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Fires and Explosions > Lecture 15 > Flashcards

Lecture 15 Flashcards

1
Q

what two things can the fire triangle be applied to

A

the initiation and the sustaining of a fire

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2
Q

if the ignition source of a fire is removed what must be enough to sustain the fire

A

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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3
Q

what are the 4 components of the fire pyramid/pentangle

A

heat
oxygen
fuel
chemical chain reaction - radicals

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4
Q

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

A

a) start
b) end

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5
Q

give the combustion equation of methane

in a fire what does this equation need to be

A

CH4 + O2 –> CO2 + 2H2O

in a fire this reaction needs to be self sustaining

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5
Q

in what molecules are bonds
a) broken
b) formed

what does the energy produced need to be greater than to continue reaction

A

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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6
Q

what type of chemical reaction is a fire

A

a radical chain reaction

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7
Q

what is heterolysis

A

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

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7
Q

what is a radical

A

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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8
Q

what are the two types of breaking bonds

which applies to radical reactions and which to polar reactions

A

heterolysis = polar reactions

homolysis = radical reactions

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9
Q

what is homolysis

A

the equal distribution of electrons between atoms

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10
Q

what is the main factor affecting the reactivity of a radical

A

its stability

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11
Q

what are the factors affecting the stability of a radical (3)

A
  1. steric/bulkiness makes them more stable
  2. electron withdrawing groups make them more stable (e.g ketones, nitriles)
  3. more conjugation e.g rings make them more stable
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11
Q

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

A

TEMPO

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12
Q

how do we tell if a reaction is a propagation

A

number of radicals is same of each side

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12
Q

how do we tell if a reaction is an initiation

A

more radicals on the right than the left

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13
Q

what are the three types of radical reaction that drive the sustainability of a fire (briefly explain each)

A
  1. initiation = generation of the reaction intermediate
  2. propagation = the intermediate attacks a stable chemical species to generate another reactive intermediate
  3. termination = two radicals combine to quench the unpaired electrons, stopping the chain reaction, produces a by product
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14
Q

how do we tell if a reaction is a termination

when does this reaction normally happen

A

number of radicals more on the left

results from a high concentration of radicals being present

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15
Q

what happens in an initiation reaction

give an example of a common initiator

A

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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15
Q

how is bromine stored to prevent it turning into radicals

A

in a brown bottle to reduce light exposure

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15
Q

what type of bonds do radicals tend to attack

A

double bonds

16
Q

what is a peroxide bond

A

o-o

17
Q

what is the delta H of a radical reaction

A

-ve
exothermic
favourable energetically

18
Q

in radical chemistry combustion reactions, what is not taken into account when calculating delta H of the reaction

A

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

19
Q

what are two things needed to create radicals

A

high energy - from UV light or high temp

a suitable initiator

19
Q

what two things allow propagation reactions to continue

A

radicals and fuel available

19
Q

in a idealised world only the desired propagation reaction will occur but what happens realistically

A

the ratio of the reagents is not always perfect so multiple side reactions are likely to also be occurring

19
Q

what is meant by a materials flammability

A

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

20
Q

when does a reaction fully stop

A

when all the radicals have been quenched

21
Q

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

A

an oxygen double bond (O=O)

oxygen free radicals

this bond can be strong

22
Q

when will termination reactions not occur so initiation or propagation reactions continue

A

when a new fuel source in introduced

23
Q

what is the generation of a flame due to (in terms of the types of reaction occurring)

A

branching reactions dominating over termination reactions

here the fire consumes a lot more of the fuel

23
Q

what is the alternative to a propagation reaction that could also occur

what is this

A

a branching reaction

where two or more radicals are produced when a radical reacts with a molecule

also called fission reactions

23
Q

what is an example of a branching reaction that can occur in the combustion of methane

A

O2 + H(radical) –> OH(radical) + O(diradical (2 unpaired electrons))

24
Q

in the combustion of methane - why is this reaction O (diradical) + H (radical) –> OH (radical) considered an initiation

A

because it is energetically favourable - the O and H radicals are very unreactive but OH radicals are very reactive

25
Q

why can oxygen exists as having 2 unpaired electrons (why do they not join)

A

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

26
Q

what are branching reactions fundamental for

A

combustion reactions

27
Q

why when a magnetic is applied round a flame does the flame intensity increase

A

because radicals are affected by magnetic fields due to the unpaired electrons

28
Q

how is the double bond in oxygen different to other double bonds

A

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

29
Q

how do we calculate bond order

what is the bond order of a double bond

A

bond order = (no electrons bonding - no electrons anti-bonding)/2

2

30
Q

what do the numbers in the name ‘Halon 1211’ represent

A

the number of each atom that makes up the molecule

1 - carbon
2 - fluorine
1 - chlorine
1 - bromine

also known as BCF

30
Q

what unusual properties does dioxygen being a diradical have (3)

A
  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
31
Q

why are we all not on fire all the time if dioxygen is so reactive

A

at room temp O2 exists in the triplet state and can only undergo reactions by making the transition to the singlet state

32
Q

what is halon 1211 used for

what is the benefit of using this

A

in fire extinguishers

good to store in a cannister - under mild pressure it can be liquified and bp is -4 degrees

33
Q

why is Halon preferred to CO2 in fire extinguishers

A

halon is under much less pressure so it is less likely to disperse the fire when it is applied

33
Q

how does halon work to extinguish a fire (3)

A
  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
34
Q

why were halon fire extinguishers banned

what type of fire are halon extinguishers good for

A

they destroy the ozone layer = bad for the environment

good for chemical fires where water or CO2 are not so good

35
Q

how do halon radicals compare to the radicals produced in the combustion of methane

A

they are more stable

36
Q

when does halon produce radicals

A

when it is heated

37
Q

do halon radicals prefer to propagate or terminate

A

terminate - they are happy to sit in the fire until they are used in termination reactions

Fires and Explosions (20 decks)

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