CH7003 Practise Problem 3

1. Consider the chain reaction A -> B + C which proceeds by the steps:

The second termination step occurs by reaction of a scavenger S. Find the expression for the rate of this reaction in terms of C_A, C_S and k_i, k_p, k_t, k_tS. Assuming C_R is small.  Under what condition does the reaction reach the maximum possible reaction orders with respect to C_A?

The reaction H₂ + O₂ -> H₂O proceeds first via adsorption of H₂ and O₂ as atoms. It has a rate-limiting step H - S + O -S -> HO - S (here S stands for the surface active sites) and the rate of the reaction is r = k_RΘ_HΘ_o. Derive a reaction rate expression r(P_H2 ,P_O2 K_H2 ,K_O2 ) for this reaction assuming negligible coverage of HO and H₂O and a Langmuir Hinshelwood kinetics with competitive adsorption, where K_i is the adsorption-desorption equilibrium constant.

3. The irreversible, gas-phase reaction 2A B+C is conducted over a solid catalyst. The reaction may be considered isothermal. The following reaction mechanism is proposed, where S represents a surface site.

(i) A + S  A-S
(ii) A-S + A-S  B-S + C + S
(iii) B-S  B + S

It is proposed that step (ii) is rate-determining and irreversible, whereas steps (i) and (iii) are equilibrated. 

1. Based on the proposed mechanism, derive the LH rate expression for the reaction.
2. Experiments with a differential reactor have been used to measure the reaction rate, tabulated in the table below.  
    

Note: Pressure are in MPa, measured rate is on arbitrary units.

Are the data consistent with the proposed mechanism? State and justify any assumptions you have made.