3 Sn + 2 K2Cr2O7 + 14 H2SO4 → 3 Sn(SO4)2 + 2 Cr2(SO4)3 + 2 K2SO4 + 14 H2O
This is an oxidation-reduction (redox) reaction:
3 Sn0 - 12 e- → 3 SnIV (oxidation)
4 CrVI + 12 e- → 4 CrIII (reduction)
Sn is a reducing agent, K2Cr2O7 is an oxidizing agent.