and difficult to handle safely

Question 30

what is a low explosive

Answer 30

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

Question 31

Name three low explosives

Answer 31

Nitrocellulose Black powder - gun powder Propellants Flares

Question 32

what is a high explosive

Answer 32

a detonating explosive that is decomposed by high pressure shock waves which move sonically through the explosive material. Split into primary and secondary

Question 33

give 3 examples of primary explosives

Answer 33

Lead azide Lead styphnate Mercury fulminate

Question 34

what properties does a primary explosive have

Answer 34

- 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

Question 35

What is a secondary explosive and how does it work

Answer 35

the main explosive - its insensitive to shock and therefore, the only way to detonate it is via the primary explosives' shockwave

Question 36

name 3 examples of secondary explosives

Answer 36

1. 2,4,6-Trinitroluene (TNT) 2. Nitroglycerin (dynamite) 3. 1,3,5-Trinitro (RDX) 4. 1,3,5,7-Tetranitro (HMX) 5. ANFO - homemade bombs

Question 37

name a type of propellent and what its used for

Answer 37

smokeless powders - to accelerate a projectile to a high speed

Question 38

do propellants detonate?

Answer 38

NO - they ignite

Question 39

explain the difference between a single, double and triple propellant

Answer 39

Single - just nitrocellulose Double - Nitrocellulose and Nitro-glycerine Triple - Nitrocellulose + Nitro-glycerine + Nitroguanidine

Question 40

what can we use to analyse explosives

Answer 40

chromatography and spectroscopy

Question 41

what is the shockwave called that is created by the nose of the bullet

Answer 41

Mach cone

Question 42

What is the shockwave called that is created from the lower end of the bullet (not the back)

Answer 42

Cannelure

Question 43

what is the shockwave called created from the back of a bullet

Answer 43

Wake

Question 44

what are the 3 factors that affect the acceleration of a bullet

Answer 44

1. Rate of production of gas 2. Burn rate 3. Burn pattern

Question 45

how does the rate of production of gas affect the acceleration of a bullet

Answer 45

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

Question 46

how does the burn rate affect the acceleration of a bullet

Answer 46

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.

Question 47

how does burn pattern affect the acceleration of a bullet

Answer 47

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

Question 48

what are the 3 components found in gunshot residue

Answer 48

1. Lead (Pb) 2. Antimony (Sb) 3. Barium (Ba)

Question 49

what does definitive identification of GSR require

Answer 49

all three elements to be present

Question 50

what is used to detect GSR

Answer 50

Scanning electron microscopy-energy dispersive X-ray (SEMEDX)

Question 51

what is used to analyse GSR and how do you use it

Answer 51

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