4103FSBMOL - Lecture 8 - Chemistry of Combustion.

Question 1

What is the Symbol and Word Equation for a Combustion Reaction?

Balance the equation.

Answer 1

• Fuel + Oxygen –> Gases + Heat + Light.
• CH₄ + 2O₂ –> CO₂ + 2H₂O + Heat + Light.

Question 2

To initiate a combustion reaction, what are the 3 things you need?

Answer 2

Fuel, oxygen and heat - they should be in the correct ratio.

Question 3

What are the 3 different types of fuel?

Give examples of each.

Answer 3

• Solids - Wood, Fabric, Paper.
• Liquids - Diesel, Petrol, Fuel.
• Gases - Natural Gas, Propane, Butane.

Question 4

Where does the oxyegn come from in a combustion reaction?

Answer 4

The air and/ or explosives.

Question 5

Is breaking bonds endothermic or exothermic?

Answer 5

Exothermic (releasing energy).

Question 6

What are the 2 factors which affect the rate of comubstion?

Answer 6

1. Physical State of the Fuel - a gaseous state provides more collisions.
2. Temperature - must be high enough to covert solvent molecules.

Question 7

What is the Definition of Detonation?

Answer 7

Speed of combustion reaction is faster than the speed of sound - sonic and produces devastating shock wave which will destroy anything in its path.

Question 8

What is the Definition of Deflagration?

Answer 8

Speed of combustion reaction is slower then the speed of sound - subsonic - produces a wave or flame front.

Question 9

What are the properties of Flammable Liquids?

Answer 9

• 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.

Question 10

What are the 2 different types of flammable solid?

Answer 10

• Pyrolyzable Solid.
• Non-Pyrolyzable Solid.

Question 11

Explain what a Pyrolyzable Solid is and give an example.

Answer 11

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).

Question 12

Explain what a Non-Pyrolyzable Solid is and give an example.

Answer 12

An example is Charcoal/ Cigarettes. Undergoes glowing combustion which consumes itself. Carbon on the surface oxidises.

Question 13

What is Gibbs Free Equation?

Answer 13

∆G = ∆H - T∆S.

Question 14

What does the ∆G mean in Gibbs Free Eqauation?

Answer 14

Change in free energy (Gibbs Constant).

Question 15

What does the ∆H mean in Gibbs Free Eqauation?

Answer 15

Change in Enthalpy (measurement of the overall amount of energy in the system).

(heat energy released).

Question 16

What does the T mean in Gibbs Free Eqauation?

Answer 16

Temperature (in Kelvin).

Question 17

What does the ∆S mean in Gibbs Free Eqauation?

Answer 17

Change in Entropy (randomness/ order of activity in a system).

Question 18

Would ∆S be positive or negative?

Answer 18

Positive (+) as molecules are moving to a highly disordered system.

Question 19

How does a Bullet get ejected from a Gun Barrel?

Answer 19

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.

Question 20

What type of energy changes occurs when firing a bullet from a gun?

Answer 20

Mechanical energy into heat energy into chemical energy into kinetic/ movement energy.

Question 21

Give an example of Deflagration Combustion.

Answer 21

A Candle - temperature is not high enough, less collisions.

Question 22

Give an example of Detonation Combustion.

Answer 22

An Explosive - temperature is high enough, more collisions.

Question 23

In a combustion reaction, what happens to the heat tarnsfer?

Answer 23

Some Heat is an initiator of the reaction, some is consumed, but most is released to the atomosphere.

Question 24

Explain the Heat Transfer in a Candle.

Answer 24

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).

Question 25

What *type of Combustion Reaction* does a Candle produce?

Answer 25

A **Self-Sustaining** Combustion Reaction

Question 26

What can a Candle also be called?

Answer 26

A **Laminar Flame.**

Question 27

What happens in the blue part of a flame?

Answer 27

Hydrocarbon **Cracking** Reigion - involves the absorption and emission of blue light.

Question 28

Give an **example** of another *Laminar Flame.*

Answer 28

A **Bunsen Burner** - methane and oxygen ratio correct to produce blue flame.

Question 29

Where is the **hottest part of a flame**?

Answer 29

The tip of the inner blue flame.

Question 30

What happens if a flame has *too much methane and not enough oxygen*?

Answer 30

It will produce a **yellow/ orange flame** - produces carbon.

Question 31

What is the name of the boundaries of the Flammable Range?

Answer 31

Upper Flammable/ Explosive Limit and Lower Flammable/ Explosive Limit.

Question 32

Is **Arson** Homogeneous or Heterogeneous?

Answer 32

**Homo**geneous - thing don't mix.

Question 33

Which is *less* dense: natural gas or air? ## Footnote What does this mean?

Answer 33

Natural Gas. ## Footnote It will dissipate upwards and cause a **Rich** Layer at the *Top* and a **Lean** layer at the *bottom.*

Question 34

# **_Practice Question:_** What material/s are required for combustion? ## Footnote 1. Fuel. 2. Oxyegn. 3. Heat. 4. Fuel, Oxygen and Heat.

Answer 34

4. Fuel, Oxygen and Heat.

Question 35

# **_Practice Question:_** A combustion reaction that travels *slower* than the speed of sound is a… ## Footnote 1. Detonation. 2. Deflagration.

