The Haber process conditions are a compromise between yield and rate
Definition
Reversible reaction
A reaction in which the products can react to form the original reactants.
Definition
Equilibrium position
The relative amounts of reactants and products present when a reversible reaction reaches equilibrium in a closed system.
Definition
Equilibrium yield
The amount or percentage of a product present when a reaction mixture has reached equilibrium under stated conditions.
- The Haber reaction, N2+3H2⇌2NH3\text{N}_2 + 3\text{H}_2 \rightleftharpoons 2\text{NH}_3N2+3H2⇌2NH3, is reversible and reaches a dynamic equilibrium in a closed system.
- The forward reaction, which makes ammonia, is exothermic.
- The chosen conditions balance a good equilibrium yield against a fast enough rate and a sensible cost.
Key Idea
- No single set of conditions gives both the highest yield and the fastest rate.
- The real conditions are a compromise that produces a reasonable amount of ammonia quickly and cheaply.
Pressure: a high pressure increases the yield
- The forward reaction turns 4 molecules of gas into 2 molecules of gas.
- Increasing the pressure shifts the equilibrium position towards the side with fewer gas molecules, which is the ammonia, so the yield rises.
- A higher pressure also speeds up the rate, because the particles are closer together and collide more often.
- Very high pressures need thick, expensive pipes and are dangerous, so about 200 atmospheres is used as a compromise.
Example
- On a graph of yield against pressure, the yield of ammonia rises as the pressure increases.
- The operating pressure is chosen where the yield is good but the equipment is still safe and affordable.
Temperature: a low temperature increases the yield but slows the reaction
- Because the forward reaction is exothermic, lowering the temperature shifts the equilibrium towards the ammonia, raising the yield.
- A lower temperature also slows the rate, so the ammonia would be made too slowly to be useful.
- About 450 degrees Celsius is a compromise: it gives a lower yield but at a workable rate.
- The unreacted gases are recycled, so a lower yield each pass is acceptable.
Common Mistake
- Do not say a high temperature gives a higher yield for this reaction.
- A lower temperature gives a higher yield but a slower rate, so 450 degrees Celsius is a balance between the two.
The catalyst speeds up the reaction without changing the yield
Definition
Catalyst
A substance that increases reaction rate without being used up by providing an alternative pathway with lower activation energy.
Definition
Activation energy
The minimum energy that reacting particles must have for a collision to result in a reaction.
- The iron catalyst provides a pathway with a lower activation energy, so the reaction is faster.
- The catalyst lets equilibrium be reached more quickly.
- It does not change the equilibrium position, so it does not change the yield of ammonia.
Exam technique
- State clearly that a catalyst changes the rate, not the yield.
- It only helps equilibrium form faster; the amount of ammonia at equilibrium is the same.
Recycling gases and reading yield graphs
Definition
Feedstock
A raw material used as the starting substance for making other chemicals or materials.
- Unreacted nitrogen and hydrogen are recycled as feedstock, which increases the overall amount of ammonia made.
- On a graph of yield against temperature, the yield falls as the temperature rises.
- On a graph of yield against pressure, the yield rises as the pressure increases.
- To explain the conditions, read the yield off the graph and describe the compromise between yield, rate and cost.
Self review
- Why does a high pressure increase the yield of ammonia?
- The forward reaction is exothermic. Why is a low temperature not used, even though it would raise the yield?
- What effect does the iron catalyst have on the yield of ammonia?
- Why are the unreacted gases recycled?
- Describe how the yield of ammonia changes as the temperature is increased.
