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A catalyst reduces the activation energy for both forward and back reactions, leaving the equilibrium position unchanged.

For example when the reaction

2HI(g) H2(g) + I2(g)

is uncatalysed, its activation energy is 185 kJ mol-1 in the forward direction and 164 kJ mol-1 in the reverse direction.

When a platinum catalyst is used, the activation energy in the forward direction is reduced to 59 kJ mol-1.

Each catalyst has a different, individual effect on a reaction. The effect of two metallic catalysts on the activation energy for the decomposition of ammonia is shown in the table
Effect on Activation Energies

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

catalyst Ea/ kJmol-1
none 73
iodide ion 54
Pt surface 46
iron(III) 40
catalase 4
2H2O2 2H2O + O2
A variety of different kinds of catalyst can be effective, as shown for the decomposition of hydrogen peroxide
2NH3(g) N2(g) + 3H2(g)
catalyst Ea /kJmol-1
none 335
tungsten 163
osmium 197

Question

Which catalyst gives the greatest increase in the rate?

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How does a catalyst change the activation energy?

In two ways:


In other words catalysts change the path of a reaction; they change its mechanism.