CI 10.2: The effect of temperature on rate
For many reactions, increasing the temperature by 10oC will DOUBLE the rate of reaction.
Extending the collision theory of reactions
Consider the reaction in the Haber process for making ammonia:
N2 (g) + 3H2 (g) 2NH3 (g)
The collision theory says that N2 and H2 will only react when they collide. The more frequently they collide, the faster the rate of reaction. Increasing the pressure brings H2 and N2 closer together, so they collide more often. Increasing the temperature makes molecules move faster, increasing the frequency of collisions.
How much more frequently do molecules collide, when temperatures rise?
The average speed of molecules is proportional to the square root of the absolute temperature.
Example:Temperature rise: 300 K to 310 K
Increase in average speed of molecules: (310/300)1/2
= 1.016
(this is 101.6%...so an increase of 1.6% faster)
Particles must also react with a certain minimum kinetic energy.
In the Haber process, (reaction between H2 and N2)
at 300 K only 1 in 1011 collisions between H2 and N2 results in a reaction!
Even at 800 K only 1 in 104 collisions results in a reaction.
The collision theory says:
Reactions occur when molecules collide with acertain minimum kinetic energy. The more frequent these collisions, the faster the rate of reaction.
The energy needed to overcome the energy barrier is called the activation enthalpy, for the reaction. It is the energy needed to start breaking bonds in the colliding molecules, so that collisions can lead to reactions.
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
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
Bromine 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 4O + O3 O2 + O2and
Reaction 6Cl + 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.