CHM370 Problem Set 9 Due Nov 17, 2008
Equilibrium Energetics
1. Calculate/Predict the equilibrium constant for the reaction
H2(g) + I2(g) 2HI(g)
a) The .log file of Opt/Freq calculations for H2, I2, and HI are given. From these output
files, determine qHI, qH2, qI2. Use Q with zero point energy correction; Find the keyword
“Total V=0”
b) From the Gaussian partition function output, calculate the equilibrium constant (Kc).
Note: Gaussian program has already calculated the electronic energy and zero-point
correction. Look for the keyword “Sum of electronic and thermal Free Energies” in your
output file (either .log or .out). This energy corresponds to the E0 in class. See the
diagram below. Keep track of units, the E0 reported in Gaussian output is in hartree,
make sure you convert it so that it is consistent with kT.
c) Is the equilibrium dominantly driven by entropy or enthalpy of the reaction? Why?
d) Calculate Kc from Hess’s Law using tabulated values with the following entropy of
formation (S0); H2 = 130.684, I2=260.69, and HI=206.59 JK-1mol-1 (Refer to the textbook
section 19-11) Note: G= -RTlnKc.
e) Compare Kc and G obtained from b) and c). Are these methods comparable? Explain
the effect of G discrepancy to the estimated Kc.
.
0
Sum of electronic and zero-point Energies (E0)
H++ H+ + e-+ e-
H2