2Al+3H_2 SO_4 \to Al_2 (SO_4)_3 +3H_2
$oxidation half equation:$ Al\to Al^{3+}+3e^-
$reduction half equation:$ 2H^+2e^-\to H_2
SO_4 ^{2-} $ is a spectator ion
For Ba + HCl acid
Ba_{s}+2HCl_{aq}\to BaCl_2 . _{aq} +H_2._{g}
$oxidation half equation:$ Ba\to Ba^{2+}+2e^-$Reduction half equation:$...