Answer 35

2. Deflagration.

Question 36

# **_Practice Question:_** Wood - What type of solid is it and how does it combust? ## Footnote 1. Non-pyrolyzable, Flaming combustion. 2. Pyrolyzable, Glowing combustion. 3. Non-pyrolyzable, Glowing combsution. 4. Pyrolyzable, Flaming combustion.

Answer 36

4. Pyrolyzable, Flaming combustion.

Question 37

What is the *3 steps* of **Fire Dynamics**?

Answer 37

1. **Ignition and Growth.** 2. **Steady State** - enough fuel and oxygen to maintain fire. 3. **Decay** - when reactant is consumed (carbon based residue undergoing glowing combustion left).

Question 38

What burn pattern is created from a Conventional Fire? | (e.g. pattern on the wall).

Answer 38

A **V-shaped** burn pattern. Gas moves *upwards and outwards* from the seat/ point of origin of the fire.

Question 39

What happens in **Flashover**?

Answer 39

**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!)

Question 40

What happens in **Backdraft**?

Answer 40

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).*

Question 41

What are the *Different Types* of **Explosives**?

Answer 41

* Molecular. * Detonation. * Deflagration. * High. * Low. * Primary. * Secondary.

Question 42

How is a **shockwave** produced?

Answer 42

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.**

Question 43

What are the *Detonation Speeds and Densities* of some common explosives?

Answer 43

* **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. ## Footnote **Higher Density = Higher Detonation Speed.**

Question 44

What are the **structural features** of *Explosives?*

Answer 44

* Carbon-Hydrogen based backbone. * Contains **Nitrogen(s) and Oxygen(s).**

Question 45

What are **Molecular Explosives**?

Answer 45

They are explosives which **detonate easily** and they are *highly unstable* and this means they are difficult to handle safely.

Question 46

What are **Examples** of a *Molecular Explosive?*

Answer 46

**Nitro-glycerine, Mercury Fulminate and Nitrocellulose.** They are in their purest form.

Question 47

What are **Low Explosives**?

Answer 47

**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.

Question 48

What are **Examples** of a *Low Explosive?*

Answer 48

Nitrocellulose, Black Powder (Gun powder), Propellants and flares.

Question 49

What are **High Explosives**?

Answer 49

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.**

Question 50

What are **Primary Explosives**?

Answer 50

They are ultra sensitive and **can detonate secondary explosives and propellants.** They can be detonated by *friction.* They are the 1^st 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.*

Question 51

What are **Examples** of *Primary Explosives?*

Answer 51

Blasting caps, military fuses and primers in cartridges.

Question 52

What are **Secondary Explosives**?

Answer 52

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.

Question 53

What are **Examples** of *Secondary Explosives?*

Answer 53

TNT, Dynamite, HMX, RDX - C4, PETN, Semtex, ANFO.

Question 54

What is the *most powerful* military garde explosive?

Answer 54

HMX.

Question 55

What is the *most common* military garde explosive?

Answer 55

RDX - C4.

Question 56

What is a **Propellant**?

Answer 56

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.

Question 57

What is the **Structure** of a *Bullet Cartridge?*

Answer 57

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.

Question 58

What follows the bullet out the end of a gun?

Answer 58

A ring of fire made of burnt and unburnt propellant.

Question 59

What are the **3 types of shockwave** which come from a *Bullet?*

Answer 59

* **Machcone** - Shockwave at the *tip* of the bullet. * **Cannelure** - Shockwave around the *middle* of the bullet. * **Wake** - Shockwave at the *trail* of the bullet.

Question 60

What are the **factors** which affect the *acceleration of a bullet?*

Answer 60

1. **Rate of Production of Gas** - rapid but steady rate. Too fast = bullet slows, Too slow = bullet pushed out. 2. **Burn Rate** - controlled by propellant particle shape and any particles. Ensures maximum kinetic energy imparted to the bullet. Ethyl centralite. 3. **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.

Question 61

What does **GSR** stand for?

Answer 61

**G**un **S**hot **R**esidue.

Question 62

For **Positive Identification** of *Gun Shot Residue*, what does it need to contain?

Answer 62

Lead, Antimony and Barium.

Question 63

What is the only **technique** that can be used to identify *Gun Shot Residue?*

Answer 63

SEM-EDX.

Question 64

What does **SEM-EDX** stand for?

Answer 64

**S**canning **E**lectron **M**icroscopy **-** **E**nergy **D**ispersive **X**-ray.

Question 65

What do you have to collect Exposive and Fire Debris in, at a crime scene?

Answer 65

You have to collect in a nylon bag and then that is placed into another nylon bag.

Question 66

What are the ways to Analyse Explosive and Fire Debris?

Answer 66

Solid Phase Microextraction, Gas Chromatograpy and Mass Spectrometry.

Question 67

What does **EOD** stand for?

Answer 67

Explosive Ordinance Disposal.

Question 68

What does **IED** stand for and what are they?

Answer 68

**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.

Question 69

What are the steps in the IEDD Philosophy?

Answer 69

1. Preservation of Life. 2. Preservation of Property. 3. Preservation and Collection of forensic evidence, without compromising saftey. 4. Return the situation to normal.

Question 70

What different **IED types** can you have? | Give examples of each.

Answer 70

* **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.