LeChâtelier’s Principle– Tutorial based on video shown in class                                  Std 9a



                        N2O4     -->     2 NO2



                 colorless                 dark brown


Observations from the video


1.  TEMPERATURE – We first saw how temperature affected the equilibrium of the reaction.  Cold water made the gas mixture turn a lighter color and hot water made it turn a darker color.


2.  PRESSURE / VOLUME   Next we saw how pressure being lowered (pulling out the plunger of the syringe) and then being increased (shoving the syringe plunger in) changed the equilibrium. 


What we had to watch for in this video was not colorless or dark brown, but lighter brown or darker brown color.  This is because a lighter color indicates

                                    SOME but NOT ALL

of the NO2  (dark brown)  had changed to N2O4 .  So lighter color means that  more N2O2 is present and less NO2 is present.   Another wonderful thing that we observed was that as NO2 was formed (darker brown mixture) that the amount of gas in the plunger increased as we poured hot water into the graduated cylinder holding the syringe.  This was caused by formation of more gas in the closed container as the

N2O4     <------>     2 NO2 . 

Remember that at any temperature the number of moles of any and all gases determines the volume it occupies at any given temperature. 


Here’s a link you can go to for fun to see a video of the second demo shown in the video we saw in class on LeChâtelier’s Principle with a nice bit of background information that will help you understand how it works:

                        Demonstration of LeChâtelier’s Principle using nitrogen dioxide . 

            The plunger reduces the volume and momentarily system of gases turns dark brown, but as the equilibrium shifts the system turns lighter as more of the dark brown NO2 converts to the colorless N2O4 , which relieves the pressure.  The change is subtle and must be watched for carefully


Here’s 2 links for the chromate – dichromate equilibrium in water, first a demo:


            2nd  is a great video of a lab with a good explanation as part of the video: