Question

Calculate S_f˚ at 298 K of ; (i) NaCl(s), (ii) NH₄ Cl(s) & (iii) diamond. The values of S˚ of Na, Cl₂, NaCl, NH₄ Cl, N₂, H₂, diamond & graphite are 51, 223, 72, 95, 192, 131, 2.43 & 5.69 JK^-1mol^-1 respectively.

Answer 1

2Na + Cl₂ → 2NaCl 

∆S = 2 × S_NaCl –S_Cl2 – 2 × S_Na

= 2 × 72 – 223 – 2 × 51

∆S = –181/2 = – 90.5 

N₂ +4H₂ + Cl₂ → 2NH₄ Cl

= 2.43 – 5.69 = – 3.26