Nearly half the nitrogen in your body came from a reaction that never finishes.
Nearly half the nitrogen atoms in your proteins were fixed from the air by the Haber-Bosch process: N2(g) + 3 H2(g) ⇌ 2 NH3(g) + heat. As soon as ammonia forms, collisions between ammonia molecules break some of it apart again. In a closed vessel, the forward and reverse reactions settle at the same rate. Engineers then push on that balance with temperature, pressure and product removal, and make millions of tons of ammonia.
Core Claims
- Dynamic equilibrium. In a closed system, a reversible reaction reaches dynamic equilibrium when the forward and reverse rates are equal and not zero.
- Steady, yet busy. At equilibrium the amounts of reactants and products stay constant, even though both reactions keep running at the particle level.
- Le Chatelier’s principle. When a change disturbs a system at equilibrium, the system shifts to work against the change. The changes are concentration, temperature and pressure.
- Concentration. Adding reactant or removing product makes the forward rate larger than the reverse rate. The system shifts right, toward more products.
- Temperature. Treat heat as a product of an exothermic reaction. Heating shifts the system left. Cooling shifts it right.
- Pressure. Compressing a gas mixture shifts the system toward the side with fewer moles of gas.
Equal rates, constant amounts
Retrieval Checklist
- Describe dynamic equilibrium for the particles and for the whole system.
- Predict the shift when you add a reactant or remove a product.
- Explain how heating or cooling shifts an exothermic equilibrium.
- Explain why a catalyst does not change the amounts at equilibrium.
Running on a molecular treadmill: the rate balance.
Imagine walking forward on a treadmill. You walk at 4.0 miles per hour and the belt moves backward at 4.0 miles per hour. You stay in the same place. A person across the room sees nothing change. Your legs are working hard, though. If you or the belt speeds up, your position shifts at once.
A reversible reaction at dynamic equilibrium works the same way.
At equilibrium the forward and reverse rates are equal, and they are not zero. The amounts stay constant, but they are not always equal.
Thinking equilibrium means the reactions have stopped, or that reactants and products must be equal in amount. Both reactions keep running at the same rate. A mixture at equilibrium can hold 99% products or 99% reactants.
Le Chatelier’s Principle: nature’s chemical pushback.
In 1884 the French chemist Henri Louis Le Chatelier described how a system at equilibrium answers a change. His idea is called Le Chatelier’s Principle.
When a change disturbs a system at equilibrium, the system shifts its reaction rates in the direction that works against the change.
1. Changing a concentration
Add reactant. More reactant means more collisions between reactant molecules. The forward rate rises above the reverse rate, and the system shifts right (→) toward products.
Remove product. With less product around, fewer product molecules collide, so the reverse rate drops. The forward rate is now larger, and the system keeps shifting right (→) to replace the missing product.
2. Changing the temperature
To predict a temperature shift, treat heat as a reactant or a product.
• Exothermic: Reactants ⇌ Products + heat. Heating adds heat, so the system shifts left (←) to use it up. Cooling shifts it right (→).
• Endothermic: Reactants + heat ⇌ Products. Heating shifts it right (→). Cooling shifts it left (←).
Temperature is the only change that alters the value of the equilibrium constant.
3. Changing the pressure or volume of a gas
Compressing the vessel raises the pressure and packs the gas particles closer. The system works against this by shifting toward the side with fewer moles of gas.
In the Haber process, N2(g) + 3 H2(g) is 4 moles of gas and 2 NH3(g) is 2 moles. Raising the pressure shifts the system right (→), toward the 2-mole side.
Thinking the system shifts toward the side that just grew. The system works against the change. Adding reactant shifts it right, toward the products.
The Haber-Bosch dilemma: cold favors yield, heat favors speed.
Here is the reaction that feeds the world:
An engineer who designs an ammonia plant faces a trade-off.
Cold gives more ammonia, slowly
The reaction is exothermic, so cooling shifts the equilibrium right. At room temperature (25 °C) and 1 atmosphere, the mixture holds over 90% ammonia. But the N≡N triple bond (945 kJ/mol) is hard to break. At 25 °C the reaction takes years to reach equilibrium.
Heat and a catalyst give speed, with less ammonia
To reach equilibrium fast, engineers heat the gases to about 450 °C and pass them over an iron catalyst. The catalyst speeds up both directions equally, so it helps the system reach equilibrium sooner. It does not change the amount of ammonia at equilibrium. But at 450 °C and 1 atmosphere, the shift to the left leaves under 1% ammonia.
