4103FSBMOL - Lecture 8 - Chemistry of Combustion. Flashcards
What is the Symbol and Word Equation for a Combustion Reaction?
Balance the equation.
- Fuel + Oxygen –> Gases + Heat + Light.
- CH4 + 2O2 –> CO2 + 2H2O + Heat + Light.
To initiate a combustion reaction, what are the 3 things you need?
Fuel, oxygen and heat - they should be in the correct ratio.
What are the 3 different types of fuel?
Give examples of each.
- Solids - Wood, Fabric, Paper.
- Liquids - Diesel, Petrol, Fuel.
- Gases - Natural Gas, Propane, Butane.
Where does the oxyegn come from in a combustion reaction?
The air and/ or explosives.
Is breaking bonds endothermic or exothermic?
Exothermic (releasing energy).
What are the 2 factors which affect the rate of comubstion?
- Physical State of the Fuel - a gaseous state provides more collisions.
- Temperature - must be high enough to covert solvent molecules.
What is the Definition of Detonation?
Speed of combustion reaction is faster than the speed of sound - sonic and produces devastating shock wave which will destroy anything in its path.
What is the Definition of Deflagration?
Speed of combustion reaction is slower then the speed of sound - subsonic - produces a wave or flame front.
What are the properties of Flammable Liquids?
- They burn at their surface.
- They only combust in their flammable range.
- Too much fuel = rich mixture.
- Too much oxygen, not enough fuel = lean mixture.
- Can only combust when the fuel-oxygen ratio is correct.
What are the 2 different types of flammable solid?
- Pyrolyzable Solid.
- Non-Pyrolyzable Solid.
Explain what a Pyrolyzable Solid is and give an example.
An example is Wood. They decompose and produce gaseous products producing heat and fire which causes decomposition. This keeps decomposing producing fire until a carbon product is left which doesn’t - glowing combustion (smouldering).
Explain what a Non-Pyrolyzable Solid is and give an example.
An example is Charcoal/ Cigarettes. Undergoes glowing combustion which consumes itself. Carbon on the surface oxidises.
What is Gibbs Free Equation?
∆G = ∆H - T∆S.
What does the ∆G mean in Gibbs Free Eqauation?
Change in free energy (Gibbs Constant).
What does the ∆H mean in Gibbs Free Eqauation?
Change in Enthalpy (measurement of the overall amount of energy in the system).
(heat energy released).
What does the T mean in Gibbs Free Eqauation?
Temperature (in Kelvin).
What does the ∆S mean in Gibbs Free Eqauation?
Change in Entropy (randomness/ order of activity in a system).
Would ∆S be positive or negative?
Positive (+) as molecules are moving to a highly disordered system.
How does a Bullet get ejected from a Gun Barrel?
The firing pin strikes the primer - mechanical energy - (primary explosion) and produces flames which ignite the propellent which produces gases which push the bullet down the gun.
Low Explosive Train - an arrangement of explosives that uses a small amount of a primary high explosive to initiate a larger amount of a secondary low explosive.
What type of energy changes occurs when firing a bullet from a gun?
Mechanical energy into heat energy into chemical energy into kinetic/ movement energy.
Give an example of Deflagration Combustion.
A Candle - temperature is not high enough, less collisions.
Give an example of Detonation Combustion.
An Explosive - temperature is high enough, more collisions.
In a combustion reaction, what happens to the heat tarnsfer?
Some Heat is an initiator of the reaction, some is consumed, but most is released to the atomosphere.
Explain the Heat Transfer in a Candle.
Wick is placed into wax. Heat melts the wax into vapour molecules. Heat radiates outwards and pushes heat back in.
(Heated to incandescence - top yellow bit of flame).
What type of Combustion Reaction does a Candle produce?
A Self-Sustaining Combustion Reaction
What can a Candle also be called?
A Laminar Flame.
What happens in the blue part of a flame?
Hydrocarbon Cracking Reigion - involves the absorption and emission of blue light.
Give an example of another Laminar Flame.
A Bunsen Burner - methane and oxygen ratio correct to produce blue flame.
Where is the hottest part of a flame?
The tip of the inner blue flame.
What happens if a flame has too much methane and not enough oxygen?
It will produce a yellow/ orange flame - produces carbon.
What is the name of the boundaries of the Flammable Range?
Upper Flammable/ Explosive Limit and Lower Flammable/ Explosive Limit.
Is Arson Homogeneous or Heterogeneous?
Homogeneous - thing don’t mix.
Which is less dense: natural gas or air?
What does this mean?
Natural Gas.
It will dissipate upwards and cause a Rich Layer at the Top and a Lean layer at the bottom.
Practice Question:
What material/s are required for combustion?
- Fuel.
- Oxyegn.
- Heat.
- Fuel, Oxygen and Heat.
- Fuel, Oxygen and Heat.
Practice Question:
A combustion reaction that travels slower than the speed of sound is a…
- Detonation.
- Deflagration.
- Deflagration.
Practice Question:
Wood - What type of solid is it and how does it combust?
- Non-pyrolyzable, Flaming combustion.
- Pyrolyzable, Glowing combustion.
- Non-pyrolyzable, Glowing combsution.
- Pyrolyzable, Flaming combustion.
- Pyrolyzable, Flaming combustion.
What is the 3 steps of Fire Dynamics?
- Ignition and Growth.
- Steady State - enough fuel and oxygen to maintain fire.
- Decay - when reactant is consumed (carbon based residue undergoing glowing combustion left).
What burn pattern is created from a Conventional Fire?
(e.g. pattern on the wall).
A V-shaped burn pattern. Gas moves upwards and outwards from the seat/ point of origin of the fire.
