# hw8 - CE 561 Homework 8(assigned due A proposed mechanism...

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CE 561 Homework 8: (assigned 10/19/09, due 10/26/09) A proposed mechanism for the water-gas shift reaction on ferrochrome catalysts at high temperatures is H 2 O + S H 2 O-S r 1 = k 1 [H 2 O] θ S - k -1 θ H2O-S (1) H 2 O-S HO-S-H r 2 = k 2 θ H2O-S - k -2 θ HO-S-H (2) HO-S-H O-S + H 2 r 3 = k 3 θ HO-S-H - k -3 [H 2 ] θ O-S (3) CO + O-S OCO-S r 4 = k 4 [CO] θ O-S - k -4 θ OCO-S (4) OCO-S CO 2 + S r 5 = k 5 θ OCO-S - k -5 [CO 2 ] θ S (5) where S is a surface site and H 2 O-S, HO-S-H, O-S and OCO-S are surface bound species. (a) What are the units of the ten elementary rate constants for these five reversible reactions? (b) Derive an expression for the rate of the overall reaction (H 2 O + CO H 2 + CO 2 ) in terms of the gas phase concentrations, assuming that reaction 3 is the rate limiting step. (c) Derive an expression for the rate of the overall reaction assuming that CO 2 desorption is the rate limiting step. (d) Derive an expression for the rate of the overall reaction assuming that H 2 O adsorption is the rate limiting step.

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