Lecture 1

Question 1

Crime Scene to Court, the Essentials of Forensic Science, P. White, RSC,

What is an explosion?

A sudden

Answer 1

“A sudden and violent release of physical or chemical energy, often accompanied by the emission of light, heat and sound”

Question 2

Chambers Science and Technology Dictionary

What is an explosion?

Answer 2

“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”

Question 3

What are the different types of explosions?

Answer 3

• Physical
• Chemical
• Nuclear

Question 4

What is the power of an explosion related to?

Answer 4

The power of the explosion is related to the amount of gas released upon its detonation

Question 5

What are chemical explosions generally related to?

Answer 5

These are generally related to thermodynamics and free energy we’re generating through detonation

Question 6

What are explosives?

Answer 6

They are energetic materials but not all explosions are caused by energetic materials

Question 7

What does an energetic material contain?

Answer 7

• An energetic material contains its own oxidant, its not relying on oxygen in the air.
• This makes it a more powerful and efficient explosion

Question 8

What do we need to cause an explosion?

Answer 8

Fuel and an oxidant

Question 9

What is an example of an explosion not caused by energetic material?

Answer 9

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.

Question 10

Typically what are the products of a detonation?

Answer 10

Gases

Question 11

What gases are produced by a TNT explosion?

Answer 11

• CO
• CO2
• N2
• H2O

Question 12

What’s the problem with aromatic rings in explosives?

Answer 12

Aromatic rings are not always favourable for explosives, partly because they’re stable which isn’t always what we want.

Question 13

What is toluene?

Answer 13

It is a hydrocarbon which means it makes a good fuel

Question 14

What is NO2?

Answer 14

• it is an oxidiser
• Nitro groups are often added to molecule to add an oxidant

Question 15

What’s the problem with methane as a fuel?

Answer 15

It isn’t favoured in terms of its enthalpy

Question 16

Why is forming nitrogen gas favourable?

Answer 16

Forming nitrogen gas is highly favoured due to it’s heat and enthalpy & entropy.

Question 17

What are the features of a primary explosive?

Answer 17

• Sensitive to shock, heat, light & electricity
• Extremely dangerous to user
• Not generally synthesised on large scale
• Can set things off easily
• Not always advantageous

Question 18

What are the features of a secondary explosive?

Answer 18

• More difficult to detonate
• Contact-insensitive
• Typically detonated by a small amount of primary explosive 🡪 ‘blasting cap’/detonator

Question 19

What is the least sensitive explosive?

Answer 19

Tertiary explosives

Question 20

What are the features of low explosives?

Answer 20

• Typically deflagrate
• They essentially burn except when pressurised (when they can detonate)
• Typically used as propellants
• They will burn quickly once initiated

Question 21

What are some examples of low explosives?

Answer 21

• Gunpowder (traditional and smokeless)
• Fuels (e.g. propane, methane, hydrogen)
• Fireworks

Question 22

What are the features of a high explosive?

Answer 22

• 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

Question 23

What can you do to a low explosive if you pressurize it?

Answer 23

If you can find a low explosive and pressurize it, it can detonate but typically the deflagrate (burn)

Question 24

What does deflagrate mean?

Answer 24

Burn

Question 25

What is deflagration?

Answer 25

A rapid oxidation reaction generating a low intensity pressure wave of moving gases

Question 26

Where does propagation occur?

Answer 26

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)

Question 27

Why are explosives typically powders?

Answer 27

They require a high surface area to have an effective burn rate

Question 28

What does the pressure of an explosive determine?

Answer 28

The pressure that the explosive is deflagrating at is important and determines how quickly the material will be used up

Question 29

What will a higher pressure result in?

Answer 29

A higher pressure will result in the material being used up quicker as more energy is generated.

Question 30

What is burn rate proportional to?

Answer 30

Burn rate is proportional to area and pressure.

Question 31

What is the purpose of low explosives?

Answer 31

Purpose of low explosives is to generate a lot of gas which is why they are typically used in propellants

Question 32

How can you make a low explosive detonate?

Answer 32

Confinement and a large surface area may lead to detonation

Question 33

How can you increase the burn rate for an explosive?

Answer 33

The same explosive could generate a faster burn rate if we increase the surface area

Question 34

What does increasing the pressure and surface area lead to and why?

Answer 34

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.

Question 35

Give the definition for detonation

Answer 35

A process of supersonic combustion in which a shock-wave is propagated forward due to energy release in a reaction zone behind it

Question 36

Where does a reaction begin for detonation

Answer 36

- 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.

Question 37

What does the high-speed shockwave cause?

Answer 37

A process of supersonic combustion in which a shock-wave is propagated forward due to energy release in a reaction zone behind it

Question 38

What is the advantage of using RDX as an explosive?

Answer 38

- 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.