- The first tube is placed in a hot water bath.
- The second is placed in ice-water.
- The third is placed in a flask filled with dry-ice/acetone slush.
- The fourth is placed in liquid nitrogen.
Compare the glass tubes.
Chemical Concept Demonstrated: Temperature's effect on equilibrium
Demonstration:
Four glass tubes are sealed with NO2 gas.
Compare the glass tubes. |
|
Observations:
NO2 is brown and N2O4 is
colorless. The intensity of the brown color decreases as the temperature decreases.
Therefore, a decrease in temperature yields and increase in N2O4.
Equilibrium is shifted to the N2O4 side upon a decrease in
temperature.
Explanation (including important chemical equations):
2 NO2 (g) <=> N2O4 (g)
The standard enthalpy (delta H° = -57.2 kJ) and the entropy (delta S° = -175.83 kJ) of reaction can be calculated from the follow standard-state enthalpies of formation and standard-state entropies.
| Compound | delta H°f (kJ/mol) | delta S° (J/mol-K) |
| NO2 | 33.18 | 240.06 |
| N2O4 | 9.16 | 304.29 |
We can then invoke the assumption that the temperature dependence of delta H° for this reaction is small to estimate the equilibrium constant at various temperatures.
| Temperature | delta G° (kJ) | Kp |
| 100 °C | 8.4 | 0.066 |
| 0 °C | -9.2 | 58 |
| -78 °C | -22.9 | 1.3 x 106 |
| -196 °C | -43.6 | 3.7 x 1029 |