Haber Process

Section 5: part d) The industrial manufacture of chemicals

-for double award you only need to know the Haber Process, Single Award people, you also need to know the Contact Process for the manufacture of Sulphuric Acid!

5.21 recall that nitrogen from air, and hydrogen from natural gas or the cracking of hydrocarbons, are used in the manufacture of ammonia

So if they ask what are the raw materials used in the Haber process, you know it. It's nitrogen from the air and hydrogen either from natural gas, which is methane (CH₄); or from the cracking of hydrocarbons. (Cracking is in the Crude Oil post.) 

5.22 describe the manufacture of ammonia by the Haber process, including the essential conditions:

i. a temperature of about 450°C

ii. a pressure of about 200 atmospheres

iii. an iron catalyst

Remember these conditions!! And remember that the reaction is reversible. Also, the forwards reactions is exothermic. 

N₂ + 3H₂ ⇌ 2NH₃

So decreasing the temperature would actually increase the yield, however, it is still done at a fairly high temperature to speed up the reaction. It makes the rate of reaction faster so the manufacturers get their ammonia quicker, as they say, time is money. The reaction would be too slow otherwise at low temperatures. It would be useless to have a low temperature and achieve a high yield of ammonia if it's going to take ages. You need the gases to reach equilibrium within the very short time that they will be in contact with the catalyst in the reactor.So 450°C is a compromise, and still produces a reasonably high proportion of ammonia. 

The catalyst does NOT affect the amount of products made. The yield of ammonia stays the same, you just get it faster because it speeds up the reaction by lowering the activation energy needed for the reaction. :) 

Increasing the pressure would favour the forwards reaction which is what is wanted, to get more ammonia. This is because if you look at the balanced equation, there are 4 moles of gas on the reactants side (left) but 2 moles of gas (ammonia) on the right hand side (products). So according to Le Chatelier's principle where you try to remove the change, if you increase pressure, the equlibrium would move to the right hand side to decrease pressure. And the products have less pressure because there are only 2 moles there.

Also, just as an extra, just thought that this will be useful to know and is very logical: :)

Rate considerations
Increasing the pressure brings the molecules closer together. In this particular instance, it will increase their chances of hitting and sticking to the surface of the catalyst where they can react. The higher the pressure the better in terms of the rate of a gas reaction.

Economic considerations
Very high pressures are very expensive to produce on two counts.
You have to build extremely strong pipes and containment vessels to withstand the very high pressure. That increases your capital costs when the plant is built.
High pressures cost a lot to produce and maintain. That means that the running costs of your plant are very high.

The compromise
200 atmospheres is a compromise pressure chosen on economic grounds. If the pressure used is too high, the cost of generating it exceeds the price you can get for the extra ammonia produced.

5.23 understand how the cooling of the reaction mixture liquefies the ammonia produced and allows the unused hydrogen and nitrogen to be recirculated

Separating the ammonia

When the gases leave the reactor they are hot and at a very high pressure. Ammonia is easily liquefied under pressure as long as it isn't too hot, and so the temperature of the mixture is lowered enough for the ammonia to turn to a liquid. The nitrogen and hydrogen remain as gases even under these high pressures, and can be recycled.

Recycling

At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%.

5.24 recall the use of ammonia in the manufacture of nitric acid and fertilisers

So ammonia is used to make nitric acid and fertilisers, as you know from bio, plants need nitrates to grow.

Just in case you want to know, here are some properties of ammonia:

• alkaline gas (turns damp red litmus paper blue, which is the test for ammonia!)
• extremely soluble in water--it forms a weak alkali-->ammonia solution
• less dense than air
• colourless gas with pungent odour