Which change require an oxidizing agent to produce the indicated product?

Question

a) S2O3^-2 -> S4O6^-2

b) zn^+2 -> Zn

c) Clo^- -> Cl^-

d) So3 -> SO4^-2

I know the answer is a, but I need some explannation in detail. The wording seems like very tricky to me. Need advise from experts

Answer 1

Look at the oxidation states in the products and reactants.

In a, O almost always is 2-, so the reactant must have S with a 2+ charge to get a total charge of 2-.  In the product, the 4 S atoms must have a total charge of 10+.  I'm not sure how the charges are divided up in this ion, but the average charge on an S atom is 10/4 = 2.5, which is higher than the original 2, so oxidation must have occurred.

In b, Zn goes from 2+ to 0, a decrease, so it was reduced, not oxidized.

In c, Cl goes from 1+ to 1-, also a decrease and therefore also reduction.

Finally, in d, the S starts and ends at a +6 oxidation state, so no redox reaction occurred in d.