When Mechanism II for ozone destruction operates with X = CI and X\' = Br, the r

Question

When Mechanism II for ozone destruction operates with X = CI and X' = Br, the radicals CIO and BrO react together to reform atomic chlorine and bromine (see Problem 2-1). A fraction of the latter process proceeds by the intermediate formation of BrCl, which undergoes photolysis in daylight. At night, however, all the bromine eventually ends up as BrCl, which does not decompose and restart the mechanism until dawn. Deduce why all the bromine exists as BrCl at night, even though only a fraction of the CIO with BrO collisions yields this product.

Explanation / Answer

BrCl is quite a stable species in absence of light. In presence of light it undergoes photolysis. So once BrCl is formed, it stays intact duting the night.

During the day, it undergoes photolysis and forms Br which forms BrO. So most of bromine stay as BrO or Br in stratosphere. Thus, Br will quickly react with ozone to form BrO, and BrO can eventually react with ClO to form the dimer. The dimer will either produce Br or BrCl, with Br atoms eventually re-reacting to form BrO and re-reacting with ClO.

following reactions takes place:

Br + O3 BrO + O2

Cl + O3 ClO + O2

BrO + ClO BrOOCl

BrOOCl + hn Br + Cl + O2

            Net rxn: 2O 3O2

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