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Fires & Explosions > Lecture 4 > Flashcards

Lecture 4 Flashcards

1
Q
A
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2
Q

What is neccessary for a hydrocarbon fire to be sustained?

A
  • Molecular fragments with unpaired electrons, free radicals, are necessary for a hydrocarbon fire to be sustained.
  • f our initial ignition source is removed, then the heat generated by the chemical reaction must be enough to sustain the fire.
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3
Q

Fire is a ?

A

Fire is a radical chain reaction

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4
Q

What is a radical?

A
  • An atomic or molecular species that possesses unpaired electrons in an otherwise open-shell configuration (open shell = un-filled valence shell).
  • Paramagnetic (unpaired electrons are attracted to a magnetic field).
  • High reactivity, but are also isolable.
  • Important to combustion and atmospheric chemistry, but also in synthesis and biological processes.
  • Attracted to magnetic field as they have a magnetic componenet.
  • Very reactive
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5
Q

Chlorine radical

A
  • Bonding orbital of Cl2 gas, has a pair of electrons, spins parallel.
  • If we break the bond evenly, we make two Chlorine radicals.
  • ## Must be spin spin parallel not in the same direction.
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6
Q

Heterolysis

A

Movement of charge from one place to another

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7
Q

Homolysis

A
  • Equal distribution of electrons between atoms
  • One electron goes to each species across the bond so we get two radical species
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8
Q

LUMO

A

Lowest unoccupied MO, lowest energy without an electrons

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9
Q

HOMO

A

Highest occupied MO, highest energy that contains an electron

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10
Q

SOMO

A
  • Singly occupied by one electron.
  • In a Radical this is the one that will react with one of the three types of orbitals!
  • Goes on to further react
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11
Q

Carbocations and carbanions

A

Carbocations = positive
Carbanions = negative

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12
Q

Forms of radicals

A
  • Planar (carbocations)
  • Tetrahedral (Carbanions)
  • Hybrid orbital
  • P orbital
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13
Q

What influence the stability of a radical?

A
  • Sterics, pi sysem, and conjugation
  • Electron donating or withdrawing groups
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14
Q

How can you stabilise radicals?

A
  • Radicals can be stabilised using either electron donating or withdrawing groups.
  • Basically, reactivity can be “tuned”
  • Stability is inveresly correlated to homolytic bond strength
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15
Q

A conjugated benezene ring

A

Less stable

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16
Q

Less steric bulk

A

more distribution of charge so more reactive

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17
Q

Double bonded species

A
  • More steric bulk
  • Less distribution of charge so more reactive
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18
Q

How do radical reactions work?

A
  1. Initiation
  2. Propagation
  3. Termination
19
Q

Initiation

A

Generation of reactive intermediate

20
Q

Propagation

A

The generated reactive intermediate attacks a stable chemical species to generate another reactive intermediate.

21
Q

Termination

A
  • Two radicals combine to quench the unpaired electrons, halting the reaction. This is often a by-product.
  • Two radicals join together to make a new bond.
  • The concentration can become too high so it’s more favourable for termination to occur.
  • If you end with less radicals that you started with then this is termination.
22
Q

Most likely source for initiation

A

Bond that isn’t carbon

23
Q

Example of compounds useful for initiation

A
  • Peroxides as they decompose with heat exposure
  • Dihalogens which split when exposed to UV light
  • This generates two radicals per molecule and is used a lot in polymer chemistry
24
Q

Bromines importance in radicals

A

Bromine is stored to reduce the amount of light getting into the bottle otherwise bromine radicals would form inside the bottle

25
Q

Radical reactions utilise

A
  • Radical reactions utilise highly reactive intermediates.
  • Typically those with unpaired electrons (to overcome activation barriers).
  • However, in order to create unpaired electrons, we need high energy and a suitable initiator.
26
Q

What influences the nature of the products in radical chemistry

A

The ratio of the reagents (chlorine, methane) in the mixture, as well as reaction time, will influence the nature of the products.

27
Q

For a combustion to occur, we need to be able to break x bond?

A

For combustion to occur, we need to be able to break the O=O bond so we can make oxygen free radicals (but this bond is relatively strong).

28
Q

A materials flammability relates to?

A
  • When considering a materials flammability, it relates to the concentration of free radicals the material needs to be subjected to before both initiation and propagation reactions become the dominant reactions.
  • Once they do, the material combusts.
29
Q

What happens once a material is consumed by a fire?

