The reaction of nitric oxide (NO(g)) with molecular hydrogen (H2(g)) results in

Question

The reaction of nitric oxide (NO(g)) with molecular hydrogen (H2(g)) results in the formation of nitrogen and water as follows:

2NO (g) + 2H2(g)...> N2(g) + 2H2O(g)

The experimentally determined rate-law expression for this reaction is first order in H2(g) and second order with NO(g).

a) Is the reaction above, as written an elementary reaction?

b) One potential mechanism for this reaction is as follows:

H2(g) + 2NO(g)...> N2O (g) + H2O (g) k1

H2(g) + N2O (g)...> N2(g) + H2O(g) k2

Is this mechanism consistent with the experimental rate law? If not, why?

c) An alternative mechanism for the reaction is:

2NO(g) <...> N2O2(g) (fast) k1(fwd) k-1(rev)

H2(g) + N2O2 (g)...> N2O(g) + H2O(g) k2

H2(g) + N2O (g)...> N2(g) + H2O(g) k3

Show that this mechanism is consistent with the experimental law.

Explanation / Answer

a) The reaction is not an elementary reaction because more than 2 reactants molecules (4 molecules) are engaged in the reaction. In addition a profound modification of structure is occured.

b) The mechanism is not consitent with the experimental rate law because in this case the reaction is second order for H2 considering the 2 steps are elementary reactions.

c) If we assume a steady state for N2O2, we can easily verify that the reaction is first order on H2 and second order for NO.

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