`a)`

$\mathop{Fe}\limits^{0}$`+H_2`$\mathop{S}\limits^{+6}$`O_4 (\text{đặc})`$\xrightarrow{t^o}$$\mathop{Fe_2}\limits^{+3}$`(SO_4)_3+`$\mathop{S}\limits^{+4}$`O_2+H_2O`

Ta có:

`xx 1|`$2\mathop{Fe}\limits^{0}$`->`$\mathop{Fe_2}\limits^{+3}$`+6e`

`xx 3|`$\mathop{S}\limits^{+6}$`+2e->`$\mathop{S}\limits^{+4}$

Cân bằng: `2Fe+6H_2SO_4`$\xrightarrow{t^o}$`Fe_2(SO_4)_3+3SO_2+6H_2O`