Catalysis

A catalyst is a substance added to a reaction that speeds up a reaction by making the activation energy lower, which makes the rate constatn larger and consequently the rate higher.  A catalyst speeds up both the forward and reverse reactions, so that no more product is made but the reaction yields the product faster.  Catalysts lower the Ea by providing a different mechanism for the reaction that requires lower energy.  The catalyst is not consumed during the reaction.  A catalyst that is in the same phase as reactants is  called a homogeneous catalyst while those that are in a different phase are referred to as heterogenous catalysts.

Homogeneous catlysis plays a key role in one of the most serious environmental problems of our time --- stratospheric ozone depletion.  The stratospheric ozone layer blocks out a considerable portion of the sun's ultraviolet radiation, radiation that can cause serious health risks to people, animals and plants if it arrive on the earch's surface without the filtering effect of the ozone layer.   Anthropogenic chlorofluorocarbons (CFC) are released into the atmosphere.  When they reach the troposphere absorb UV photons Cl atoms are released in the reaction:

                                                    CF2Cl2(g) + UV photon CF2Cl(g)  + Cl(g)

The dots () represent unpaired electrons resulting from bond breakages.  The chlorine radical is a very reactive species and readily reacts with stratospheric ozone to produce the intermedicate chlorine monoxide ( ClO ), that goes on to react with free O atoms to regenerate Cl atoms:

                                                    O3(g)  + Cl(g) ClO(g)  +  O2(g)

                                      ClO(g)  +  O(g)Cl(g)    +   O2(g)

The sum of these two elementary reactions gives the overall reaction ozone breakdown:

                        

The Cl atom acting as a homogeneous catalyst (since it is in gas phase like the reactants), speeds up the reaction by introducing a new mechanism. Chlorine gas is regenerated as part of the final reaction.   Because of its ability to regenerate, a single Cl atom can remain in the stratosphere for about 2 years destroying as many as 100,000 ozone molecules.
 

 





   

Credit: TOMS science team and Scientific Visualization Studio, NASA GSFC and Robert Simmons, Research and Professional Services.
NASA Website

These images were produced by Total Ozone Mapping Spectrometry instruments aboard NASA's Earth Probe satellite. The image on the left shows the largest ozone hole detected by NASA over the Antarctic in September 2000.  The figure on the right shows the ozone hole over the antarctic in October of 2000.  The hole expanded to  approximately 11 million square miles in September.  The previous record was approximately 10.5 square miles.  Despite cutbacks in the production of CFCs under international agreements, ozone-destroying gases in the stratospere are only now reaching their peaks due to their persistence in the atmosphere.


 

Quick Quiz: Does a catalyst result in an increase in the reaction rate by the same route as a rise in temperature ? Why?

Yes. They both increase the number of collisions in a gas.
Yes. They both reduce the activation energy necessary for a reaction to occur.
No. A catalyst provides an alternate mechanism that reduced the activation energy while a rise in temperature adds more energy to the system increasing the probability of a collision of sufficient energy to bring about a reaction occurring.
none of the above.

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