\(a.O_2\underrightarrow{\left(1\right)}Fe_3O_4\underrightarrow{\left(2\right)}Fe\underrightarrow{\left(3\right)}H_2\)
\(\left(1\right)3Fe+2O_2\underrightarrow{t^o}Fe_3O_4\\ \left(2\right)Fe_3O_4+4H_2\underrightarrow{t^o}3Fe+4H_2O\\ \left(3\right)Fe+2HCl\rightarrow FeCl_2+H_2\)
\(b.S\underrightarrow{\left(1\right)}SO_2\underrightarrow{\left(2\right)}SO_3\underrightarrow{\left(3\right)}H_2SO_4\)
\(\left(1\right)S+O_2\underrightarrow{t^o}SO_2\\ \left(2\right)2SO_2+O_2\underrightarrow{t^o,xt}2SO_3\\ \left(3\right)SO_3+H_2O\rightarrow H_2SO_4\)