I’m not sure on which elements to compare to ensure that this is a redox reaction:
$$\ce{2 H2SO4 (aq) + 2 NaBr (s) -> Br2 (l) + SO2 (g) + Na2SO4 (aq) + 2 H2O}$$
I’m trying to indicate whether this involve oxidation/reduction. I’m looking at the oxidation state of $\ce{S}$.
What do I compare $\ce{H2SO4}$ with: $\ce{SO2}$ or $\ce{Na2SO4}$?
I tried both. Oxidation state of $\ce{S}$ in $\ce{SO2}$ is $+4$ and in $\ce{Na2SO4}$ is $+6$. One of them tells me it’s reduced, and the other tells me that no redox reaction is happening.
Which do I compare with and why? I’m not sure.