Question

Consider the following reaction:
2NO(g)+O2(g)→2NO2(g)
Estimate ΔG∘ for this reaction at each of the following temperatures and predict whether or not the reaction will be spontaneous. (Assume that ΔH∘ and ΔS∘ do not change too much within the give temperature range.)

A)298 K

B)735 K

C)855 K

Answer 1

A) at 298K

   calculate standard G^o

G^o = Gproducts - Greactants

          =(2x G_NO2 - 2 xG_NO + G_O2 )

             = 2 x 51.2 - (2 x 86.6 + 0)

             = -70.8 kJ/mol

G^o value negative so at 298K reaction is spontaneous

B) at 735 K

H^o = Hproducts - Hreactants

          = (2x H_NO2 - 2 xH_NO + H_O2 )

= 2 x 33.2 - (2 x 90.2 + 0)

          = - 114 kJ/mol

S^o = Sproducts - Sreactants

          = (2x S_NO2 - 2 xS_NO + S_O2 )

= 2 x 240 - (2 x 210.7 + 205)

          = -146.4 J/mol

G^o = H^o - TS^o

        = -114 - 735 x (-146.4 x 10^-3)

         = - 6.396 kJ/ mol

G^o value negative so at 735K reaction is spontaneous

C) at 855K

G^o = H^o - TS^o

        = -114 - 855 x (-146.4 x 10^-3)

         = 11.17 kJ/ mol

G^o value positivetive so at 855K reaction is non-spontaneous