Chemistry · Chapter 13

Push the Piston, Watch It Answer

A real N2O4 and NO2 equilibrium runs in this piston. Add gas, squeeze it, heat it or drop in a catalyst, then watch the mixture settle into a new balance.

The piston brown tint tracks the NO2

A piston holding N2O4 and NO2 at equilibrium

Running at equilibrium. Pick a stress and watch what happens.

Amounts against time dashed lines mark each stress

Concentrations against time with stress markers

Each stress makes the lines jump or bend. Then they flatten again at new values. That flattening is the system reaching a fresh equilibrium, which is Le Chatelier's principle happening in front of you.

Rates against time they split apart, then rejoin

Forward and reverse rates with stress markers

Live numbers

0.00460Q at equilibrium
K at this temperature = 0.00460
[N2O4] = 0.0430 M   [NO2] = 0.0141 M
Volume = 1.00 L   Temperature = 298 K
Forward rate = 0.0128 mol/L·s
Reverse rate = 0.0128 mol/L·s
Total pressure = 1.40 atm
Share that is NO2 = 24.7%

Apply a stress

Change an amount
Change the volume
Change the temperature
Tricks that do not shift it
Speed

Why it moved that way

The reaction
heat + N2O4 ⇌ 2 NO2
One mole of gas on the left, two on the right, and the forward step takes in heat. Those two facts explain every answer this piston gives.

ΔH for the forward step is +57.2 kJ/mol, so warming it raises K. Squeezing favors the N2O4 side, which holds fewer gas molecules.