The collision theory of reactions


The distribution of energies



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The distribution of energies

Molecules are moving at different speeds, at any given time. Some have higher energies, some medium energies, others have lower energies.



Maxwell-Boltzmann distribution: distribution of kinetic energies in a gas, at a given temperature.

As the temperature increases, more molecules move at higher speeds and have higher kinetic energies.






300 K


310 K

What is the significance of this for reaction rates?

Eg. the activation enthalpy for a reaction is: E­a­­ is +50 kJ mol-1.

How many collisions have MORE energy than +50 kJ mol-1?



Only collisions with energies in this region can lead to a reaction.


At higher temperatures, a significantly higher proportion of molecules will have energies above +50 kJ mol-1.





300 K



310 K

Kinetic energy (E)


About twice as many molecules have enough energy to react –so the reaction goes twice as fast.



SL: What is removing the ozone?

Several radicals remove ozone, in the stratosphere:

Chlorine atom, Cl

B
From the oceans, burning coal / vegetation (eg forest fires); they are the source of Br and Cl in the atmosphere.


romine atom, Br

Chloromethane, CH3Cl

Bromomethane, CH3Br
Several reactions occur in the stratosphere:
Chlorine-containing molecules absorb high energy solar radiation and break down to give chlorine atoms.
Reaction 6: Cl + O3  ClO + O2

(chlorine atoms react with ozone, forming radicals.)



Reaction 7: ClO + O  Cl + O2

(The ClO radical then reacts with oxygen atoms.)


We now have TWO reactions, competing to remove ozone:

Reaction 4 O + O3  O2 + O2 and

Reaction 6 Cl + O3  ClO + O2
The concentration of Cl atoms in the stratosphere is much less than the concentration of O atoms. How significant is reaction 6?
 do assignment 7, p.69
Chlorine atoms are particularly effective at removing ozone. A single atom can remove about 1 million ozone molecules.

Add equations 6 and 7 together to produce the equation for the overall reaction caused by chlorine atoms.

Comment on the result. What role are Cl atoms playing in the overall reaction?


  • (O + O3  O2 + O2 )

  • The radicals are regenerated, and so acting as catalysts.

It is important for chemists to know which reaction (6 or 7) is happening fastest, to understand whether oxygen atoms or chlorine atoms are responsible for the removal of ozone.
In fact, Cl atoms react 1500 faster with ozone, compared to O atoms. Even though Cl atoms have a much lower concentration in the stratosphere than O atoms, the fact that they can be regenerated in a catalytic cycle can have devastating effects.
Bromine is 100 times more effective at destroying ozone than chlorine, despite being much less concentrated than chlorine.
CI 10.5 How do catalysts work?

Collision theory and enthalpy profiles can help us to understand how catalysts work.

Bond breaking is endothermic (requires an energy input to overcome the activation enthalpy barrier). Bonds first stretch and then break, reactants are made and new bonds instantly form. If few molecules have enough energy to overcome this activation enthalpy, then the reaction will be slow.

Catalysts provide an alternative reaction pathway for breaking and remaking bonds that has a lower activation enthalpy.




Catalysts and equilibrium
Catalysts only affect the RATE and not the position of equilibrium in a reversible reaction. The composition of the reaction mixture remains unchanged.
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