Engineers win some of it back. They raise the pressure to 200 atmospheres, which shifts the system right, toward fewer moles of gas. At 450 °C that lifts the yield to about 25%. They also cool the gas to liquefy the ammonia and remove it, then recycle the unreacted N2 and H2.
Thinking a catalyst raises the amount of ammonia at equilibrium. A catalyst speeds up both directions equally. The mixture reaches equilibrium sooner, and the equilibrium amount stays the same.
Change one condition at a time.
The chamber starts at equilibrium. Make one change and watch which way the mixture shifts. The other controls stay locked until the forward and reverse rates are equal again.
When you disturb an equilibrium, which way does it move?
The explanation appears after you predict.
- Nitrogen, N₂ (two joined circles)
- Hydrogen, H₂ (two small joined circles)
- Ammonia, NH₃ (a large circle with three small ones)
- Ring: a forward reaction. A green ring is a reverse reaction.
- The moment you made a change
- Press Start over, then press Add N₂ and H₂. Wait for Settled, then press Record. Look for: a shift to the right. NH₃ rises from 10 to 16 molecules.
- Press Start over, then press Remove NH₃. Wait for Settled, then press Record. Look for: NH₃ drops to 5 at once, then climbs back to 7 as the system shifts right.
- Press Start over, then press Heat by 50 °C. Wait for Settled, then press Record. Look for: a shift to the left. NH₃ falls from 10 to 8.
- Press Start over, then press Compress. Wait for Settled, then press Record. Look for: a shift to the right as the piston squeezes the gas. NH₃ rises from 10 to 13.
Use your evidence
Fill the blanks from memory.
Type each blank from memory, then press Check. Press Show answers after your first check if you are stuck.
1. A system reaches chemical equilibrium when the rate of the forward reaction equals the rate of the .
2. At dynamic equilibrium the amounts of reactants and products stay , even though collisions continue in both directions.
3. When you add extra reactant, the system shifts .
4. Compressing a gas mixture shifts the equilibrium toward the side with .
5. In an exothermic reaction, raising the temperature shifts the equilibrium to the .
Two questions about the Haber process.
Each question changes one condition. Write your answer first, then open the key points.
Temperature
Pressure
Write your answer first. Then grade yourself.
Give yourself 1 mark for each idea you wrote down.
Dinitrogen tetroxide (N2O4) and nitrogen dioxide (NO2) reach dynamic equilibrium in a sealed glass syringe:
A student does two experiments with this mixture at 25 °C.
Experiment 1 (temperature): The student puts the sealed syringe in ice water at 0 °C. Within two minutes the dark brown gas turns almost colorless.
Experiment 2 (pressure): The student pushes the plunger in and halves the gas volume from 60 mL to 30 mL. The gas flashes darker brown for one second. Over the next 15 seconds the brown color fades until the gas is lighter than it was at the start.
(a) Use Le Chatelier’s principle and the heat in the equation to explain why the gas turns colorless in Experiment 1. [1 mark]
(b) Explain why the gas flashes darker the instant the plunger moves in Experiment 2. [1 mark]
(c) Explain why the color fades over the next 15 seconds. Use the moles of gas and Le Chatelier’s principle. [2 marks]
- Cooling (1 mark): You say cooling shifts the endothermic reaction left, which makes colorless N2O4.
- Flash (1 mark): You say halving the volume doubles the particles per volume at once, before any shift.
- Moles of gas (1 mark): You say N2O4 is 1 mole of gas and 2 NO2 is 2 moles.
- Shift (1 mark): You say pressure shifts it toward fewer moles, so NO2 becomes N2O4 and the color fades.
Equilibria also set the carbon cycle of the planet. Carbon dioxide from the air dissolves in the oceans: CO2(g) ⇌ CO2(aq) ⇌ H2CO3(aq) ⇌ H+ + HCO3−. Rising CO2 in the air raises the pressure of the gas. Under Le Chatelier’s principle, more CO2 dissolves in seawater. The forward shift releases H+ ions, which acidify the ocean and threaten coral reefs.
Adaptive Competency Practice: HS-PS1-6
Test your mastery of reversible reactions and Le Chatelier shifts one question at a time. The adaptive engine covers equal rates at equilibrium, concentration changes, temperature shifts with heat as a reactant or product, gas compression, and industrial yield.