Lecture 15 Flashcards
what two things can the fire triangle be applied to
the initiation and the sustaining of a fire
if the ignition source of a fire is removed what must be enough to sustain the fire
the heat generated by the chemical reaction within the fire
- molecules with unpaired electrons (free radicals) drive these reactions to sustain the fire
what are the 4 components of the fire pyramid/pentangle
heat
oxygen
fuel
chemical chain reaction - radicals
what is meant by
a) inception of a fire
b) extinguishment of a fire
a) start
b) end
give the combustion equation of methane
in a fire what does this equation need to be
CH4 + O2 –> CO2 + 2H2O
in a fire this reaction needs to be self sustaining
in what molecules are bonds
a) broken
b) formed
what does the energy produced need to be greater than to continue reaction
a) the fuel and oxygen
b) the water and carbon dioxide
energy produced in products needs to be greater than the activation energy of the remaining reactant for the reaction to continue
what type of chemical reaction is a fire
a radical chain reaction
what is heterolysis
the movement of charge from one place to another in a reaction
what is a radical
a molecule that has an unpaired electron in an open-shell configuration
they are paramagnetic = unpaired electrons being attracted to a magnetic field
highly reactive species
isolable = capable of being isolated
what are the two types of breaking bonds
which applies to radical reactions and which to polar reactions
heterolysis = polar reactions
homolysis = radical reactions
what is homolysis
the equal distribution of electrons between atoms
what is the main factor affecting the reactivity of a radical
its stability
what are the factors affecting the stability of a radical (3)
- steric/bulkiness makes them more stable
- electron withdrawing groups make them more stable (e.g ketones, nitriles)
- more conjugation e.g rings make them more stable
what is an example of a radical that is stable and therefore isolable
TEMPO
how do we tell if a reaction is a propagation
number of radicals is same of each side
how do we tell if a reaction is an initiation
more radicals on the right than the left
what are the three types of radical reaction that drive the sustainability of a fire (briefly explain each)
- initiation = generation of the reaction intermediate
- propagation = the intermediate attacks a stable chemical species to generate another reactive intermediate
- termination = two radicals combine to quench the unpaired electrons, stopping the chain reaction, produces a by product
how do we tell if a reaction is a termination
when does this reaction normally happen
number of radicals more on the left
results from a high concentration of radicals being present
what happens in an initiation reaction
give an example of a common initiator
a non radical decomposes to give radicals
usually do to het or light exposure e.g UV
double bonds are commonly broken
AIBN or peroxides
how is bromine stored to prevent it turning into radicals
in a brown bottle to reduce light exposure
what type of bonds do radicals tend to attack
double bonds
what is a peroxide bond
o-o
what is the delta H of a radical reaction
-ve
exothermic
favourable energetically
in radical chemistry combustion reactions, what is not taken into account when calculating delta H of the reaction
the activation energy barrier
if this is high and can’t be overcome no reaction will occur
if this is too large the reaction may not occur at all and other competing reactions may occur instead
what are two things needed to create radicals
high energy - from UV light or high temp
a suitable initiator
what two things allow propagation reactions to continue
radicals and fuel available
in a idealised world only the desired propagation reaction will occur but what happens realistically
the ratio of the reagents is not always perfect so multiple side reactions are likely to also be occurring
what is meant by a materials flammability
the concentration of free radicals the materials needs to be subjected to before both initiation and propagation reactions are the dominant reactions
when they become dominant the material combusts
when the material is consumed by the fire there is a shift from initiation and propagation reactions to termination and the fire goes out
when does a reaction fully stop
when all the radicals have been quenched
for a combustion to occur in the context of fires what type of bond needs to be broken
what is made from this
why is this difficult
an oxygen double bond (O=O)
oxygen free radicals
this bond can be strong
when will termination reactions not occur so initiation or propagation reactions continue
when a new fuel source in introduced
what is the generation of a flame due to (in terms of the types of reaction occurring)
branching reactions dominating over termination reactions
here the fire consumes a lot more of the fuel
what is the alternative to a propagation reaction that could also occur
what is this
a branching reaction
where two or more radicals are produced when a radical reacts with a molecule
also called fission reactions
what is an example of a branching reaction that can occur in the combustion of methane
O2 + H(radical) –> OH(radical) + O(diradical (2 unpaired electrons))
in the combustion of methane - why is this reaction O (diradical) + H (radical) –> OH (radical) considered an initiation
because it is energetically favourable - the O and H radicals are very unreactive but OH radicals are very reactive
why can oxygen exists as having 2 unpaired electrons (why do they not join)
due to the ground state of oxygen and it being a paramagnetic species
oxygen has 2 unpaired electrons in its molecular orbitals (Pi anti-bonding orbitals) and these are both radical in nature
what are branching reactions fundamental for
combustion reactions
why when a magnetic is applied round a flame does the flame intensity increase
because radicals are affected by magnetic fields due to the unpaired electrons
how is the double bond in oxygen different to other double bonds
the electrons in the antibonding pi orbitals are split between two orbitals (x and y) and they have the same spin
so technically there are two half pi bonds but this is confusing so we draw it as one pi bond
how do we calculate bond order
what is the bond order of a double bond
bond order = (no electrons bonding - no electrons anti-bonding)/2
2
what do the numbers in the name ‘Halon 1211’ represent
the number of each atom that makes up the molecule
1 - carbon
2 - fluorine
1 - chlorine
1 - bromine
also known as BCF
what unusual properties does dioxygen being a diradical have (3)
- every element reacts exothermically with dioxygen - apart from gold
- it is highly energetic (can be used as rocket fuel)
- essential for the combustion of organic matter
why are we all not on fire all the time if dioxygen is so reactive
at room temp O2 exists in the triplet state and can only undergo reactions by making the transition to the singlet state
what is halon 1211 used for
what is the benefit of using this
in fire extinguishers
good to store in a cannister - under mild pressure it can be liquified and bp is -4 degrees
why is Halon preferred to CO2 in fire extinguishers
halon is under much less pressure so it is less likely to disperse the fire when it is applied
how does halon work to extinguish a fire (3)
- it forms a dense cloud over a fire that works to smother the fire - halon is more dense than oxygen so halon will sit lower than oxygen in the air removing the oxygen source from the fire so it dies down (deprives fire of oxygen)
- halon can undergo a reaction to form free radicals but this requires energy. the energy is taken from the fire which would otherwise be used to propagate it
- these radical can also do something called radical scavenging - the free radicals produced by halon can react with radicals produced by the fire = quenches them in a termination reaction
why were halon fire extinguishers banned
what type of fire are halon extinguishers good for
they destroy the ozone layer = bad for the environment
good for chemical fires where water or CO2 are not so good
how do halon radicals compare to the radicals produced in the combustion of methane
they are more stable
when does halon produce radicals
when it is heated
do halon radicals prefer to propagate or terminate
terminate - they are happy to sit in the fire until they are used in termination reactions
