The reaction for which ∆ng≠0,∆H≠∆U because ∆H=∆U+∆ngRT ∆ng=nP−nR (where, np and nR represent the number of moles of gaseous products and reactants respectively.) (a) C(graphite) +O2(g)⟶CO2(g); ∆ng=np−nR=1−1=0 (b) C2H4(g)+H2(g)⟶C2H6(g) ∆ng=1−2=−1 (c) 2C( graphite )+H2(g)⟶C2H2(g); ∆ng=1−1=0
(d) H2(g)+I2(g)⟶2HI(g);∆ng=2−2=0
(e) N2(g)+O2(g)⟶2NO(g) ∆ng=2−2=0 Thus, for the reaction given in option (b), ∆H≠∆U