Lecture 1 Flashcards
Crime Scene to Court, the Essentials of Forensic Science, P. White, RSC,
What is an explosion?
A sudden
“A sudden and violent release of physical or chemical energy, often accompanied by the emission of light, heat and sound”
Chambers Science and Technology Dictionary
What is an explosion?
“A rapid increase of pressure in a confined space…generally caused by the occurrence of exothermic chemical reactions in which gases are produced in relatively large amounts”
What are the different types of explosions?
- Physical
- Chemical
- Nuclear
What is the power of an explosion related to?
The power of the explosion is related to the amount of gas released upon its detonation
What are chemical explosions generally related to?
These are generally related to thermodynamics and free energy we’re generating through detonation
What are explosives?
They are energetic materials but not all explosions are caused by energetic materials
What does an energetic material contain?
- An energetic material contains its own oxidant, its not relying on oxygen in the air.
- This makes it a more powerful and efficient explosion
What do we need to cause an explosion?
Fuel and an oxidant
What is an example of an explosion not caused by energetic material?
An example of an explosion not caused by energetic material is chemical explosion based on fuel reaction with oxygen in the atmosphere, we’re relying on the amount of oxygen in the atmosphere.
Typically what are the products of a detonation?
Gases
What gases are produced by a TNT explosion?
- CO
- CO2
- N2
- H2O
What’s the problem with aromatic rings in explosives?
Aromatic rings are not always favourable for explosives, partly because they’re stable which isn’t always what we want.
What is toluene?
It is a hydrocarbon which means it makes a good fuel
What is NO2?
- it is an oxidiser
- Nitro groups are often added to molecule to add an oxidant
What’s the problem with methane as a fuel?
It isn’t favoured in terms of its enthalpy
Why is forming nitrogen gas favourable?
Forming nitrogen gas is highly favoured due to it’s heat and enthalpy & entropy.
What are the features of a primary explosive?
- Sensitive to shock, heat, light & electricity
- Extremely dangerous to user
- Not generally synthesised on large scale
- Can set things off easily
- Not always advantageous
What are the features of a secondary explosive?
- More difficult to detonate
- Contact-insensitive
- Typically detonated by a small amount of primary explosive 🡪 ‘blasting cap’/detonator
What is the least sensitive explosive?
Tertiary explosives
What are the features of low explosives?
- Typically deflagrate
- They essentially burn except when pressurised (when they can detonate)
- Typically used as propellants
- They will burn quickly once initiated
What are some examples of low explosives?
- Gunpowder (traditional and smokeless)
- Fuels (e.g. propane, methane, hydrogen)
- Fireworks
What are the features of a high explosive?
- Typically detonate
- The chemical reaction yields a supersonic shock wave that propagates through the material
- Tend to contain the oxidiser in one molecule
- Detonates efficiently
- They will detonate once initiated
What can you do to a low explosive if you pressurize it?
If you can find a low explosive and pressurize it, it can detonate but typically the deflagrate (burn)
What does deflagrate mean?
Burn
What is deflagration?
A rapid oxidation reaction generating a low intensity pressure wave of moving gases
Where does propagation occur?
Propagation of the reaction takes place at the surface of the material and is subsonic (slower than the speed of sound 🡪 ~1000 m s-1)
Why are explosives typically powders?
They require a high surface area to have an effective burn rate
What does the pressure of an explosive determine?
The pressure that the explosive is deflagrating at is important and determines how quickly the material will be used up
What will a higher pressure result in?
A higher pressure will result in the material being used up quicker as more energy is generated.
What is burn rate proportional to?
Burn rate is proportional to area and pressure.
What is the purpose of low explosives?
Purpose of low explosives is to generate a lot of gas which is why they are typically used in propellants
How can you make a low explosive detonate?
Confinement and a large surface area may lead to detonation
How can you increase the burn rate for an explosive?
The same explosive could generate a faster burn rate if we increase the surface area
What does increasing the pressure and surface area lead to and why?
If we increase the pressure and surface area, this would increase the burn rate as we’re generating energy more rapidly and the gases are unable to escape.
Give the definition for detonation
A process of supersonic combustion in which a shock-wave is propagated forward due to energy release in a reaction zone behind it
Where does a reaction begin for detonation
- Energy is generated in the reaction zone
- As the detonation process begins, the gases expand quickly and start to leave the reaction zone towards the unreacted explosive
- The shockwave propagates from the left to right and rapidly detonates all the unreacted material.
What does the high-speed shockwave cause?
A process of supersonic combustion in which a shock-wave is propagated forward due to energy release in a reaction zone behind it
What is the advantage of using RDX as an explosive?
- One mole of RDX generates nine moles of gas which is very entropically favoured
- The heat of explosives is entropically favoured
- It doesn’t have enough oxygen to produce CO2 so when it detonates it will react with the air.
