On heating a mixture of NaBr and conc. ${\mathrm{H}}_2{\mathrm{SO}}_4$ we obtain:

$$\begin{gathered} 2\mathrm{NaBr}+{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{Na}}_2{\mathrm{SO}}_4 + 2\mathrm{HBr} \\ {\mathrm{H}}_2{\mathrm{SO}}_4 \to {\mathrm{H}}_2\mathrm{O}+{\mathrm{SO}}_2+\left(\mathrm{O}\right) \\ 2\mathrm{HBr} + \left(\mathrm{O}\right) \to {\mathrm{H}}_2\mathrm{O} + {\mathrm{Br}}_2 \\ ------------- \\ 2\mathrm{NaBr}+2{\mathrm{H}}_2{\mathrm{SO}}_4\to {\mathrm{Na}}_2{\mathrm{SO}}_4+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{SO}}_2+{\mathrm{Br}}_2 \end{gathered}$$

HBr acts as reducing agent and H₂SO₄ acts as oxidizing agent

HBr undergoes oxidation and H₂SO₄ under goes reduction