Fires 4 - Kinetics of Fire - Radical Chemistry

Question 1

Other than the fire triangle being applied to the initiation of a fire, but what else can it be applied to?

Answer 1

sustaining it

for example if the initial ignition source is removed, the heat generated by the chemical reaction must be enough to sustain the fire

Question 2

What other component is needed alongside the fire triangle for a hydrocarbon fire to be sustained? Why?

Answer 2

self-sustaining chemical chain reaction - makes fire pyramid

free radicals (molecular fragments with unpaired electrons) are necessary for a hydrocarbon fire to be sustained

without the radicals there is nothing for anything to react with to sustain fire

Question 3

What are four stages of fire?

Answer 3

inception
sustainment
acceleration
extinguishment

Question 4

what type of reaction is a fire?

Answer 4

radical chain reaction

Question 5

Define ‘radical’

Give two properties of them

Explain what they are important to.

Answer 5

an atomic or molecular species that possesses unpaired electrons in an otherwise open-shell (un-filled valence shell) configuration

1. paramagnetic - unpaired electrons are attracted to a magnetic field
2. high reactivity but isolable

important to combustion and atmospheric (CFC’s) chemistry
and synthesis and biological processes

Question 6

What are the spins like in the bonding orbital of chlorine?

Answer 6

the spins are parallel

Question 7

What is a heterolysis reaction?

What is the type of reaction?

What type of cleavage does it involve?

What movement does it involve?

Answer 7

it is the movement of charge from one place to another

polar

heterolytic cleavage

electron pair movement

Question 8

What is a homolysis reaction?

What is the type of reaction?

What type of cleavage does it involve?

What movement does it involve?

Answer 8

equal distribution of electrons between atoms

radical

homolytic cleavage

single electron movement

Question 9

What are the three stages of a radical reaction?

Answer 9

initiation - generation of the reactive intermediate

propagation - the generated intermediate attacks a stable chemical species to generate another reactive intermediate

termination - two radicals combine to quench the unpaired electrons, halting the reaction

Question 10

Why do we need to generate radicals?

How are they generated?

What are they generated from?

Give an example

Answer 10

because stable radicals are uncommon

generated using energy (heat or light)

non-radical decomposes to give radicals. typically a X-Y bond breaks where X and Y are not C

dihalogens e.g. chlorine gas which split when being exposed to UV light

Question 11

what is the most common reaction after initiation?

Answer 11

propagation

Question 12

When will the process stop?

Answer 12

when the radicals run out and they have all self-quenched

this happens when there are enough radicals generated so that the concentration of them is too high and it becomes more favourable for termination processes to occur

Question 13

What is a polymerisation radical reaction?

What type of bonds are most likely in polymerisation processes?

Give an example of this

Answer 13

initial radical formed (initiator) reacts with a monomer to generate a new radical

the newly formed radical can react with another monomer and again and again

double bonds called vinyl bond

casting polyacrylamide gels for electrophoresis

Question 14

What is the radical reaction of methane and chlorine?

Exothermic/endothermic?

What does the enthalpy of dissociation not take into account?

Answer 14

methane + chlorine –> methyl chloride + hydrogen chloride
- need high energy and a suitable initiator (heat above 300 degrees and UV)

enthalpy of dissociation is exothermic so it is favourable but needs a lot of energy to break Cl2

(at ambient temperature, nothing happens as need a lot of energy to make something happen)

it doesn’t take into account the energy barrier that needs to be overcome before the reaction will occur

Question 15

What happens if the activation energy for a reaction is sufficiently high and enough energy cannot be provided to overcome it?

Even if the required energy is put in what can sometimes happen?

Answer 15

no reaction will occur

competing reactions will preferentially occur even if you put in the required energy

Question 16

What influences the nature of the products in the reaction between methane and chlorine?

Answer 16

the ratio of the reagents (methane and chlorine) and the reaction time

Question 17

What can be said about the idealised and real reaction that occurs?

Give an example

Answer 17

in reality there will be multiple chain reactions occurring

for example in petrol and diesel combustion processes there are multiple components and will therefore be able to produce many different types of radicals and therefore will be multiple chain reactions occurring

Question 18

What needs to happen for combustion to occur?

Answer 18

need to break the O=O bond so we can make oxygen free radicals but this bond is very strong

Question 19

What does the materials flammability relate to?

Answer 19

relates to the concentration of free radicals the material needs to be subjected to before both initiation and propagation reactions become the dominant reactions

when they are the most frequently occurring reactions, the material combusts

Question 20

What happens to the proportion of initiation/propagation reactions compared to termination reactions when the material is consumed by fire?

