Chain Reactions Flashcards
What is a linear chain reaction?
This is a radical chain reaction which involves a sequence of steps of reactions in which a radical reacts and then is regenerated in subsequent steps
What 3 steps does a chain reaction consist of?
1) initiation step
2) propagation step
3) termination step
What is an example of a chain reaction?
H2 + Br2 -> 2HBr
The stoichiometry is much more complex than is show
This is indicated by the observed rate law
d[HBr]/dt= kobs[H2][Br2]1/2
What is the initiation step of H2 + Br2 -> 2HBr
Br2 + M -> 2Br• +M (k1)
This forms Br• which is the chain carrier
This reaction is endothermic and very slow
The reaction goes from no radicals to two radicals
What is the propagation step of H2 + Br2 -> 2HBr
Br• + H2 -> HBr + H• (k2)
H• + Br2 -> HBr + Br• (k3)
The product HBr is formed and the carrier Br• is regenerated for another cycle
One radical to one radical
What is the termination step for H2 + Br2 -> 2HBr?
Br•+Br•+M -> Br2 +M (k4)
The chain carrier is removed ( the reverse of reaction 1)
Two radicals to no radicals
How can the overall rate of formation of HBr be expressed? Br2 + M -> 2Br• +M (k1) Br• + H2 -> HBr + H• (k2) H• + Br2 -> HBr + Br• (k3) Br•+Br•+M -> Br2 +M (k4)
d[HBr]/dt= k2[Br][H2] + k3[H][Br2]
Why can the concentration of the intermediates Br• and H• not easily be measured?
This is because their concentrations are very low as they are very reactive
This means they react as soon as they are formed
How can you calculate the concentrations of the intermediates H• and Br•?
1) use steady state on Br and H (find rate in regards to intermediates and then make it equal 0)
2) add the two equations together and cancel
3) rearrange to find the concentration of one of the intermediates
What is chain length?
This is the number of times a carrier is recycled
How do you calculate chain length?
Chain length= rate of propagation/ rate of termination
What are branched chain reactions and give generalised form
These are reactions that involve elementary reactions called branching steps
R + reactant -> product + xR
What is an example of a branching reaction?
2H2 + O2 -> 2H2O
What is the initiation step for the branching reaction: 2H2 + O2 -> 2H2O
1) H2 + 2OH- (k1)
What is the propagation step for the branching reaction: 2H2 + O2 -> 2H2O
2) propagation
OH + H2 -> H2O + H• (k2)
H + O2 -> O• + OH- (k3)
What is the chain branching step in 2H2 + O2 -> 2H2O
O• + H2 -> H• + OH (k4)
Each initial H atom can generate 10^15 atoms in 0.3 seconds at 700K and 11kPa of O2
What is autocatalytic?
This is a reaction whereby catalysis of the reaction is by one of its products
How do you find stoichiometry for chain carrier
The chain carrier is H•
1) balance the reactions to give what 1 H• atom reacts to
2) add the reactions together in a way that eliminates the intermediates (add 3+4 and x2 of 2)
4) from this you can see the net production of the chain carrier per cycle
What is the rate equation for H•
There are 2H atoms per cycle, and equation 3 is the only one with H• as a reactant
d[H]/dt= 2k3[O2][H]
How do you show H• is an autocatalyst?
d[H]/dt= 2k3[O2][H]
The positive sign shows the rate of production of H• increases with [H] which shows H• catalyses its own production
Why are not all combinations of H2 and O2 explosives
Sometimes they act in termination reactions
What are the possible termination steps?
H• + O2 + M -> HO2 +M
(Gas phase termination)
H• + wall -> removal
O• + wall -> removal
OH- + wall -> removal
Wall termination)
What does the wall removal process rate depend on?
The wall removal processes have a rate that depends on pressure, the size of the vessel and the materials it is made of
What does the rate of gas phase termination depend on?
Pressure (eg for number of collisions)
How does a low pressure affect the termination of the reaction
2H2 + O2 -> 2H2O
Slow general reaction because:
Rate of wall termination> rate of branching
How does a medium pressure affect the termination of the reaction
2H2 + O2 -> 2H2O
There will be an explosion as
Rate of branching > rate of termination
How does a high pressure affect the termination of the reaction
2H2 + O2 -> 2H2O
There will be a slow reaction as
Rate of gas phase termination > rate of branching
How would increasing the vessel size of the reactor affect the system?
2H2 + O2 -> 2H2O
Increasing the vessel size will decrease diffusion of radicals to the walls, so therefore lower the explosion limit
How would changing the wall material of the reactor affect the system?
2H2 + O2 -> 2H2O
Pacifying the walls to make them inert to radicals will lower the first explosion limit
How would adding an inert bath gas affect the system?
2H2 + O2 -> 2H2O
Adding an inert bath gas would reduce diffusion and hence loss of radicals to the walls, so will lower the first explosion limit
How do you find the rate equation for H2 + Br2 -> 2HBr?
1) find expression for [Br•], use steady state
2) find relationship between propagation step rates (they are equal)
3) rewrite the rate of formation of HBr (write in terms of the propagation step with H• in it)
4) sub all into rate of formation so you can find what k is in terms of the other rate constants
