Chemical equilibrium – N₂O₄ ⇌ 2NO₂ shift

What it shows

Dinitrogen tetroxide and nitrogen dioxide form a dynamic equilibrium, N₂O₄ ⇌ 2NO₂, whose forward reaction is endothermic. Because NO₂ is reddish-brown and N₂O₄ is colorless, the color of the gas shows which way the equilibrium moves. The simulation applies Le Chatelier's principle quantitatively: heating raises K and favors NO₂; compression favors N₂O₄, the side with fewer gas molecules; adding NO₂ makes Q larger than K, so the reverse reaction runs until Q = K. Only temperature changes K; pressure and concentration only move the position of equilibrium.

How to use

Let the system settle, then change one factor at a time while watching both the flask and the concentration graph. Move Temperature up and down and note how the color and K respond. Reduce the Flask volume: the color first darkens because the gas is compressed, then fades a little as the system shifts toward N₂O₄. Press Add NO₂ and ask students to predict the new equilibrium before the graph levels out.

Parameters you can change

• Temperature 0–120 °C
• Flask volume 0.5–4 L
• Initial amount of N₂O₄ 0.02–0.4 mol
• Show concentration graph

Questions to explore

1. What happens to the color of the flask and to the value of K when the temperature is lowered, and why?
2. Why does the gas darken immediately after compression and then become slightly paler?
3. After NO₂ is added, is the new equilibrium concentration of NO₂ higher or lower than before the addition?