5 Na2SO3 + K2Cr2O7 + 2 H2SO4 → Cr2(SO4) + 5 Na2SO4 + K2SO4 + 2 H2O
This is an acid-base reaction (neutralization): Na2SO3 is a base, H2SO4 is an acid.
This is an oxidation-reduction (redox) reaction:
SIV + 2 e- → SII (reduction)
4 SIV - 8 e- → 4 SVI (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
Na2SO3 is an oxidizing agent, Na2SO3 is a reducing agent, K2Cr2O7 is an oxidizing agent.