What happens in Flashover?
Smoke is produced (layer formed around the top of room - radiates heat downwards. Decomposes flammable items. Produces pyrolyzable vapours. Temperature rises and when temp gets to 600°c, the whole room will combust and go to flashover.
(It will destroy EVERYTHING!)
What happens in Backdraft?
In a tightly closed room, the fire is starved of oxygen - will subside. Room rich in fuel vapours. Temperature in room is high enough to form vapours. As soon as** oxygen (door opened) it will cause a boom (ignition)** as oxygen is reintroduced. Person opening a door will fall backwards (backdraft).
What are the Different Types of Explosives?
- Molecular.
- Detonation.
- Deflagration.
- High.
- Low.
- Primary.
- Secondary.
How is a shockwave produced?
Spark produces a wave front and heat causes another combustion, causing another wave front. A shockwave is where a wave front goes faster than the other wavefronts. Shockwave is just air but can be a devastating force. It is a Wall of High Pressure.
What are the Detonation Speeds and Densities of some common explosives?
- HMX Explosive - Density = 1.89g/ml, Speed of Detonation = 9100m/s.
- RDX Explosive - Density = 1.70g/ml, Speed of Detonation = 8440m/s.
- Nitroglycerin - Density = 1.60g/ml, Speed of Detonation = 7900m/s.
- TNT Explosive - Density = 1.55g/ml, Speed of Detonation = 6850m/s.
Higher Density = Higher Detonation Speed.
What are the structural features of Explosives?
- Carbon-Hydrogen based backbone.
- Contains Nitrogen(s) and Oxygen(s).
What are Molecular Explosives?
They are explosives which detonate easily and they are highly unstable and this means they are difficult to handle safely.
What are Examples of a Molecular Explosive?
Nitro-glycerine, Mercury Fulminate and Nitrocellulose. They are in their purest form.
What are Low Explosives?
Deflagration explosive which is decomposed by a flame front which moves sub-sonically through the explosive material. They won’t detonate but can detonate in a confined space.
What are Examples of a Low Explosive?
Nitrocellulose, Black Powder (Gun powder), Propellants and flares.
What are High Explosives?
Detonation explosive decomposed by high pressure shock waves which move sonically through the explosive material. A detonation can be up to a shockwave speed of 1000m/s.
What are Primary Explosives?
They are ultra sensitive and can detonate secondary explosives and propellants. They can be detonated by friction.
They are the 1st step in low and high explosive trains. 2-step high explosive train has a blasting cap with a secondary explosive on the end. 3-step high explosive train has a blasting cap, secondary explosive and a main charge.
What are Examples of Primary Explosives?
Blasting caps, military fuses and primers in cartridges.
What are Secondary Explosives?
They are the MAIN EXPLOSIVE. They will burn and are insensitive to shock. They can only be detonated from a shockwave from a primary explosive.
What are Examples of Secondary Explosives?
TNT, Dynamite, HMX, RDX - C4, PETN, Semtex, ANFO.
What is the most powerful military garde explosive?
HMX.
What is the most common military garde explosive?
RDX - C4.
What is a Propellant?
They are used to accelerate a projective to a high speed (e.g. a bullet). THEY DON’T DETONATE, THEY IGNITE! We use Smokeless powders: Single, Double or Triple Base.
What is the Structure of a Bullet Cartridge?
The main tip is the bullet. There is the cartridge case which contains the propellant. The Primary explosive is located at the bottom covered by the rim.
What follows the bullet out the end of a gun?
A ring of fire made of burnt and unburnt propellant.
What are the 3 types of shockwave which come from a Bullet?
- Machcone - Shockwave at the tip of the bullet.
- Cannelure - Shockwave around the middle of the bullet.
- Wake - Shockwave at the trail of the bullet.
What are the factors which affect the acceleration of a bullet?
- Rate of Production of Gas - rapid but steady rate. Too fast = bullet slows, Too slow = bullet pushed out.
- Burn Rate - controlled by propellant particle shape and any particles. Ensures maximum kinetic energy imparted to the bullet. Ethyl centralite.
- Burn Pattern - if there is a deterrent it will burn one end to the other consuming the deterrent - progressive pattern. If burns outside in (degressive pattern). Neutral pattern will burn and produce pores.
What does GSR stand for?
Gun Shot Residue.
For Positive Identification of Gun Shot Residue, what does it need to contain?
Lead, Antimony and Barium.
What is the only technique that can be used to identify Gun Shot Residue?
SEM-EDX.
What does SEM-EDX stand for?
Scanning Electron Microscopy - Energy Dispersive X-ray.
What do you have to collect Exposive and Fire Debris in, at a crime scene?
You have to collect in a nylon bag and then that is placed into another nylon bag.
What are the ways to Analyse Explosive and Fire Debris?
Solid Phase Microextraction, Gas Chromatograpy and Mass Spectrometry.
What does EOD stand for?
Explosive Ordinance Disposal.
What does IED stand for and what are they?
Improvised Explosive Device - They are devices which are placed or fabricated in an improvised manner, incorporating destructive, lethal, noxious, pyrotechnic or incendiary chemicals, and designed to destroy, incapacitate, harass or distract. It may incorporate military stores, but they are normally devised from non-military components.
What are the steps in the IEDD Philosophy?
- Preservation of Life.
- Preservation of Property.
- Preservation and Collection of forensic evidence, without compromising saftey.
- Return the situation to normal.
What different IED types can you have?
Give examples of each.
- Time - detonated by a timer (e.g. watch).
- Command - detonated by command (e.g. mobile phone).
- Victim Operated - device strapped to a victims chest while the victim holds the detonator trigger.
