10.07.1 The Haber Process
Syllabus
The Haber process is used to manufacture Ammonia, which can be used to produce Nitrogen-based fertilisers.
The raw materials for the Haber process are Nitrogen and Hydrogen.
Students should be able to recall a source for the Nitrogen and a source for the Hydrogen used in the Haber process.
The purified gases are passed over a catalyst of Iron at a high temperature (about 450°C) and a high pressure (about 200 atmospheres).
Some of the Hydrogen and Nitrogen reacts to form Ammonia.
The reaction is reversible so some Ammonia breaks down into Nitrogen & Hydrogen:
Nitrogen + Hydrogen ⇌ Ammonia
The Ammonia is cooled, liquefies & is removed. Remaining Hydrogen & Nitrogen is recycled.
(HT only) Students should be able to:
interpret graphs of reaction conditions versus rate
apply the principles of dynamic equilibrium to the Haber process
explain the trade-off between rate of production and position of equilibrium
explain how the conditions used for the Haber process are related to the availability and cost of raw materials & energy supplies, control of equilibrium position & rate.
What does this mean?
Raw Materials - Nitrogen & Hydrogen
The Haber Process makes Ammonia (NH3) from Nitrogen (N2) and Hydrogen (H2)
Nitrogen makes up 78% of dry air. If the air is cooled then the different gases will condense at different temperatures.
Oxygen will condense at -183oC, Nitrogen will condense at -196oC.
Separating substances by their boiling/condensing points is Fractional Distillation.
There is no Hydrogen in the air.
But it can be produced during catalytic cracking of Crude oil, or from Natural Gas
The Reactor
You won't have to draw this reactor or label it.
But you might be asked what goes on in it and what the operating temperature and pressure is, along with what is used as a catalyst (see below)
Hydrogen and Nitrogen are compressed to 200 times atmospheric pressure and heated to around 450oC.
The mixed gases then pass through beds of Iron powder which catalyses the reaction.
N2 + 3H2 ⇌ 2NH3
The ⇌ symbol means that this reaction is reversible - so it's not possible to turn all the Nitrogen and Hydrogen into Ammonia.
However, the Ammonia can be separated from unreacted Nitrogen and Hydrogen by cooling.
Ammonia condenses (turns into a liquid) at -33oC and is piped away.
The unreacted gases are then pumped back into the reactor (recycled) and given another chance to react.
So, it's not that important that the yield is only 20 to 30% as eventually all the reactants are used.
Animation
Click the icon below to watch an animation about the historical background to the discovery of the Haber process
Videos
Choosing the right conditions.
Manufacturers need to pick conditions that provide a good yield (lots of product) and a good rate of reaction (make the product quickly.
Catalyst
Adding a catalyst (in this case Iron) increases the rate of reaction by providing a different reaction route with a lower Activation Energy - (see Rates of Reaction)
But catalysts do not change the yield of a reaction - at 450°C and 200 atmospheres the Haber Process might give a 25% yield.
Adding the catalyst still gives a 25% yield but does so faster.
But changing Pressures and Temperatures changes both rate and yield.
Pressure.
Nitrogen + Hydrogen ⇌ Ammonia
N2 + 3H2 ⇌ 2NH3
Under Pressure
(pushing down you, pushing down on me.. etc)Raising pressures always increases rates of reaction of gases because it pushes the gas particles closer together where they collide more frequently.
But raising pressure may raise or lower yield depending on the reaction.
In the reaction above there are 4 moles of gas on the left, and only two moles of gas on the right.
So a high yield (lots of Ammonia, less Nitrogen and Hydrogen) provides a lower pressure than a low yield (less Ammonia, lots of Nitrogen and Hydrogen)
Le Chatalier's Principle says that the equilibrium will move to oppose any changes you make to the conditions.
If we raise the pressure, the equilibrium will move right (towards the product) to lower the pressure.
So using 200 atmospheres is good for yield and rate.
A higher pressure would be even better but the cost of building a safe factory and the energy costs of operating the pumps needed to raise pressure higher than 200 atm means that it is not worth it.
Temperature
Nitrogen + Hydrogen ⇌ Ammonia (ΔH = −92.22 kJ·mol−1)
N2 + 3H2 ⇌ 2NH3
Raising temperatures always increases rates of reaction of gases because it allows the gas particles to move faster causing them to collide more frequently.
But raising temperatures may raise or lower yield depending on the reaction.
ΔH = −92.22 kJ·mol−1 means that the forward reaction is exothermic.
The backward reaction will be endothermic ΔH = +92.22 kJ·mol−1
Le Chatalier's principle says that raising the temperature will make the equilibrium move to lower it again.
So a higher temperature favours the backwards (endothermic) reaction, lowering the yield.
So 450oC is chosen because it is hot enough to give a good rate of reaction but not so hot that the yield is too low.
It is the Optimum temperature.
At higher temperatures the rate is better but the yield is lower.
At lower temperatures the rate is worse but the yield is higher.
Rumours that Le Chatelier was reincarnated as Ned Flanders abound.
The graph shows yields rising as pressure is increased along the x-axis.
Higher pressure forces the equilibrium to the right to make more Ammonia
The five lines show that yield is always lower at higher temperatures.
The forwards reaction is exothermic, so higher temperatures force the equilibrium to the left and less Ammonia is made
Revision Notes and Quiz.
Click the icon below to read revision notes and take the quiz
Videos
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