$1)$
$a)$
$2Ba+O_{2} \xrightarrow{t^o} 2BaO$
$BaO+H_{2}O \to Ba(OH)_{2}$
$Ba(OH)_{2}+H_{2}SO_{4} \to BaSO_{4}+2H_{2}O$
$b)$
$C+O_{2} \xrightarrow{t^o} CO_{2}$
$CO_{2}+H_{2}O \to H_{2}CO_{3}$
$H_{2}CO_{3}+2NaOH \to Na_{2}CO_{3}+2H_{2}O \uparrow$
$Na_{2}CO_{3} +2HCl \to 2NaCl+H_{2}O+CO_{2} \uparrow$
$CO_{2}+CaO \to CaCO_{3}$
