Question

Classify each of the following reactions as one of the four possible types summarized in Table 19.3: (i) spontanous at all temperatures; (ii) not spontaneous at any temperature; (iii) spontaneous at low T but not spontaneous at high T ; (iv) spontaneous at high T but not spontaneous at low T . (a) N 2 1 g 2 + 3 F 2 1 g 2 ¡ 2 NF 3 1 g 2 ∆ H ° = - 249 kJ; ∆ S ° = - 278 J > K

(b) N 2 1 g 2 + 3 Cl 2 1 g 2 ¡ 2 NCl 3 1 g 2 ∆ H ° = 460 kJ; ∆ S ° = - 275 J > K

(c) N 2 F 4 1 g 2 ¡ 2 NF 2 1 g 2 ∆ H ° = 85 kJ; ∆ S ° = 198 J > K

Answer 1

a)   answer : (iii) spontaneous at low T but not spontaneous at high T

N2 + 3F2 --------------------> 2NF3 ∆ H ° = - 249 kJ; ∆ S ° = - 278 J / K = -0.278 kJ / K

∆ G ° = ∆ H ° - T ∆ S °

          = -249 - T (-0.278)

          = -249 + 0.278 T

for spontaneous ∆ G ° should be negative. for that in this reaction T (temperature should be low)

spontaneous at low T

b)   answer : (ii) not spontaneous at any temperature

N2 + 3 Cl2 -------------------> 2 NCl3 , ∆ H ° = 460 kJ; ∆ S ° = - 275 J / K = - 0.275 kJ/K

∆ G ° = ∆ H ° - T ∆ S °

           = 460   - T (-0.275)

           = 460 + 0.275 T

this reaction never spontaneous because we get ∆ G ° positive at any temperature.

c) answer : (iv) spontaneous at high T but not spontaneous at low T

N2F4 ----------------------> 2 NF2 , ∆ H ° = 85 kJ; ∆ S ° = 198 J > K = 0.198 kJ /k

∆ G ° = ∆ H ° - T ∆ S °

          = 85 - T (0.198)

at high temperature ∆ G ° negative and reaction is spontaneous.