A
  • 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.
  • A shift where termination becomes more favourable which is when the fire will start to go out unless a new fuel is introduced
30
Q

Branching reaction

A
  • This is where two (or more) radicals are produced when a radical reacts with a molecule
  • The generation of a flame is due to branching reactions dominating over termination reactions
  • When branching reactions dominate over termination reactions, the fire will consume a lot of fuel.
  • It is a different way of doing a propagation which increases the rate of reaction.
  • This is what happens in flames
31
Q

Why are flames so destructive?

A
  • They have branching reactions
  • They can accelerate and grow
  • When you’ve got these branching reactions dominating over termination reactions, which is very common in a flame, you’re going to have a fire which can really consume a lot of fuel. In this case, more radicals are formed instead of quenched.
32
Q

fission reactions

A

Breaking up reactions

33
Q

O (2 radicals) + H (1 radical) –> OH (1 radical)

A
  • Going from 3 radicals to 1 which is more reflective for termination but it is actually a initiation process as it is very energetically favoured .
  • The end process is a really receive radical instead of two unreactive ones
  • The generated radical is very reactive so it is an exception.
34
Q

Oxygen

A
  • The only abundant, paramagnetic molecule found in our atmosphere with a triplet ground state.
  • Triplet oxygen contains two unpaired electrons.
  • There is such a thing as singlet oxygen, which is the excited state of it!
  • Oxygen is thus a di-radical species.
35
Q

Hunds rule

A

When adding electron to an MO diagram, equivalent orbitals must be singly occupied before being doubly occupied, and electrons must have identical spin to minimise energy

36
Q

Bond order calculation

A

Bond order = (no. electrons bonding – no. electrons anti-bonding) / 2

37
Q

Dioxygen

A
  • Has equivalent x and y orbitals, and two electrons to spare.
  • These consequently don’t pair
  • They also share an identical spin
  • The only solution is for two radicals to exist, one located on the x orbital, the other on the y orbital.
38
Q

Diradical oxygen

A
  • Unusual properties
  • Highly energetic so used as rocket fuel
  • Is essential for combustion of organic matter
  • Every single element reacts exothermically with oxygen, with the exception of gold
  • At room temperature, O2 exists in a triplet state, which can only undergo a chemical reaction by making the forbidden transition into a singlet state.
39
Q

Halon 1211

A
  • Numbers relate to C, F, Cl, Br
  • Boiling point = - 4 degrees celcius
  • The benefit is that under mild pressure, Halon 1211 is liquified.
  • It has a vapour pressure of 2.3 atm. at 20 °C, and a vapour density of 16.5 g/L-1
  • This means a dense cloud forms over a fire that is difficult to disperse, aiding smothering.
  • The canister containing Halon 1211 is under much less pressure than a CO2 extinguisher, meaning it wont disperse a fire when applied
  • Fire retardant, the radical counter the fires
  • Much denser than air so oxygen gets pushed up stopping the fire as oxygen can’t access it.
40
Q

Halon Fire extinguishers

History

A
  • Halon fire extinguishers target the chemical chain reaction in fires
  • They were banned following the Montreal Protocol of 1987 owing to their use of CFCs
  • Came into force in UK in 2003, but is not a complete ban – some critical uses are still allowed
  • Bad for environment
  • Sometimes still used in labs as they are unreactive with most species
41
Q

Actions of Halon

A
  • Halon is not an oxidant – it will naturally smother a fire by depriving it of oxygen
  • Halon undergoes reaction to form free radicals but it requires energy to do this.
  • It takes this energy from the fire, absorbing some energy of activation that would otherwise propagate the fire
  • There is lots of initation but not propagation
  • It starves the fire of oxygen and then it starves the fire of heat.
  • We get increased radicals but halon radicals are more stable
  • Radicals steal energy from the fire.
42
Q

Chemistry of halons

A
  • Halons are really stable not reactive so its happy to sit in the fire as a radical so it doesn’t propagate into products
  • It waits to terminate with another radical, it prefers to do this over propagating
  • It takes the termination step and we end up with less radicals in the fire
  • It steals the more stable radicals and terminates them.
  • The free radicals produced by Halon react with the free radicals produced by the fire, quenching them a termination reaction.
  • These radicals would normally branch and add to the fire but instead they are quenched by the halons.
43
Q

Why does O2 participate in branching reactions?

A

Because O2 has a triplet ground state, it participates in branching reactions which accelerate the overall reaction.

Fires & Explosions (18 decks)

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