Classify each of the following reactions as one of the four possible types summa
ID: 882262 • Letter: C
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
Explanation / Answer
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.