1)
$1|\quad \mathop{I_2}\limits^0+2e\to 2\mathop{I}\limits^{-1}$
$1|\quad 4\mathop{S}\limits^{+2}\to 4\mathop{S}\limits^{+2,5}+2e$
$\Rightarrow I_2+2Na_2S_2O_3\to Na_2S_4O_6+2NaI$
Chất khử: $Na_2S_2O_3$
Chất oxi hoá: $I_2$
2)
$1|\quad 2\mathop{Cr}\limits^{+3}\to 2\mathop{Cr}\limits^{+6}+6e$
$3|\quad \mathop{N}\limits^{+5}+2e\to\mathop{N}\limits^{+3}$
$\Rightarrow Cr_2O_3+3KNO_3+4KOH\to 2K_2CrO_4+ 3KNO_2+2H_2O$
Chất khử: $Cr_2O_3$
Chất oxi hoá: $KNO_3$