# Is it a redox reaction if the oxidation state of the element has changed only in one of the products?

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.

• I edited the question's title to make it more definitive (feel free to rollback if you think this was wrong). Also, I improved the formatting a bit. If you are interested, please visit this page, this page and this one on how to format your future posts better with MathJax and Markdown. – andselisk Jan 11 '18 at 14:17
• In a redox reaction you must both an oxidation and a reduction. It is impossible to have only a reduction, or only an oxidation. – MaxW Jan 11 '18 at 16:25

One sulphuric acid is used to form sodium sulphate and the other one is used for oxidizing. So you have a reduction:

$\ce{4H+ + SO4^2- +2e- -> SO2 + 2H2O}$

and an oxidation:

$\ce{2Br- -> Br2 +2e^-}$

The sodium sulphate is formed with:

$\ce{H2SO4 + 2NaBr -> Na2SO4 + 2HBr}$

This last one is not a redox reaction.

• @Nilay Ghosh Thank you for editing. I really should learn that if I want to write here from my mobile. Is that the LaTeX mhchem package? – Raoul Kessels Jan 11 '18 at 14:36
• You can find this page of help. – Nilay Ghosh Jan 11 '18 at 16:44