Alkynes are reduced to cis-alkenes on reacting with H2 in presence of Lindlar's catalyst. So, P is cis-pent-2-ene. This addition of H2 is syn-addition. CH3−C≡C−CH2−CH3
Alkynes are reduced to trans-alkenes on treating with Na in liquid NH3. Here, anti-addition of hydrogen takes place. So, Q is trans-2-pentene. CH3−C≡C−CH2−CH3