a/
$Fe+2HCl \to FeCl_{2}+H_{2}$
$FeCl_{2}+1/2Cl_{2} \to FeCl_{3}$
$FeCl_{3}+3NaOH \to Fe(OH)_{3}+3NaCl$
$2Fe(OH)_{3} \buildrel{{t^o}}\over\longrightarrow Fe_{2}O_{3}+3H_{2}O$
b/
$2Fe+3Cl_{2} \buildrel{{t^o}}\over\longrightarrow 2FeCl_{3}$
$2FeCl_{3}+Fe \buildrel{{t^o}}\over\longrightarrow 3FeCl_{2}$
$FeCl_{2}+2AgNO_{3} \to 2AgCl+Fe(NO_{3})_{2}$
$2Fe(NO_{3})_{2}\buildrel{{t^o}}\over\longrightarrow 2FeO+4NO_{2}+O_{2}$