Practice Problem 11

Use values of  H^o and S^o for the following reaction at 25°C to estimate the equilibrium constant for this reaction at the temperature of boiling water (100°C), ice(0°C), a dry ice-acetone bath (-78°C), and liquid nitrogen (-196°C):

2 NO₂(g)  N₂O₄(g)

Answer

100°C:    K_p = 0.067

0°C:        K_p = 58

-78°C:    K_p = 1.4 x 10⁶

-196°C:   K_p   = 3.8 x 10²⁹

At 100°C, the unfavorable entropy term is relatively important, and the equilibrium lies on the side of NO₂.  As the reaction is cooled, the entropy term becomes less important, and the equilibrium shifts toward N₂O₄.   At the temperature of liquid nitrogen all of the NO₂ condenses to form N₂O₄.