Equilibrium & Industry
Reversible reactions and dynamic equilibrium, and how the Haber process makes ammonia using conditions chosen as a compromise between yield, rate and cost.
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Reactions that go both ways
Most reactions we meet go one way: reactants turn into products and stop. But some reactions are reversible (shown by the ⇌ symbol): the products can react to reform the reactants. In a closed container, a reversible reaction reaches a balance called dynamic equilibrium. This module explores equilibrium and how it is used in industry, in the Haber process that makes ammonia.
The words of equilibrium
The key ideas for reversible reactions:
What is dynamic equilibrium?
What is happening at dynamic equilibrium in a closed container?
- The forward and backward reactions happen at the same rate
- Both reactions have completely stopped
- Only the forward reaction is happening
- All the reactants have been used up
Balance the Haber equation
In the Haber process, nitrogen reacts with hydrogen to make ammonia: N2 + _____ H2 ⇌ _____ NH3. The nitrogen comes from the _____ and the hydrogen from natural gas. Because the reaction is _____, not all of the gas reacts, so the unreacted gases are _____.
The Haber process
The Haber process makes ammonia (NH3), which is used to make fertilisers that help feed the world. It combines nitrogen (from the air) with hydrogen (from natural gas) in a reversible reaction: N2 + 3H2 ⇌ 2NH3 Because it is reversible, only some of the gas turns into ammonia each time, so the unreacted nitrogen and hydrogen are recycled.
The Haber conditions
- Pressure
- Temperature
- Catalyst
- Unreacted gases
- About 200 atmospheres, a high pressure
- About 450 degrees C, a moderate temperature
- An iron catalyst to speed up the reaction
- Are recycled back into the reactor
True about equilibrium?
Select the TWO true statements about a reaction at dynamic equilibrium in a closed container.
- Both the forward and backward reactions are still happening
- The amounts of reactants and products stay constant
- The reaction has stopped completely
- Only products are left
A compromise: yield vs rate
The conditions in the Haber process are chosen as a compromise between getting a good yield and a fast enough rate.
Why 450 degrees C?
Why does the Haber process use a moderate temperature of about 450 degrees C, rather than a low one?
- A low temperature would make the reaction too slow
- A low temperature would give too much ammonia
- Low temperatures are more expensive to reach
- A low temperature would make it explode
Steps of the Haber process
Put the steps of the Haber process into order.
- Get nitrogen from the air and hydrogen from natural gas
- Pass the gases over an iron catalyst at high pressure
- Some of the nitrogen and hydrogen react to form ammonia
- Cool the mixture so the ammonia condenses to a liquid
- Recycle the unreacted nitrogen and hydrogen
Spot the conditions
Tap the TWO conditions actually used in the Haber process.
- About 200 atmospheres pressure
- A temperature of 2000 degrees C
- An iron catalyst
- No catalyst at all
- A very low pressure
Set the conditions
You are running an ammonia plant. Choose the best option each time.
- You want the reaction to go fast enough to be useful. What helps the rate?
- A very low temperature would give more ammonia. Why is it not used?
- Some nitrogen and hydrogen leave the reactor without reacting. What is done with them?
Explain the compromise
The Haber process uses a pressure of about 200 atmospheres, a temperature of about 450 degrees C and an iron catalyst. Explain why these conditions are a compromise. Write in clear, full sentences.
- State the pressure, temperature and catalyst used
- Explain why a moderate temperature is used, not a low one (yield vs rate)
- Explain why a catalyst is used
- Explain what happens to the unreacted gases