Question

11. Ozone can be decomposed in the upper atmosphere 3 (g)+0 (g)-2 02 (g) One possible pathway is 3 (g) +NO (g) NO2 (g)+0209) NO2 (g) +0 (g)NO (g)+02(9) 4 (1) Which compound is a catalyst, which compound is an intermediate? (2) If Ea for un-catalyzed reaction is 16.5 kJ while Ea for the same reaction when catalyzed is 12.0 k. Assume the frequency factor A is the same. What is the ratio of the rate constant for the catalyzed and un-catalyzed reaction at 25.0°C?

Answer 1

1) The compound which is catalyst is nitric oxide (NO) because it does not involve in the reaction but catalyses the reaction.

We can observe in the above reactions where NO is present in the initial step and recovered in the final step.

The intermediate compound formed is NO₂ (nitrogen dioxide). It is formed at intermediate step and absent at final step.

2) Ratio of rate constant of catalysed reactions and uncatalyzed reactions = (Kcat/K)

Kcat = Ae^-Ea,cat/RT

K = Ae^-Ea/RT

Kcat/K = Ae^-Ea,cat/RT /Ae^-Ea/RT

Kcat/K = e^-Ea,cat/RT /e^-Ea/RT

Kcat/K = e^{-12 kJ/mol/0.008314 kJ/mol K x 298 K} / e^{-16.5 kJ/mol/0.008314 kJ/mol K x 298 K)}

Kcat/K = e^-(12/2.48) /e^{-(16.5 /2.48)}

Kcat/K = 0.00792 / 0.00129 = 6.14

This shows that the reaction in presence of catalyst is 6 times faster than uncatalyzed reaction