Home Revision A-Level Chemistry Physical Chemistry Rates of Reaction

THE EFFECT OF CATALYSTS ON REACTION RATES


          
This page describes and explains the way that adding a catalyst affects the rate of a reaction.  It assumes that you are already familiar with basic ideas about the collision theory of reaction rates, and with the Maxwell-Boltzmann distribution of molecular energies in a gas.

      

      

        
        
          

          

          Note:  If you haven't already read the page about collision theory, you should do so before you go on.
          

          
Use the BACK button on your browser to return to this page, or come back via the rates of reaction menu.

          
          

          

          

      

      

        
Note that this is only a preliminary look at catalysis as far as it affects rates of reaction.  If you are looking for more detail, there is a separate section dealing with catalysts which you can access via a link at the bottom of the page.

          

          

            
The facts
          

          
What are catalysts?

          
A catalyst is a substance which speeds up a reaction, but is chemically unchanged at the end of the reaction.  When the reaction has finished, you would have exactly the same mass of catalyst as you had at the beginning.

          
Some examples

          
Some common examples which you may need for other parts of your syllabus include:

          

            

              reaction
              catalyst
            

            

              Decomposition of hydrogen peroxide
              manganese(IV) oxide, MnO2
            

            

              Nitration of benzene
              concentrated sulphuric acid
            

            

              Manufacture of ammonia by the Haber Process
              iron
            

            

              Conversion of SO2 into SO3 during the Contact Process to make sulphuric acid
              vanadium(V) oxide, V2O5
            

            

              Hydrogenation of a C=C double bond
              nickel
            

          

      

      

        
        
          

          

          Note:  You can find details of these and other catalytic reactions by exploring the menu for the main section on catalysis.  You will find a link at the bottom of this page.

          
          

          

          

      

      

        

          
The explanation

          
The key importance of activation energy

          
Collisions only result in a reaction if the particles collide with a certain minimum energy called the activation energy for the reaction.

      

      

        
        
          

          

          Note:  What follows assumes you have a reasonable idea about activation energy and its relationship with the Maxwell-Boltzmann distribution.  This is covered on the introductory page about collision theory.
          

          
If you aren't confident about this, follow this link, and use the BACK button on your browser to return to this page.

          
          

          

          

      

      

        
You can mark the position of activation energy on a Maxwell-Boltzmann distribution to get a diagram like this:

          

          
Only those particles represented by the area to the right of the activation energy will react when they collide.  The great majority don't have enough energy, and will simply bounce apart.

          
Catalysts and activation energy

          
To increase the rate of a reaction you need to increase the number of successful collisions. One possible way of doing this is to provide an alternative way for the reaction to happen which has a lower activation energy.

          
In other words, to move the activation energy on the graph like this:

          

          

          
Adding a catalyst has exactly this effect on activation energy.  A catalyst provides an alternative route for the reaction.  That alternative route has a lower activation energy.  Showing this on an energy profile:

          

          
A word of caution!

          

            

              
Be very careful if you are asked about this in an exam.  The correct form of words is

            

            

              
"A catalyst provides an alternative route for the reaction with a lower activation energy."

            

          

          
It does not "lower the activation energy of the reaction".  There is 
            a subtle difference between the two statements that is easily illustrated with a simple analogy.

          
Suppose you have a mountain between two valleys so that the only way for people to get from one valley to the other is over the mountain.  Only the most active people will manage to get from one valley to the other.

          
Now suppose a tunnel is cut through the mountain.  Many more people will now manage to get from one valley to the other by this easier route.  You could say that the tunnel route has a lower activation energy than going over the mountain.

          
But you haven't lowered the mountain!  The tunnel has provided an alternative route but hasn't lowered the original one.  The original mountain is still there, and some people will still choose to climb it.

          
In the chemistry case, if particles collide with enough energy they can still react in exactly the same way as if the catalyst wasn't there.  It is simply that the majority of particles will react via the easier catalysed route.

          

          

            

              
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