Question

Which of the following chemical reactions is/are related to smog formation in the troposphere?

• A. \(CFCl_3 + h\nu \rightarrow CFCl_2 + Cl\cdot\)
• B. \(CH_3O_2 + NO + O_2 \rightarrow HCHO + HO_2 + NO_2\)
• C. \(NO_2 + O_2 + h\nu \xrightarrow{M} O_3 + NO\)
• D. \(CH_4 + 8O_2 \rightarrow CO + H_2O + 2OH\cdot + 4O_3\)

Hint:

Photochemical smog forms in the troposphere through NOx and hydrocarbon oxidation cycles that generate ozone; CFC photolysis is a stratospheric ozone depletion reaction, not a tropospheric smog reaction.

Answer 1

The Correct Option is B, C, D

Step 1: Understand what photochemical smog formation involves.
Photochemical smog forms in the troposphere, the lowest layer of the atmosphere where we live, through a chain of reactions involving nitrogen oxides (NOx), volatile organic compounds and hydrocarbons, sunlight, and oxygen, which together generate ground level ozone (O3) and other secondary pollutants. The key feature of these reactions is that they occur in sunlight in the lower atmosphere and involve NOx-hydrocarbon-ozone cycles.

Step 2: Examine option (A).
The reaction \(CFCl_3 + h\nu \rightarrow CFCl_2 + Cl\cdot\) is the photolysis of a chlorofluorocarbon (CFC-11) that occurs in the stratosphere under high energy UV radiation, releasing a chlorine radical that goes on to destroy stratospheric ozone. This is the mechanism of stratospheric ozone depletion, not tropospheric smog formation, so option (A) is NOT related to smog and is excluded.

Step 3: Examine option (B).
The reaction \(CH_3O_2 + NO + O_2 \rightarrow HCHO + HO_2 + NO_2\) shows a methylperoxy radical, formed from hydrocarbon oxidation, reacting with NO to regenerate NO2 while producing formaldehyde (HCHO) and a hydroperoxy radical (HO2). This is a classic step in the photochemical smog cycle where hydrocarbon oxidation products convert NO to NO2 without consuming ozone, exactly the pathway that allows ozone to build up in smog. This reaction IS related to smog formation.

Step 4: Examine option (C).
The reaction \(NO_2 + O_2 + h\nu \xrightarrow{M} O_3 + NO\) represents, in combined form, the photolysis of NO2 to NO and atomic oxygen, followed by the atomic oxygen combining with molecular oxygen in the presence of a third body M to form ozone. This is the central ozone generating step of the photochemical smog cycle. This reaction IS related to smog formation.

Step 5: Examine option (D).
The reaction \(CH_4 + 8O_2 \rightarrow CO + H_2O + 2OH\cdot + 4O_3\) is a net stoichiometric representation of the hydrocarbon (methane) oxidation chain in the presence of NOx and sunlight, showing how a single hydrocarbon molecule, through repeated radical chain steps, ends up generating multiple ozone molecules and hydroxyl radicals. This overall equation is used specifically to summarize how hydrocarbons drive net ozone formation in smog. This reaction IS related to smog formation.

Step 6: Conclude.
Reactions (B), (C), and (D) all represent steps or net outcomes of the tropospheric photochemical smog cycle, while reaction (A) represents stratospheric CFC photolysis relevant to ozone layer depletion, an entirely different phenomenon. Hence the correct answers are options (B), (C), and (D).