1. ANSWER SCHEME ASSIGNMENT 4

2. QUESTION 1
Catalysis is the study of a chemical reaction due to the participation of an additional substance called a catalyst.
Define a catalyst and describe its properties.
Differentiate between homogeneous and heterogeneous catalytic process.
Describe all steps involved in a catalytic reaction and illustrate the diagram.

3. CATALYST-DEFINITION
A catalyst is a substance that affects the rate of a reaction but emerges from the process unchanged.
A catalyst usually changes a reaction rate by promoting a different molecular path ("mechanism") for the reaction.
Catalyst affect yield and selectivity
Changes only the rate of reaction; it does not affect the equlibrium.

4. CATALYST-PROPERTIES
Porous-Catalyst with large area resulting from pores
Molecular sieve - Small pores
Monolithic- can be either porous or nonporous
Supported-consists of minute particles of an active material dispersed over a less active substance.
Unsupported- mainly promoters that increase activity

5. CATALYST-TYPE HOMOGENEOUS HETEROGENEOUS
more than one phase – typical solid catalyst in liquid/gaseous reactants
Example;
The dehydrogenation of cyclohexane
A catalyst is in solution with at least one of the reactants
Example;
0xo process for manufacturing normal isobutylaldehyde

6. Steps in Catalytic Reaction
Mass transfer (diffusion) of the reactant from the bulk to the external transport.
Internal diffusion of the reactant
Adsorption of the reactant
Reaction on the surface
Desorption of the product
Diffusion of the product
Mass transfer of the product
* Steps 3, 4 and 5 are MOST important

9. QUESTION 2
Discuss the Rate-Limiting Step or Rate-Controlling Step in heterogeneous reactions. Give an example of the mechanisms involved in a process for:
Adsorption-limited reaction
Surface-limited reaction

10. Rate-Limiting Step
Three reaction steps in series (adsorption, surface reaction and desorption) involved in heterogeneous reactions.
One particular step in series is usually found to be rate-limiting or rate controlling.
If we could make this particular reaction faster, the entire reaction would proceed at an accelerated rate.

11. Adsorption-limited reaction

12. Surface-limited reaction

13. QUESTION 3
The overall reaction of the decomposition of cumene to form benzene and propylene is shown as equation below:
C6H5CH(CH3) C6H C3H6
According to the step in a Langmuir-Hinshelwood kinetic mechanism, propose the three steps represent the mechanism for cumene decomposition by denoting the various species in the catalytic reaction as C = cumene, B = benzene, and P = propylene.