Which of the following graphs correctly represents the variation of thermodynamic properties of Haber's process?
- [figure]
- [figure]
- [figure]
- [figure]
Answer
(1) [figure]
Solution
Haber's process: N2(g) + 3H2(g) 2NH3(g)
rH -92 kJ mol-1 (negative, nearly independent of T)
rS -198 J K-1 mol-1 (4 mol gas 2 mol gas; negative, nearly constant)
- H T = + 92000T: positive ( +300 J K-1 mol-1 at 300 K) and falls as T rises.
G = H - T S
- G T = - H T + S
- G T 300 - 200 = +100 at 300 K; it falls with T and becomes negative above T = H S 465 K.
Graph (1) shows exactly this: - H /T from about +300 falling, - G /T from about +100 falling through zero, H about −100 and S about −200, both nearly constant.
(2) plots H /T as negative and - H as a falling curve; (3) puts - G /T above - H /T, which needs S > 0; (4) shows S and H positive. All three are wrong.
Answer: (1)