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
