ANSWER SCHEME ASSIGNMENT 4

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.

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.

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

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

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

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

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.

Adsorption-limited reaction

Surface-limited reaction

QUESTION 3 The overall reaction of the decomposition of cumene to form benzene and propylene is shown as equation below:   C6H5CH(CH3)2 C6H6 + 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.