Chemistry · Chapter 13

Watch Two Rates Meet and Hold

A sealed tube of N2O4 turns brown as it makes NO2, then stops changing. Press play and watch the forward and reverse rates close in on each other.

The sealed tube big teal pairs are N2O4, small gold dots are NO2

Molecules in a sealed tube at equilibrium

Each teal pair that splits apart hands you two gold dots. The brown tint tracks how much NO2 is present. Once the tint stops deepening, the tube has reached equilibrium, but the dots are still swapping the whole time.

Rates against time they meet, then run together

Forward and reverse rates against time

The gold line is the forward rate, which starts high and falls as N2O4 is used up. The teal line is the reverse rate, which starts at zero and climbs. Equilibrium is the moment they touch.

Amounts against time both lines go flat

Concentrations against time

Live numbers

0.0000Q right now forward is winning
Equilibrium constant K = kf ÷ kr = 0.00460
[N2O4] = 0.0500 M
[NO2] = 0.0000 M
Forward rate = kf[N2O4] = 0.01495 mol/L·s
Reverse rate = kr[NO2]2 = 0.00000 mol/L·s
Gap between the rates = 100%
Model clock = 0.0 s

Run it

Speed
Start the tube with

Both starting points land on the same final mixture. That is one of the clearest signs of a true equilibrium.

Rate constants

0.299 per second
65 L/mol·s

Raise kf and K goes up, so the tube ends browner. Raise kr and K goes down. Both constants change how fast you get there.

What you are seeing
Only the forward step can run
There is no NO2 yet, so the reverse rate is zero. The forward rate has nothing to balance it.