Lecture 15 Flashcards by Georgia Parker

what two things can the fire triangle be applied to

the initiation and the sustaining of a fire

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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

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what are the 4 components of the fire pyramid/pentangle

heat
oxygen
fuel
chemical chain reaction - radicals

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what is meant by
a) inception of a fire
b) extinguishment of a fire

a) start
b) end

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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

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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

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what type of chemical reaction is a fire

a radical chain reaction

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what is heterolysis

the movement of charge from one place to another in a reaction

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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

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what are the two types of breaking bonds

which applies to radical reactions and which to polar reactions

heterolysis = polar reactions

homolysis = radical reactions

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what is homolysis

the equal distribution of electrons between atoms

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what is the main factor affecting the reactivity of a radical

its stability

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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

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what is an example of a radical that is stable and therefore isolable

TEMPO

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how do we tell if a reaction is a propagation

number of radicals is same of each side

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how do we tell if a reaction is an initiation

more radicals on the right than the left

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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

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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

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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

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how is bromine stored to prevent it turning into radicals

in a brown bottle to reduce light exposure

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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)

1. every element reacts exothermically with dioxygen - apart from gold 2. it is highly energetic (can be used as rocket fuel) 3. 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)

1. 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) 2. 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 3. 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