Fires 4 Flashcards
What is the fire pyramid?
- heat
- oxygen
- fuel
- chain reaction
What is necessary for a hydrocarbon fire to be sustained?
free radicals - molecular fragments with unpaired electrons
* no free radicals = no fire
What is a radical?
- an atomic or molcular species that possesses unpaired electrons in an otherwise open-shell configuration
- paramagnetic
- high reactivity but also can be isolated
What is paramagnetic?
unpaired electrons are attracted to a magnetic field
What is an open shell?
unfilled valence shell
What are polar reactions?
- heterolysis
- movement of charge from one place to another
- heterolytic cleavage
- double headed arrow - electron pair movement
- arrow can only go in one direction
What are radical reactions?
- homolysis
- equal distribution of electrons between atoms
- homolytic cleavage
- fish-hook arrow - single electron movement
- arrow can be in either direction
What are some types of radicals?
- planar (carbocations) - unpaired electron in the p-orbital
- tetrahedral (carbanions) - unpaired electron in the hybrid orbital
What are the three types of molecular obitals are what is the one that will react further in a radical?
- LUMO - lowest unoccupied MO
- HOMO - highest occupied MO
3. SOMO - singly occupied MO
What are some factors that can influence the stability of a radical?
- sterics and conjugation
- inversely correlated to homolytic bond strength
- electron donating/withdrawing groups
What are the three types of radical reactions?
- initiation
- propagation
- termination
What is an initation radical reaction?
- 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
What is a propagation radical reaction?
- the generated reactive intermediate attacks a stable chemical species to generate another reactive intermediate
- same amount of radicals on both sides of the reaction
What is a termination reaction?
- 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
How does free radical polymerisation work?
- 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…
What is the equation for change in enthalpy under standard conditions using the enthalpy change associated with the dissolution of a substance?
enthalpy change of dissolution of breaking bonds - enthalpy change of dissolution of forming bonds
When we look at a reaction in terms of it bond strengths what doesnt this take into account?
the energy barrier that needs to be overcome before the reaction will occur
What will happen if the activation energy is sufficiently high?
no reaction will occur
What do radical reactions utilise?
highly reactive intermediates
What two things do we need to create unpaired electrons?
- high energy
- suitable initiator
What does a materials flammability relate to?
the concentration of free radicals the material meeds to be subjected to before both initation and propagation reactions become the dominant reactions
What is a branching reaction?
- 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
What is the generation of a flame due to?
- branching reactions dominating over termination reactions
- when this happens the fire will consume a lot of fuel
What does branching do with oxygen?
makes oxygen with two unpaired electrons
Why dont the two unpaired electrons pair in the branching of oxygen?
- ground state of oxygen
What is dioxygen?
- 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
What is a singlet oxygen?
- excited state of dioxygen
- in which the two oxygen atoms are in a spin-paired configuration
What is the Hund’s rule?
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
How is fire and oxygen affected by magnetic fields?
- unpaired electron effect
What is the equation for bond order?
(no. of electrons bonding - no. of electrons antibonding)/2
What properties does dioxygen have dye to being a diradical?
- every element reacts exothermically with it except for gold
- highly energetic
- essential for combustion
Why arent we all on fire all the time?
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
What is Halon 1211?
- numbers relate to the number of C, F, Cl, Br
How does Halon 1211 put out a fire?
- 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)
Why were halon fire extinguishers banned?
bad for the environment
* labs can still use them in scenarios
Why does halon not propagate?
it is stable and not reactive
