2 KCrO7 + 11 Na2SO3 + 4 H2SO4 → K2SO3 + 11 Na2SO4 + Cr2(SO4)3 + 4 H2O
This is an acid-base reaction (neutralization): Na2SO3 is a base, H2SO4 is an acid.
This is an oxidation-reduction (redox) reaction:
10 SIV - 20 e- → 10 SVI (oxidation)
2 CrXIII + 20 e- → 2 CrIII (reduction)
Na2SO3 is a reducing agent, KCrO7 is an oxidizing agent.