Why can sulfate be oxidized to peroxodisulfate?

Question

I am currently in high school and have encountered the following reaction in my course on electrochemistry:

$$\ce{2 SO4- -> S2O8^2- + 2 e-}$$

Why this reaction happens in the first place? Isn’t the maximum oxidation state of sulfur only +6?

Answer 1

The oxidation number of sulfur in peroxydisulfate is still +6.

But the oxidation number of oxygen in peroxy group $\ce{\bond{-}O-O\bond{-}}$ is not −2, but −1.

$$\ce{O^{-}-SO2-O-O-SO2-O^{-}}$$

The reaction $\ce{2 SO4^2- -> S2O8^2- + 2 e-}$ does not oxidize sulfur. It oxidized two oxygen atoms and joins them to the peroxy group.