3.\(QToxh:Cu_2^{+1}\rightarrow2Cu^{2+}+2e|\times1\\ QTkhử:N^{+5}+1e\rightarrow N^{+4}|\times2\\ PT:Cu_2O+6HNO_3\rightarrow2Cu\left(NO_3\right)_2+2NO_2+3H_2O\)
4. \(QToxh:Fe_3^{+8/3}\rightarrow3Fe^{3+}+1e|\times1\\ QTkhử:N^{+5}+1e\rightarrow N^{+4}|\times1\\ PT:Fe_3O_4+10HNO_3\rightarrow3Fe\left(NO_3\right)_3+NO_2+5H_2O\)