Answer: The H₂SO₃ + H₂O₂ → H₂SO₄ + H₂O reaction in droplets proceeds rapidly even at low pH, so acidification is self-reinforcing - the more acidic the droplet becomes, the more this pathway (relative to pH-sensitive pathways) dominates, enabling very low pH values of 3–4.
- A The H₂SO₃ + H₂O₂ reaction is self-limiting: as pH drops, the reaction slows to zero, preventing the pH from falling below 5 under normal conditions as generally observed in typical laboratory settings under usual circumstances according to most studies in the majority of documented cases
- B The gas-phase oxidation of SO₂ by ozone is usually faster and dominates; the aqueous pathway is negligible as widely reported in standard reference material under most conditions studied in most observed cases under typical physiological conditions according to standard texts
- C The H₂SO₃ + H₂O₂ → H₂SO₄ + H₂O reaction in droplets proceeds rapidly even at low pH, so acidification is self-reinforcing - the more acidic the droplet becomes, the more this pathway (relative to pH-sensitive pathways) dominates, enabling very low pH values of 3–4
- D H₂O₂ reacts with H₂SO₃ to form H₂SO₃ × H₂O₂ complexes that are insoluble and precipitate rather than forming acid rain in general clinical practice as frequently documented in most reference accounts under normal conditions as generally observed in typical laboratory settings
Correct answer: C. The H₂SO₃ + H₂O₂ → H₂SO₄ + H₂O reaction in droplets proceeds rapidly even at low pH, so acidification is self-reinforcing - the more acidic the droplet becomes, the more this pathway (relative to pH-sensitive pathways) dominates, enabling very low pH values of 3–4
Explanation: Unlike some aqueous oxidants (e.g., O₃, which reacts with HSO₃⁻ at a rate that slows as pH decreases), the reaction H₂SO₃ + H₂O₂ → H₂SO₄ + H₂O involves the undissociated form of sulphurous acid and is relatively pH-independent at the pH range of interest (3–5). This makes it self-accelerating: as the droplet acidifies, H₂SO₃ (pKa 1.81) predominates over HSO₃⁻, and the H₂O₂ pathway continues unimpeded, allowing cloud droplets to reach pH 3–4 in heavily polluted regions.
Classical smog is a reducing mixture of SO₂ and fog, while photochemical smog is an oxidising mixture generated by UV-driven reactions of NOx and hydrocarbons.
Concept context
Understand how human activities alter the atmosphere, water, and soil. Covers air pollutants, smog types, ozone depletion, acid rain, greenhouse effect, water and soil pollution, and the principles of green chemistry.