This is an oxidation-reduction (redox) reaction:
Rb0 - 1 e- → RbI (oxidation)
2 HI + 2 e- → 2 H0 (reduction)
O-II - 1 e- → O-I (oxidation)
Rb is a reducing agent, H2O is an oxidizing agent, H2O is a reducing agent.
Reactants:
Products: