2 K3AsO4 (aq) + 8 Zn (s) + 11 H2SO4 (aq) → 2 AsH3 (aq) + 8 ZnSO4 (aq) + 3 K2SO4 (aq) + 8 H2O (l)
This is an oxidation-reduction (redox) reaction:
2 AsV + 16 e- → 2 As-III (reduction)
8 Zn0 - 16 e- → 8 ZnII (oxidation)
K3AsO4 is an oxidizing agent, Zn is a reducing agent.