| | ΔfG°/kJmol–1
at 298 K |
| SO2(g) | -300 |
| H2S(g) | -33 |
| H2O(g) | -229 |
The reaction below is used to remove SO
2 from stack gases. It is also the origin of deposits of sulfur in thermal areas like Rotorua in New Zealand.
Calculate
K at 25 °C from the given data.
R = 8.314 J K
–1 mol
–1SO
2(
g) + 2H
2S(
g) → 3S(
s) + 2H
2O(
g)
To solve the problem:
Step 1: ΔG° = ΣΔfG°(products) – ΣΔfG°(reactants)
Step 2: ΔG° = -RTln K
|
ΔG° = –458 kJ mol–1 – (–366 kJ mol–1) = –92 kJ mol–1
–92,000 J mol–1 = –8.314 J mol–1 K–1 × 298 K × ln K
ln K = 37.1
K = 1.3 × 1016
Step 1:
Step 2:
- Conversion to joules is critical in step 2 because of the units of R
- Because of the logarithmic relationship between standard free energy change and the value of the equilibrium constant, that fairly small negative values of ΔG correspond to large equilibrium constants.