Chain Reactions

Question 1

What is a linear chain reaction?

Answer 1

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

Question 2

What 3 steps does a chain reaction consist of?

Answer 2

1) initiation step
2) propagation step
3) termination step

Question 3

What is an example of a chain reaction?

Answer 3

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

Question 4

What is the initiation step of H2 + Br2 -> 2HBr

Answer 4

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

Question 5

What is the propagation step of H2 + Br2 -> 2HBr

Answer 5

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

Question 6

What is the termination step for H2 + Br2 -> 2HBr?

Answer 6

Br•+Br•+M -> Br2 +M (k4)

The chain carrier is removed ( the reverse of reaction 1)
Two radicals to no radicals

Question 7

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)

Answer 7

d[HBr]/dt= k2[Br][H2] + k3[H][Br2]

Question 8

Why can the concentration of the intermediates Br• and H• not easily be measured?

Answer 8

This is because their concentrations are very low as they are very reactive
This means they react as soon as they are formed

Question 9

How can you calculate the concentrations of the intermediates H• and Br•?

Answer 9

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

Question 10

What is chain length?

Answer 10

This is the number of times a carrier is recycled

Question 11

How do you calculate chain length?

Answer 11

Chain length= rate of propagation/ rate of termination

Question 12

What are branched chain reactions and give generalised form

Answer 12

These are reactions that involve elementary reactions called branching steps
R + reactant -> product + xR

Question 13

What is an example of a branching reaction?

Answer 13

2H2 + O2 -> 2H2O

Question 14

What is the initiation step for the branching reaction: 2H2 + O2 -> 2H2O

Answer 14

1) H2 + 2OH- (k1)

Question 15

What is the propagation step for the branching reaction: 2H2 + O2 -> 2H2O

Answer 15

2) propagation

OH + H2 -> H2O + H• (k2)
H + O2 -> O• + OH- (k3)

Question 16

What is the chain branching step in 2H2 + O2 -> 2H2O

Answer 16

O• + H2 -> H• + OH (k4)

Each initial H atom can generate 10^15 atoms in 0.3 seconds at 700K and 11kPa of O2

Question 17

What is autocatalytic?

Answer 17

This is a reaction whereby catalysis of the reaction is by one of its products

Question 18

How do you find stoichiometry for chain carrier

The chain carrier is H•

Answer 18

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

Question 19

What is the rate equation for H•

Answer 19

There are 2H atoms per cycle, and equation 3 is the only one with H• as a reactant

d[H]/dt= 2k3[O2][H]

Question 20

How do you show H• is an autocatalyst?

Answer 20

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

Question 21

Why are not all combinations of H2 and O2 explosives

Answer 21

Sometimes they act in termination reactions

Question 22

What are the possible termination steps?

Answer 22

H• + O2 + M -> HO2 +M
(Gas phase termination)

H• + wall -> removal
O• + wall -> removal
OH- + wall -> removal
Wall termination)

Question 23

What does the wall removal process rate depend on?

Answer 23

The wall removal processes have a rate that depends on pressure, the size of the vessel and the materials it is made of

Question 24

What does the rate of gas phase termination depend on?

Answer 24

Pressure (eg for number of collisions)

Question 25

How does a low pressure affect the termination of the reaction
2H2 + O2 -> 2H2O

Answer 25

Slow general reaction because:

Rate of wall termination> rate of branching

Question 26

How does a medium pressure affect the termination of the reaction
2H2 + O2 -> 2H2O

Answer 26

There will be an explosion as

Rate of branching > rate of termination

Question 27

How does a high pressure affect the termination of the reaction
2H2 + O2 -> 2H2O

Answer 27

There will be a slow reaction as

Rate of gas phase termination > rate of branching

Question 28

How would increasing the vessel size of the reactor affect the system?

2H2 + O2 -> 2H2O

Answer 28

Increasing the vessel size will decrease diffusion of radicals to the walls, so therefore lower the explosion limit

Question 29

How would changing the wall material of the reactor affect the system?

2H2 + O2 -> 2H2O

Answer 29

Pacifying the walls to make them inert to radicals will lower the first explosion limit

Question 30

How would adding an inert bath gas affect the system?

2H2 + O2 -> 2H2O

Answer 30

Adding an inert bath gas would reduce diffusion and hence loss of radicals to the walls, so will lower the first explosion limit

Question 31

How do you find the rate equation for H2 + Br2 -> 2HBr?

Answer 31

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