Question

Compare the functions of homogeneous and heterogeneous catalysts.

Short answer

Homogeneous Catalysts	Heterogeneous Catalysts
It exists in the same phase as the reactants	It exists in different phase than the reactants.
It does not provide an active surface for the reaction to takes place but reacts directly with reactants.	It provides an active surface for the reaction to takes place.
It reacts with reactants in one or many steps.	It reacts with reactants in atleast four steps.

Step-by-step solution

Definition of catalyst

Catalyst are substances which increases the rate of the reaction without getting used up by themselves while decreasing the activation energy of the reaction.

Homogeneous Catalyst  

• Homogeneous catalyst is one which exists in the same phase of the reactants.
• It reacts directly with the reactants forming an intermediate substance.
• This further undergo decomposition with another reactant in one or many steps forming product and reproducing the catalyst.

Let us explain homogeneous catalyst with the help of an example, Earth’s ozone layer.Ozone is formed when oxygen molecule absorbs ultraviolet radiations.This formed triatomic ozone is highly unstable which undergoes reverse decomposition reaction to form diatomic oxygen with mechanism as shown below: This reaction is catalysed by nitric oxide (NO) by the following mechanism:The overall reaction which is catalysed by nitric oxide, will be:Nitric oxide is reproduced in these reactions; as it is not used up completely it acts as catalyst by increasing the rate of reaction.

Heterogeneous Catalyst   

• Heterogeneous catalyst is one which exists in different phase (generally solid) than the reactants (which may be gas or liquid phase).
• It provides an active surface for the reaction to takes place.
• It reacts with reactants in at least four steps. They are:

1. Reactant adsorption
2. Activation of the reactant that is adsorbed
3. Reaction of the activated adsorbed reactant
4. Product diffusion from surface to gas or liquid phase

Thus, the rate of the reaction will be increased.Let us explain heterogeneous catalyst with the help of an example, Hydrogenation of polyunsaturated (compounds with double bonds) fats and oils to saturated (compounds with single bonds) fats and oils in the presence of catalyst Nickel.The four steps involved in the catalysis hydrogenation of ethylene with nickel as catalyst to form ethane are:

• Adsorption of Hydrogen on the surface involving breaking of H-H bonds along with formation of Ni-H bonds.
• Adsorption of ethylene on the surface involving breaking of\(\pi \)bonds along with formation of Ni-C bonds.
• Atoms spread across the surface forming new C-H bonds by colliding each other. The formed ethane molecules\(\left( {{{\rm{C}}_{\rm{2}}}{{\rm{H}}_6}} \right)\)separate from the nickel surface as their bonding with nickel is weak.

Therefore, in the presence of nickel as catalyst the rate of the reaction is increased in this reaction. Thus, homogeneous catalysts exist in the same phase as the reactants while heterogeneous catalysts exist in different phase than the reactants.