Answer 20

once a material is consumed by fire, there is a shift between the frequency of initiation/propagation reactions towards termination reactions, the fire goes out

Question 21

What are the typical radical propagation reactions?

What is an alternative to this?

Describe this reaction

Answer 21

radical-molecule reaction that produces a radical

an alternative is a branching reaction where two or more radicals are produced when a radical reacts with a molecule

a different way of doing propagation which increases the rate of reaction

Question 22

How can the generation of a flame be explained?

How does this explain the destructivity of flames?

Answer 22

when branching reactions dominate over termination reactions, the fire will consume a lot of fuel (solid liquid or gaseous fuel)

because they are things which can accelerate and grow and grow and grow

Question 23

Describe the weird reaction step in the combustion of methane?

What is this more reflective of rather than what it is considered? Why?

Answer 23

oxygen diradical + hydrogen radical = OH radical

more reflective of a termination process as going from 3 radicals to 1 radical but considered an initiation reaction due to it being favourable

Because OH radical is most reactive so this is better than having two less reactive radicals

Question 24

What types of reactions are fundamental in combustion processes?

What are the two options for this?

Answer 24

branching reactions

generation of radical species and a diradical species from a non-radical and a radical species

generation of two radical species from one radical

Question 25

Describe dioxygen in terms of magneticity and ground state

Also explain it in terms of radicals

Answer 25

• only paramagnetic molecule found in our atmosphere with a triplet ground state
• it contains two unpaired electrons in anti-bonding orbitals so it is a di-radical species

Question 26

As liquid oxygen and flames have radicals, are they affected by magnetic fields?

Answer 26

liquid oxygen wants to stay within magnetic field

flames have radicals so also want to stay within magnetic field. When EM on, flame goes down

Question 27

Explain the double bond in oxygen?

Therefore how does this explain dioxygen

Answer 27

bond order of O2 is 2 so a double bond

but unlike typical double bond the pi component is split over two orbitals x and y and these are both magnetic due to unpaired electrons

has equivalent x and y orbitals with two electrons to spare so they do not pair and they share an identical spin
therefore the only solution is for two radicals to exist one on the x orbital and one on the y orbital

Question 28

As dioxygen is a diradical, what are the four unusual properties of dioxygen?

Answer 28

• every single element reacts exothermically with oxygen with the exception of gold
• highly energetic so it can be used as rocket fuel
• is essential for combustion of organic matter
• at room temp it exists in triplet state which can only undergo an chemical reaction by making the transition into a singlet state

Question 29

What does Halon 1211 mean?

Answer 29

1 carbon, 2 fluorine, 1 chlorine and 1 bromine

goes in order of increasing MW

Question 30

what state is halon 1211 under mild pressure?

Answer 30

it is liquified so can store in a firefighter canister

Question 31

At 20 degrees what is its vapour pressure and density and what does this mean?

Answer 31

2.3 atm. vapour pressure
16.5 g/L-1 vapour density
means when it is dispersed it sits really low to the ground

Question 32

How do halon fire extinguishers work?

Answer 32

by targeting the chemical chain reaction in fires

Question 33

Explain their use in UK

Answer 33

they were banned in 2003 in UK following Montreal protocol of 1987 owing to their use of CFCs which destroy the ozone layer/bad for environment

not complete ban as critical uses are still allowed

Question 34

Explain the three ways that halon 1211 fire extinguishers work

Answer 34

1 - can slow a fire by smothering:

Halon is not an oxidant which means that it will naturally smother a fire by depriving it of oxygen. Halon causes a dense cloud to form over the fire which is difficult to disperse. One component of the fire quadrangle is oxygen, and since Halon has deprived the fire of oxygen, this component of the fire is no longer being satisfied so the fire is slowed and eventually put out.

2 - can also slow a fire by stealing energy from a fire to generate
radicals:

Halon undergoes reactions to form free radicals but in order to form free radicals energy is required. The heat, which is an energy component of the fire, is taken away from the fire in
order to provide energy to make these radicals. However this means there is less heat in the fire and therefore less energy. Less energy in the fire means there is less energy in the fire that
can be used to propagate the fire. Therefore this means there is less ability for the fire to continue to accelerate.

3 - can also slow a fire by quenching radicals in termination reactions:

The free radicals produced by both the Halon and the fire can react with each other via a termination reaction which quenches the radicals. This means the two radicals react to form a non-radical species. A non-radical species doesn’t have the ability to continue to undergo propagation, unlike the two radical species that did have this ability. Therefore, there are less radicals in the fire that can undergo propagation to continue to accelerate the fire which means there is less fire happening