Ammonia is formed from nitrogen and hydrogen
- The Haber process brings nitrogen and hydrogen together to form ammonia.
- The equation is written with the reversible arrow: N2(g)+3H2(g)⇌2NH3(g)\text{N}_2(\text{g}) + 3\text{H}_2(\text{g}) \rightleftharpoons 2\text{NH}_3(\text{g})N2(g)+3H2(g)⇌2NH3(g)
- One molecule of nitrogen combines with three of hydrogen to give two of ammonia.
- Both reactants are gases, and so is the ammonia formed.
- Ammonia is the starting point for fertiliser manufacture, which is why the process matters.
Key Idea
The double arrow is the whole point of this reaction: the ammonia can break apart again.
The reaction runs in both directions at once
Definition
Reversible reaction
A reaction in which the products can react together to re-form the reactants, shown by the symbol ⇌.
- Nitrogen and hydrogen combine, and at the same time ammonia breaks back down into them.
- The forward reaction forms ammonia and the backward reaction destroys it.
- Once some ammonia has formed, both run simultaneously under the same conditions.
- As ammonia builds up, the backward reaction speeds up.
- As nitrogen and hydrogen are used up, the forward reaction slows down.
Note
A single arrow is the convention for a reaction that goes essentially to completion, which this one does not.
The mixture reaches a dynamic equilibrium
Definition
Dynamic equilibrium
The state of a reversible reaction in a closed system in which the forward and backward reactions happen at the same rate, so the amounts of reactants and products stay constant.
- The two rates eventually become equal, and the amounts then stop changing.
- Both reactions are still running at that point, which is what dynamic means.
- The mixture at equilibrium holds ammonia together with unreacted nitrogen and hydrogen.
- The reactor is sealed while the gases are inside it, which is what allows equilibrium to be reached.
- Reaching equilibrium sooner does not change what the equilibrium mixture holds.
Common Mistake
Equilibrium means unchanging amounts, not equal amounts of reactants and products.
No single pass converts all of the reactants
- Because the backward reaction is always running, some nitrogen and hydrogen remain.
- The proportion converted in one pass is therefore well below one hundred percent.
- Ammonia is separated by cooling the mixture until it condenses to a liquid.
- The nitrogen and hydrogen stay as gases and are recycled into the reactor.
- Recycling means the raw materials are eventually used up even though each pass converts only part.
Example
- Leaving the reactor: ammonia mixed with unreacted nitrogen and hydrogen.
- After cooling: liquid ammonia run off, the two gases returned to the reactor.
A reversible reaction is still worth running
- A low conversion per pass would waste raw material if the gases were simply discarded.
- Recycling removes that objection, so the overall use of raw material is efficient.
- The nitrogen is drawn from the air, which costs nothing but the energy to separate it.
- Running the reaction continuously rather than in batches keeps the plant working at full output.
- The scale of world demand for fertiliser is what makes the process worth the energy it uses.
Self review
- Write the equation for the Haber process, including the correct arrow.
- What does the reversible arrow tell you about ammonia in the reactor?
- What happens to the backward rate as ammonia builds up?
- What does the mixture at equilibrium contain?
- How are the unreacted gases dealt with?
