Equilibrium and LeChatelier's Principle - Background

LeChatelier's Principle - Observable Changes



In some instances the direction of an equilibrium shift can be readily observed. Our example system is one of those instances.

heat + H2 + I2 2 HI

Hydrogen and hydrogen iodide are both colorless gases. Iodine vapor is a deep purple color. While using the intensity of the purple color to determine the absolute amount of iodine present is difficult, determining relative concentrations is straight forward. Consider the two flasks below. While we cannot tell exactly how much iodine vapor is present in each flask, there is obviously more iodine in the flask on the right.



If we reexamine some of the stresses we have previously applied to this system we should be able to predict whether the observed color should be darker or lighter. If we predicted that the concentration of iodine increased we should also predict that the flask would appear darker purple. If our prediction said that the concentration of iodine decreased we should also predict that the flask would appear lighter purple.

applied
stress 		direction
of shift 		effect on
[H2] 		effect on
[I2] 		effect on
[HI] 		observed
color change
temperature raised right decreased decreased increased lighter purple
H2 removed left decreased increased decreased darker purple
I2 removed left increased decreased decreased lighter purple


Practice using LeChatelier's Principle by predicting what should happen to the color of the system when (1) hydrogen iodide is added and (2) when iodine is removed. When you are done check your answers.

In addition to changes in the intensity of color we might observe a change in color (from one color to another, or from one shade to another), the formation or disappearance of a precipitate or the formation of a gas depending on the equilibrium system observed.


This completes the background material. Proceed to the prelab.


introduction background prelab experiment postlab