$a) 2H_2 + O_2 \xrightarrow{t^o} 2H_2O$
$b) CuO + H_2 \xrightarrow{t^o} Cu + H_2O$
$c) Fe_2O_3 + 3H_2 \xrightarrow{t^o} 2F e+ 3H_2O$
$d) HgO + H_2 \xrightarrow{t^o} Hg + H_2O$
\(a.\)
\(2O_2+H_2\underrightarrow{^{t^0}}2H_2O\)
\(b.\)
\(CuO+H_2\underrightarrow{^{t^0}}Cu+H_2O\)
\(c.\)
\(Fe_2O_3+3H_2\underrightarrow{^{t^0}}2Fe+3H_2O\)
\(d.\)
\(HgO+H_2\underrightarrow{^{t^0}}Hg+H_2O\)