Ammonia decomposes to form nitrogen and hydrogen gases according to the overall…

CHEMISTRY · Multi-part · A Level · Chemistry · Kinetics · Kinetics

Problem

Ammonia decomposes to form nitrogen and hydrogen gases according to the overall stoichiometry

2\,\text{NH}_{3}(g) \rightarrow \text{N}_{2}(g) + 3\,\text{H}_{2}(g)

. At

700\,\text{K}

the uncatalyzed decomposition has an activation energy of

210\,\text{kJ\,mol}^{-1}

and an enthalpy change of

\Delta H = +46\,\text{kJ\,mol}^{-1}

. When the reaction occurs on the surface of a porous platinum catalyst, it proceeds through the following mechanistic sequence. 1. Adsorption of

\text{NH}_{3}

on Pt surface (exothermic by

65\,\text{kJ\,mol}^{-1}

) 2. Surface reaction forming adsorbed

\text{N}

and

\text{H}

atoms (rate-determining,

E_{a}=85\,\text{kJ\,mol}^{-1}

relative to the adsorbed state) 3. Recombination and desorption of

\text{N}_{2}

and

\text{H}_{2}

(fast) Answer the following. (a) Construct a fully labeled potential-energy diagram for the uncatalyzed decomposition. Include numerical values for the reactant energy, transition-state energy, and product energy relative to an arbitrary zero set at the energy of the gaseous reactants. (b) On the SAME set of axes, superimpose the catalyzed pathway that follows the sequence above. Include the adsorption well, the surface transition state, and the energies of the desorbed products. Indicate the numerical values given. (c) Justify why this catalytic pathway is classified as heterogeneous rather than homogeneous, referencing specific features of the mechanistic sequence and your diagram. (d) Platinum surfaces are susceptible to partial poisoning by strongly adsorbing sulfur species. Predict qualitatively how such poisoning would alter the shape of the catalyzed energy curve and discuss the resulting effect on the observed reaction rate.

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