3 NaNO2 + 2 K2CrO4 + 5 H2SO4 → 3 NaNO3 + Cr2(SO4)3 + 2 K2SO4 + 5 H2O
This is an acid-base reaction (neutralization): K2CrO4 is a base, H2SO4 is an acid.
This is an oxidation-reduction (redox) reaction:
3 NIII - 6 e- → 3 NV (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
NaNO2 is a reducing agent, K2CrO4 is an oxidizing agent.