Le Chatelier's Principle
When a system at dynamic equilibrium is subjected to a change in temperature, pressure, or concentration, the position of equilibrium shifts to oppose (counteract) that change.
N2O4(g) ⇋ 2 NO2(g) ΔH = +57 kJ mol⁻¹

Equilibrium constant

Shifting…
Kc = [NO2]² / [N2O4]
= (0.28)² / (0.52)
Q = 0.15 mol dm⁻³
Kc at 300 K = 0.15 mol dm⁻³
Kc changes only with temperature. Pressure and concentration change the position of equilibrium but not Kc.

What just happened

Heat the tube, drag the piston, or inject gas. Watch the colour shade shift, then settle as a new dynamic equilibrium is reached.
300 K

Temperature changes Kc

The forward reaction is endothermic (ΔH = +57 kJ mol⁻¹). Heating shifts the position of equilibrium right (browner) and raises Kc. Cooling shifts it left (paler) and lowers Kc. Temperature is the only change that alters Kc.

Pressure / volume

There are 2 moles of gas on the right, 1 on the left. Compressing deepens the colour at once (higher concentration), then the position shifts left toward fewer moles. Kc is unchanged — only Q is disturbed, then returns to Kc.

Concentration

Inject NO₂ and the system opposes the increase, shifting left to remove it. Inject N₂O₄ and it shifts right. Either way the system moves until Q returns to the same Kc for that temperature.

Q versus Kc

The expression [NO₂]²/[N₂O₄] is the reaction quotient Q at any instant. After a disturbance Q ≠ Kc, so the reaction shifts. When Q = Kc again, the system is at dynamic equilibrium and the colour stops changing.