$C + {O_2} \to C{O_2};\Delta H = - 393\,J$
$2Zn + {O_2} \to 2ZnO;\Delta H = - 412\,J$
$I.\,C{r_2}{O_7}^{2 - }\, + \,2O{H^ - }\, \to \,Cr{O_4}^{2 - }\, + \,{H_2}O$
$II.\,Zn\, + \,CuS{O_4}\, \to \,ZnS{O_4}\, + \,Cu$
$III.\,2Mn{O_4}^ - \, + \,3M{n^{2 + }}\, + \,4O{H^ - }\, \to \,5Mn{O_2}\, + \,2{H_2}O$
$IV.\,2C{u^ + }\, \to \,Cu\, + \,C{u^{2 + }}$
$2F{e^{3 + }} + S{n^{2 + }} \to 2F{e^{2 + }} + \,\,A$
$14{H^ + } + C{r_2}O_7^{2 - } + 3Ni \to 2C{r^{3 + }} + 7{H_2}O + 3N{i^{2 + }}$