Explain the preparation of alkenes by the hydrogenation of alkynes with examples.

(N/A) Alkenes can be prepared from alkynes by partial hydrogenation using specific catalysts:
$(a)$ Formation of $cis$-alkene:
Alkynes on partial reduction with dihydrogen in the presence of Lindlar's catalyst (palladised charcoal partially deactivated with poisons like sulphur compounds or quinoline) give $cis$-alkenes.
General reaction: $RC \equiv CR' + H_2 \xrightarrow{Pd/C, \text{poison}} \text{cis-alkene}$
Example: $CH_3-C \equiv C-CH_3 + H_2 \xrightarrow{Pd/C, \text{poison}} \text{cis-but-2-ene}$
$(b)$ Formation of $trans$-alkene (Birch Reduction):
Alkynes on reduction with sodium in liquid ammonia $(Na/NH_3(l))$ give $trans$-alkenes.
General reaction: $RC \equiv CR' + H_2 \xrightarrow{Na/NH_3(l)} \text{trans-alkene}$
Example: $CH_3-C \equiv C-CH_3 + H_2 \xrightarrow{Na/NH_3(l)} \text{trans-but-2-ene}$