[Win,odd Dhamnekar]

Consider this scenario and answer the following questions: Chlorine atoms resulting from decomposition of chlorofluoromethanes, such as CCl₂F₂,catalyze the decomposition of ozone in the atmosphere. One simplified mechanism for the decomposition is:

O₃ [itex]\xrightarrow{\text{sunlight}}[/itex] O₂ +O
O₃ + Cl  O₂ + ClO
ClO +O  Cl + O₂
(a) Explain why the Chlorine atoms are catalyst in the gas-phase transformation :
2O₃   3O₂ ?

(b)Nitric oxide is also involved in the decomposition of ozone by the mechanism:

O₃[itex]\xrightarrow{\text{sunlight}}[/itex] O₂ + O
O₃ + NO  NO₂ + O₂
NO₂ + O  NO + O₂

Is NO catalyst for the decomposition of ozone? Explain your answer.

My answer:

 (a) Why are chlorine atoms catalysts in the gas-phase transformation?

Chlorine atoms act as catalysts in the decomposition of ozone because they facilitate the reaction without being consumed in the overall process. The mechanism is as follows:

1. O₃ + Cl → O₂ + ClO
2. ClO + O  Cl + O₂

In this cycle, chlorine atoms react with ozone to form chlorine monoxide (ClO) and oxygen (O₂). The ClO then reacts with atomic oxygen (O) to regenerate the chlorine atom and produce more oxygen (O₂). Since the chlorine atom is regenerated and can participate in multiple cycles, it acts as a catalyst.

 (b) Is NO a catalyst for the decomposition of ozone?

Yes, nitric oxide (NO) is a catalyst for the decomposition of ozone. The mechanism is:

1. NO + O₃ → NO₂ + O₂
2. NO₂ → NO + O(under sunlight)
3. NO + O → NO₂

In this process, NO reacts with ozone to form nitrogen dioxide (NO₂) and oxygen (O₂). Under sunlight, NO₂ dissociates back into NO and atomic oxygen (O). The NO is regenerated and can continue to catalyze the decomposition of more ozone molecules, making it a catalyst.