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Fires and Explosions > Fires 4 > Flashcards

Fires 4 Flashcards

1
Q

What is the fire pyramid?

A
  • heat
  • oxygen
  • fuel
  • chain reaction
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2
Q

What is necessary for a hydrocarbon fire to be sustained?

A

free radicals - molecular fragments with unpaired electrons
* no free radicals = no fire

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3
Q

What is a radical?

A
  • an atomic or molcular species that possesses unpaired electrons in an otherwise open-shell configuration
  • paramagnetic
  • high reactivity but also can be isolated
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4
Q

What is paramagnetic?

A

unpaired electrons are attracted to a magnetic field

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5
Q

What is an open shell?

A

unfilled valence shell

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6
Q

What are polar reactions?

A
  • heterolysis
  • movement of charge from one place to another
  • heterolytic cleavage
  • double headed arrow - electron pair movement
  • arrow can only go in one direction
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7
Q

What are radical reactions?

A
  • homolysis
  • equal distribution of electrons between atoms
  • homolytic cleavage
  • fish-hook arrow - single electron movement
  • arrow can be in either direction
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8
Q

What are some types of radicals?

A
  • planar (carbocations) - unpaired electron in the p-orbital
  • tetrahedral (carbanions) - unpaired electron in the hybrid orbital
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9
Q

What are the three types of molecular obitals are what is the one that will react further in a radical?

A
  1. LUMO - lowest unoccupied MO
  2. HOMO - highest occupied MO
    3. SOMO - singly occupied MO
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10
Q

What are some factors that can influence the stability of a radical?

A
  • sterics and conjugation
  • inversely correlated to homolytic bond strength
  • electron donating/withdrawing groups
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11
Q

What are the three types of radical reactions?

A
  • initiation
  • propagation
  • termination
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12
Q

What is an initation radical reaction?

A
  • generation of the reactive intermediate
  • from 0 radicals (reactants) to 1 or more radicals (products)
  • non-radical decomposes to give radicals
  • usually breaking an X-Y bond where they arent carbon
  • generates to radicals per molecule
  • put energy in to form radicals
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13
Q

What is a propagation radical reaction?

A
  • the generated reactive intermediate attacks a stable chemical species to generate another reactive intermediate
  • same amount of radicals on both sides of the reaction
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14
Q

What is a termination reaction?

A
  • two radicals continue to quench the unpaired electrons, halting the reaction
  • often a by-product
  • from more radicals to less
  • run out of molecules to react with
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15
Q

How does free radical polymerisation work?

A
  • the intial radical formed reacts with a monomer generating a new radical
  • the newly formed radical can react with another monomer and again and again…
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16
Q

What is the equation for change in enthalpy under standard conditions using the enthalpy change associated with the dissolution of a substance?

A

enthalpy change of dissolution of breaking bonds - enthalpy change of dissolution of forming bonds

17
Q

When we look at a reaction in terms of it bond strengths what doesnt this take into account?

A

the energy barrier that needs to be overcome before the reaction will occur

18
Q

What will happen if the activation energy is sufficiently high?

A

no reaction will occur

19
Q

What do radical reactions utilise?

A

highly reactive intermediates

20
Q

What two things do we need to create unpaired electrons?

A
  • high energy
  • suitable initiator
21
Q

What does a materials flammability relate to?

A

the concentration of free radicals the material meeds to be subjected to before both initation and propagation reactions become the dominant reactions

22
Q

What is a branching reaction?

A
  • where two or more radicals are produced when a radical reacts with a molecule
  • different way of propagation which increases the rate of reaction
  • similar to fission reactions
23
Q

What is the generation of a flame due to?

A
  • branching reactions dominating over termination reactions
  • when this happens the fire will consume a lot of fuel
24
Q

What does branching do with oxygen?

A

makes oxygen with two unpaired electrons

25
Q

Why dont the two unpaired electrons pair in the branching of oxygen?

A
  • ground state of oxygen
26
Q

What is dioxygen?

A
  • paramagnetic molecule
  • has a triplet ground state
  • ground state of oxygen (O2)
  • two electrons occupy the pi* anti-bonding orbital
  • di-radical species
  • two electrons dont pair because they are in the x and y orbitals
  • participates in branching reactions which accelerate the overall reaction
27
Q

What is a singlet oxygen?

A
  • excited state of dioxygen
  • in which the two oxygen atoms are in a spin-paired configuration
28
Q

What is the Hund’s 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

29
Q

How is fire and oxygen affected by magnetic fields?

A
  • unpaired electron effect
30
Q

What is the equation for bond order?

A

(no. of electrons bonding - no. of electrons antibonding)/2

31
Q

What properties does dioxygen have dye to being a diradical?

A
  • every element reacts exothermically with it except for gold
  • highly energetic
  • essential for combustion
32
Q

Why arent we all on fire all the time?

A

O2 exists in a triplet state at room temperature, which can only undergo a chemical reaction by making the forbidden transition into a singlet state

33
Q

What is Halon 1211?

A
  • numbers relate to the number of C, F, Cl, Br
34
Q

How does Halon 1211 put out a fire?

A
  • vapour pressure is more dense than air so it means a cloud forms over the fire that is difficult to disperse - aiding smothering
  • oxygen gets pushed up because the Halon is more dense so stops the oxygen part of the triangle
  • not an oxidant - will naturally smother fire by depriving it of oxygen
  • forms free radicals but requires oxygen to do this so takes energy from the fire - more initation and less propagation, less heat
  • free radicals produced by Halon react with the free radicals produced by the fire, quenching them (termination)
35
Q

Why were halon fire extinguishers banned?

A

bad for the environment
* labs can still use them in scenarios

36
Q

Why does halon not propagate?

A

it is stable and not reactive

Fires and Explosions (20 decks)

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