12 Na2SO3 + K2Cr2O7 + 4 H2SO4 → Cr2(SO)3 + 12 Na2SO4 + K2SO4 + 4 H2O
This is an acid-base reaction (neutralization): Na2SO3 is a base, H2SO4 is an acid.
This is an oxidation-reduction (redox) reaction:
3 SIV + 18 e- → 3 S-II (reduction)
9 SIV - 18 e- → 9 SVI (oxidation)
Na2SO3 is both an oxidizing and a reducing agent (disproportionation (dismutation)).