Concentrated H2SO4's dual role as acid and oxidant with halides Steps: - Concentrated H2SO4 acts as a strong acid by donating H+ and as an oxidizing agent by accepting electrons, reducing to SO2. - It reacts with metal halides: acid behavior displaces HX, while oxidation occurs if X- is easily oxidized (Br- or I-, not Cl-). - For KBr, acid reaction forms HBr; then H2SO4 oxidizes 2Br- to Br2, with S reduced from +6 to +4 in SO2. - Compare options: only KBr shows both via the redox reaction 2KBr + 3H2SO4 → 2KHSO4 + Br2 + SO2 + 2H2O. Why C is correct: - KBr undergoes acid displacement to HBr, followed by oxidation to Br2 (E° for Br2/Br- = 1.07 V, feasible for H2SO4 oxidant). Why the others are wrong: - A. MgCO3: only acid-base reaction (MgCO3 + H2SO4 → MgSO4 + CO2 + H2O); no oxidation. - B. KCl: acid displaces HCl (2KCl + H2SO4 → 2KHSO4 + 2HCl), but no oxidation (Cl- requires stronger oxidant). - D. SO3: forms oleum (H2SO4 + SO3 → H2S2O7); neither acid-base …