Fires 4 - Kinetics of Fire - Radical Chemistry Flashcards
Other than the fire triangle being applied to the initiation of a fire, but what else can it be applied to?
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
What other component is needed alongside the fire triangle for a hydrocarbon fire to be sustained? Why?
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
What are four stages of fire?
inception
sustainment
acceleration
extinguishment
what type of reaction is a fire?
radical chain reaction
Define ‘radical’
Give two properties of them
Explain what they are important to.
an atomic or molecular species that possesses unpaired electrons in an otherwise open-shell (un-filled valence shell) configuration
- paramagnetic - unpaired electrons are attracted to a magnetic field
- high reactivity but isolable
important to combustion and atmospheric (CFC’s) chemistry
and synthesis and biological processes
What are the spins like in the bonding orbital of chlorine?
the spins are parallel
What is a heterolysis reaction?
What is the type of reaction?
What type of cleavage does it involve?
What movement does it involve?
it is the movement of charge from one place to another
polar
heterolytic cleavage
electron pair movement
What is a homolysis reaction?
What is the type of reaction?
What type of cleavage does it involve?
What movement does it involve?
equal distribution of electrons between atoms
radical
homolytic cleavage
single electron movement
What are the three stages of a radical reaction?
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
Why do we need to generate radicals?
How are they generated?
What are they generated from?
Give an example
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
what is the most common reaction after initiation?
propagation
When will the process stop?
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
What is a polymerisation radical reaction?
What type of bonds are most likely in polymerisation processes?
Give an example of this
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
What is the radical reaction of methane and chlorine?
Exothermic/endothermic?
What does the enthalpy of dissociation not take into account?
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
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?
no reaction will occur
competing reactions will preferentially occur even if you put in the required energy
What influences the nature of the products in the reaction between methane and chlorine?
the ratio of the reagents (methane and chlorine) and the reaction time
What can be said about the idealised and real reaction that occurs?
Give an example
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
What needs to happen for combustion to occur?
need to break the O=O bond so we can make oxygen free radicals but this bond is very strong
What does the materials flammability relate to?
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
What happens to the proportion of initiation/propagation reactions compared to termination reactions when the material is consumed by fire?
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
What are the typical radical propagation reactions?
What is an alternative to this?
Describe this reaction
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
How can the generation of a flame be explained?
How does this explain the destructivity of flames?
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
Describe the weird reaction step in the combustion of methane?
What is this more reflective of rather than what it is considered? Why?
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
What types of reactions are fundamental in combustion processes?
What are the two options for this?
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
Describe dioxygen in terms of magneticity and ground state
Also explain it in terms of radicals
- 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
As liquid oxygen and flames have radicals, are they affected by magnetic fields?
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
Explain the double bond in oxygen?
Therefore how does this explain dioxygen
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
As dioxygen is a diradical, what are the four unusual properties of dioxygen?
- 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
What does Halon 1211 mean?
1 carbon, 2 fluorine, 1 chlorine and 1 bromine
goes in order of increasing MW
what state is halon 1211 under mild pressure?
it is liquified so can store in a firefighter canister
At 20 degrees what is its vapour pressure and density and what does this mean?
2.3 atm. vapour pressure
16.5 g/L-1 vapour density
means when it is dispersed it sits really low to the ground
How do halon fire extinguishers work?
by targeting the chemical chain reaction in fires
Explain their use in UK
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
Explain the three ways that halon 1211 fire extinguishers work
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
