======Nitrogen Dioxide and Dinitrogen Tetroxide Equilibrium======


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**Alternative titles:** Brown to Colorless Gas Equilibrium Demonstration
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====Summary====                   
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====Procedure====  
  - Generate nitrogen dioxide (NO2) ([[demonstrations:making_nitrogen_dioxide|Link]]).
  - Place a sample of nitrogen dioxide gas in a sealed gas syringe to create a closed system.  
  - Adjust the volume of the syringe by pushing or pulling the plunger to change pressure and concentration.  
  - Observe the color changes as the system shifts toward either more brown NO2 (at lower pressures/greater volume) or more colorless N2O4 (at higher pressures/lower volume).  
  - Place the syringe in warm water to increase temperature, and observe the mixture turning browner as more NO2 forms (endothermic direction).  
  - Place the syringe in ice water to decrease temperature, and observe the mixture turning paler as more N2O4 forms (exothermic direction).



====Links==== 
Volume Effect on Equilibrium - LeChatelier's Principle Lab Extension - North Carolina School of Science and Mathematics: {{youtube>pnU7ogsgUW8?}}\\

Nitrogen Dioxide and Dinitrogen Tetroxide Equilibrium - HSC Chemistry - Science Ready: {{youtube>TdlwI4hlEPs?}}\\

====Variations==== 

  * Shine a light through the gas mixture to make color intensity changes more visible to larger groups.  
  * Compare with other equilibrium systems such as iron(III) thiocyanate to reinforce the concept of Le Chatelier’s principle.



====Safety Precautions====
  * Perform the experiment in a fume hood, as nitrogen dioxide is toxic.  
  * Ensure the syringe or container is securely sealed to prevent gas leaks.  
  * Wear safety glasses and gloves.  
  * Handle warm and ice baths carefully to avoid burns or frostbite.




====Questions to Consider====
  * Why does the color intensity change when the pressure is increased? (Because equilibrium shifts toward N2O4, which has fewer moles of gas, making the mixture less brown.)  
  * What does the observed color change reveal about the enthalpy change of the reaction? (Formation of NO2 is endothermic; formation of N2O4 is exothermic.)  
  * How does this experiment demonstrate Le Chatelier’s principle? (The system shifts to counteract changes in pressure, volume, or temperature, visible through the color changes.)  
  * Why is nitrogen dioxide brown while dinitrogen tetroxide is colorless? (NO2 absorbs visible light due to its electronic structure, while N2O4 does not significantly absorb in the visible spectrum.)