1. Write three equivalent fractions in each of the
following
3
5
(i)
(iii) (iv)
5
4
2
2
7
(ii)
(iv) 15 (1)
(v) 3
Answers
Answer:
The SO2 + H2O reaction is proposed to be the starting process for the oxidation of sulfur dioxide to sulfate in liquid water, although the thermal reaction displays a high activation barrier. Recent studies have suggested that the reaction can be promoted by light absorption in the near UV. We report ab initio calculations showing that the SO2 excited triplet state is unstable in water, as it immediately reacts with H2O through a water-assisted proton coupled electron transfer mechanism forming OH and HOSO radicals. The work provides new insights for a general class of excited-state promoted reactions of related YXY compounds with water, where Y is a chalcogen atom and X is either an atom or a functional group, which opens up interesting chemical perspectives in technological applications of photoinduced H-transfer.
Graphical abstract: Triplet state promoted reaction of SO2 with H2O by competition between proton coupled electron transfer (pcet) and hydrogen atom transfer (hat) processes
Introduction
Hydrogen atom transfer originated by radicals is one of the most fundamental processes for several areas of chemistry, from biological processes, to chemical synthesis, materials science, hydrocarbon combustion or atmospheric chemistry.1–12 In the context of the chemistry of the troposphere, the oxidation processes involving abstraction of hydrogen atoms by radicals are among the most important reactions and many of these processes are well known.7,13 Very recently, it has been shown that the triplet excited state of SO2 can react with water producing hydroxyl radicals and is the most oxidizing species in the Earth's atmosphere,14 and it has also been predicted that this reaction is enhanced by several orders of magnitude at the air–water (air–clouds) interface.15 Because the SO2 molecule is an air pollutant formed as a byproduct of fossil fuel combustion, there is broad interest in the chemistry of SO2 ranging from its industrial clean-up to understanding the chemistry associated with its oxidation to sulfuric acid, a major constituent of acid rain.7,16,17 On a fundamental level, the oxidation of SO2 is thought to proceed by the following steps in the gas-phase:
SO2(g) + OH(g) → HOSO2(g) (1)
HOSO2(g) + O2(g) → SO3(g) + HO2(g) (2)
SO3(g) + H2O(g) → H2SO4(g) (3)or an aqueous-phase
SO2(g) + H2O(l) → H2SO3(aq) (4)
H2SO3(aq) + H2O2(aq) → H2O(l) + H2SO4(aq) (5)