We already know of two ways to increase the rate of a chemical reaction
 Can you name them?
o Increasing the concentration and increasing the temperature
A third way to increase the rate of a reaction is to use a catalyst.
A catalyst increases the rate of a chemical reaction by providing a
different reaction route, with lower activation energy.
Consider that you are trying to cross thanon Sukumwit. It takes a lot of
time and effort to use the footbridge, which climbs high above the road.
However, there maybe a policeman, who will kindly stop the traffic for
you. You can quickly dash across the road, without having to spend all
that time and energy climbing the footbridge. The policeman will have
acted as a catalyst. He provides another way across the road, which
does not involve crossing a high energy barrier, and you can cross the
road much more quickly as a result.
Something else to note about catalysts is that even though they take part
in reactions, they reappear unchanged, at the end of the reaction as
product, and are then available to catalyse further reactions.
The conversion of ozone (O3) into oxygen (O2) is normally a slow reaction.
There is a relatively high activation energy to the reaction (Figure 1)
Uncatalysed pathway
Ozone
O3
Oxygen
O2
Figure 1: Uncatalysed conversion of ozone into oxygen
(1)
When chlorine acts as a catalyst however, the activation energy, is
reduced, so the energy barrier is lowered as seen in Figure 2.
Catalysed pathway, with chlorine catalyst
Ozone
O3
Oxygen
O2
Figure 2: chlorine catalysed conversion of ozone into oxygen
Atomic chlorine (Cl) provides an alternative route for the conversion of
ozone into oxygen, involving intermediate reactions, each having a lower
energy barrier than the uncatalysed reaction.
The alternate pathway involves two steps.
First the chlorine reacts with the ozone, forming chlorine monoxide, and
an oxygen molecule:
Cl + O 3
ClO + O 2
The new chlorine monoxide anion then reacts with another ozone
molecule to restore the chlorine atom and two more oxygen molecules:
ClO + O 3
Cl + 2 O 2
Although chlorine is used up in the first reaction, it reappears in the final
step, and there is no net consumption of chlorine. At the same time, two
ozone molecules are rapidly converted into three oxygen molecules.
The chlorine is therefore a catalyst for the conversion of ozone to oxygen,
because chlorine increases the rate of the reaction, whilst not being
consumed in the reaction.
(2)
Note that because chlorine is regenerated at the end of the conversion, it
is free to convert another ozone molecule, and another, and another,
and keep on doing so until the chlorine removed by natural processes. A
single chlorine molecule can convert around 100,000 molecules of ozone
into oxygen before being made ‘safe’ in this way.
Chlorine atoms in the stratosphere catalyse the destruction of the earth’s
ozone layer. This layer is important because it protects us from harmful
radiation from the sun. Evidence suggests that chlorine atoms are formed
in the stratosphere as a by-product of human-made clorofluorocarbons
(CFC’s). These have been widely used as the cooling fluid in refrigerators,
and air conditioners, and were also used as the propellant in spray cans.
Destruction of the ozone layer is a serious issue because the ozone layer
protects us from the suns harmful ultraviolet rays.
Catalysts are not all bad. Chemists can use catalysts for many beneficial
purposes. For example, the exhaust pipe in a car contains many harmful
pollutants, such as nitrogen monoxide, carbon monoxide, and even
unburned fuel (hydrocarbons). Most cars are now fitted with catalytic
converters, which reduce the amount of these harmful gasses that enter
the atmosphere. The catalytic converter uses metal catalysts, usually
platinum, palladium and ruthenium, to speed up reactions that convert
the pollutants into less toxic compounds.
Nitrogen monoxide is converted into nitrogen and oxygen, carbon
monoxide is converted into carbon dioxide, and unburned fuel is
converted into carbon dioxide and water.
It is because catalysts are not consumed by the reactions they help, that
a single catalytic converter can operate effectively for the lifetime of the
car.
(3)