a)
\(\text{ K2Cr2O7+6FeSO4+14NaHSO4 → Cr2(SO4)3+K2SO4+3Fe2(SO4)3+7Na2SO4+7H2O}\)
Ta có
Fe+2 \(\rightarrow\)Fe3+ +1e
Cr+6 +3e\(\rightarrow\)Cr+3
b)
4Zn+NO3-+7OH- \(\rightarrow\)4ZnO2 -2+NH3+2H2O
Zn \(\rightarrow\)Zn+2 +2e
N+5+8e \(\rightarrow\)N-3