Chemistry
A sealed flask reaches a colour that doesn't change. Does that mean the reaction has stopped? Drag the piston in the diagram below to compress the Haber process vessel, and click the temperature toggle — watch the equilibrium composition shift.
Before you read on: at equilibrium, how many molecules react each second?
A — Zero · B — A few · C — Billions
The answer is C. The colour stops changing because the forward and reverse rates are equal — not because anything has stopped. Billions of molecules react every second; the system sits at a balance point, not at rest.
Most common mistake
"Once equilibrium is reached, the reaction stops." Students write this in exams. The rate of the forward reaction equals the rate of the reverse reaction — neither rate is zero.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g), ΔH = −92 kJ/mol. You may change exactly one variable — drag the piston (pressure) or toggle the temperature. Which gives the most NH₃?
Compressing the vessel increases collision frequency and favours the side with fewer gas molecules — 2 moles of NH₃ versus 4 moles of N₂ + H₂. The system shifts toward NH₃ not because it "wants" balance, but because that side relieves the imposed pressure more effectively. This is arithmetic, not intention.
Raising temperature speeds up both the forward and reverse reactions — but for this exothermic reaction, it speeds up the reverse reaction more. Equilibrium shifts left, and yield falls even though the system reaches equilibrium faster. Industrial Haber plants run at 400–500 °C and 150–300 atm: a compromise between yield and a workable reaction rate, with NH₃ continuously removed to pull the equilibrium forward.
| Change | NH₃ yield | Rate |
|---|---|---|
| ↑ Pressure | ✓ Higher (fewer gas moles) | ✓ Faster |
| ↑ Temperature | ✗ Lower (exothermic) | ✓ Much faster |
| Remove NH₃ | ✓ Higher (Le Châtelier) | — no effect |
| Catalyst | — no effect on K | ✓ Faster |
5 questions · Dynamic equilibrium, Le Châtelier, the Haber process.
Question 1
At dynamic equilibrium, the rate of the forward reaction is:
Question 2
Increasing pressure on N₂(g) + 3H₂(g) ⇌ 2NH₃(g) shifts equilibrium:
Question 3
The Haber process is exothermic. Raising temperature:
Question 4
A catalyst added to a reaction at equilibrium:
Question 5
"The system wants to restore equilibrium" is best replaced with: