Equilibrium & Le Chatelier Lab
Colourless N₂O₄ and brown NO₂ turn into each other all the time. Disturb the flask by adding gas, heating it or squeezing it, and watch the reaction push back.
A dynamic balance
In N₂O₄(g) ⇌ 2NO₂(g), both reactions keep going. At equilibrium they run at the same rate, so the amounts stop changing even though molecules are still reacting. The reaction quotient Q = [NO₂]² / [N₂O₄] then equals the equilibrium constant Kc, which depends only on temperature. If Q < K the reaction moves forward (right); if Q > K it moves backward (left).
Le Chatelier's principle
Disturb a system at equilibrium and it shifts to partly undo the change. Add NO₂ and some of it turns back into N₂O₄. Squeeze the flask and it favours the side with fewer gas molecules. The forward reaction is endothermic (ΔH = +57.2 kJ/mol), so heating it favours brown NO₂: K itself gets bigger.
Wait for the flask to settle, then disturb it.
Kc = 4.6 × 10⁻³ mol/L at 298 K, changed with temperature by the van 't Hoff equation. Time is sped up.