12. Which of the following substances can be both an oxidizing agent and a reducing agent and why:
MnO2; Mn; KJ; KClO4; K2Cr2O7?
Based on the electronic equations, arrange the coefficients in the equations of the reactions that follow the schemes:
Fe(OH)2 O2 H2O → Fe(OH)3
Na3AsO3 I2 H2O → Na3AsO4 HI
Zn KNO2 KOH → K2ZnO2 NH3 H2O

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