In the transformation of thiosulfate to sulfate, what is the oxidation state change for sulfur in S2O3^2−?

• A. +2 to +6
• B. +4 to +6
• C. 0 to +6
• D. -2 to +6

Answer: (A)

This question tests how to track oxidation states in a sulfur-containing species that has two different sulfur atoms. In thiosulfate, the two sulfur atoms share the oxidation-state assignment so that their sum matches the molecule’s charge: if you let the two sulfur oxidation states be x and y, x + y − 6 = −2, so x + y = +4. The common and useful assignment is +6 for one sulfur and −2 for the other, which gives an average sulfur oxidation state of (+6 + −2)/2 = +2. When thiosulfate is oxidized to sulfate, every sulfur ends up at +6. So the average oxidation state of sulfur goes from +2 to +6. That is why the change is from +2 to +6. Remember that, on the level of individual sulfur atoms, the −2 sulfur is the one that increases to +6 while the +6 sulfur remains at +6, but the overall concept being tested is the shift of the average sulfur oxidation state from +2 to +6.