A1258607617quwVY

1
Equilibrium Isotope Quiz, Topic 54 The
equilibrium isotope effect can also be calculated
from thermodynamic data, if it is available
(which it often isnt). For the two isotopically
related reactions at 600.0 K KH F (g)
H2 (g) ltgt HF (g) H (g) KD F (g)
HD (g) ltgt HF (g) D (g) calculate the
equilibrium isotope effect KH / KD ? from
the following thermodynamic data
?Hf, 298 K (kJ/mol) S298 K (J/ (mol K))
H (g) 217.998 114.717
D (g) 221.72 123.35
H2 (g) 130.680
HD (g) 0.32 143.80
2
The equilibrium isotope effect as defined by
KH/KD is equivalent to the equilibrium constant
for the sum of the following two reactions
KH F (g) H2 (g) ltgt HF (g) H
(g) 1/ KD HF (g) D (g) ltgt F (g)
HD (g) ------------------------------------------
----------------- H2 (g) D (g) ltgt HD
(g) H (g) KH / KD e - ?G 600.0 K / R
T ?G600.0 K ?H298 K - (600.0 K) ?S298
K ?H298 K ( ?Hf, HD, 298 K ?Hf, H,
298 K ) - (?Hf, D, 298 K ) 0.32 kJ/mol
217.988 kJ/mol - 221.72 kJ/mol - 3.41
kJ/mol ?S298 K ( SHD, 298 K SH, 298 K
) - (SD, 298 K SH2, 298 K ) 143.80
J/mol K 114.717 J/mol K - 130.680
J/mol K - 123.35 J/mol K 4.49 J/(mol
K) ?G600.0 K (- 3.41 kJ/mol) - (600.0 K)
(0.00449 kJ/mol) - 6.10 kJ/mol KH / KD
e ( - 6.10 kJ/mol ) / (0.008314 kJ/mol K)
(600.0 K) 3.4