Write the following reactions:
$(1)$ Reaction of acetylene with $HCN$.
$(2)$ Hydrogenation of ethyne in the presence of Lindlar catalyst $(Pd, BaSO_4)$.

(N/A) $(1)$ Reaction of acetylene with $HCN$:
$HC \equiv CH + HCN \xrightarrow{Ba(CN)_2} CH_2 = CHCN$ (Acrylonitrile or Vinyl cyanide)
$(2)$ Hydrogenation of ethyne in the presence of Lindlar catalyst:
$HC \equiv CH + H_2 \xrightarrow{Pd, BaSO_4} CH_2 = CH_2$ (Ethene)