Chemistry: The Haber Process

Welcome, readers, to another exciting instalment of "The HSC with AB," where I'll be doing my first post on Chemistry, which is most likely my worst subject bar Extension 2. Today I'll be going through the Haber process, which is the process which synthesises ammonia (NH₃) from nitrogen and hydrogen. I decided to do a bit about the Haber process today because it's one that I'm having some trouble with, and there's a whole section in Chemical Monitoring and Management dedicated to it. It's an important one.

The Haber process was invented by a German chemist, Fritz Haber, in the 1900s (this is a dot point we need to know, and I'll try to make it as exciting as I can.). Before 1914, ammonia was mainly used in fertilisers, and Germany was the only one synthesising it, as Britain was having it imported. Then WWI rolled around, and ammonia needed to be used in explosives. Now, usually Germany would not have been able to produce ammonia, and might not have been able to fight in the war, but thanks to Fritz Haber, it could suddenly generate a lot of the stuff. So now it could fight in WWI.

Clearly, ammonia was very important in world history.

Now our history lesson is out of the way, we can get to the science involved. Ammonia, nowadays, is used in industry, most commonly in the production of fertilisers, plastics, cleaners, and non-ionic detergents, and as such, is pretty important in day-to-day life.

The Haber process can be summarised as N_{2 (g)} + 3H_{2 (g)} ⇌ 2NH_{3 (g)} [ΔH = —92 kJ/mol], but it's a lot more detailed than that (that was an equilbrium symbol, by the way. It doesn't show up brilliantly, but it's the best I can get). There's a lot of stuff to be absorbed here, but I don't think much of it is particularly difficult. So, with that in mind, let's get to the Haber process.

First, we need our nitrogen and our hydrogen. Nitrogen is easy to get: you breathe it. Air is mostly nitrogen, and so all that really needs to be done is getting the oxygen out of it, leaving the nitrogen. We do this by burning methane: CH₄ + 3O₂ → CO₂ + H₂O. Hydrogen is also, funnily enough, generated using methane, except we react it with steam: CH₄ + 2H₂O → CO₂ + 3H₂. And then we filter out the carbon dioxide and trace gases (more on why that is later), and we have our nitrogen and hydrogen.

So now we get into the Haber process itself. This can seem confusing at first, since textbooks like to have overly complex diagrams, so I found this one to the right on the net. It's just that simple. What the syllabus needs though, is an explanation of that diagram, so let's get to it.

Firstly, the simple stuff. The pressure is 200 atm because it favours the product side of the reaction. Le Chatelier's Principle 101. Moving along. The catalyst, Fe₃O₄ or magnetite, is there because it speeds the reaction up (by lowering the activation energy). Easy. The ratio of N₂:H₂ is kept at 1:3, so all the reactants are used up. Ammonia is also regularly removed, as this shifts equilibrium to the right, as per Le Chatelier's Principle.

Now, the tricky part. You'll notice that the temperature is 400°C. While that is fairly hot, it's not very hot (for industrial production). This is because an increase in heat does two things to this reaction. First, it speeds it up (that's basic chemistry), but it also favours the reactant side of the equation, due to the fact that this reaction is an exothermic one (Le Chatelier's Principle strikes again). Something's gotta give. So in order to balance out reaction rate and yield for maximum production, the process is kept at a moderate 400°C.

Finally, we need our monitoring part of the Haber process. This is fairly straightforward. Temperature, pressure, and the ratio of the reactants all have to be monitored for the reasons above. The produced ammonia needs to be monitored to ensure sufficient quality. The process also needs to be monitored to remove unwanted gases. Argon and methane will lower the efficiency of the process (if they're there, nitrogen and hydrogen aren't). CO, CO₂ and sulfur compounds will "poison" the catalyst. Oxygen can react explosively. We definitely don't want these gases in.

OK, I think that about does it for the Haber process. I think this went a little better than MRI yesterday, if only because MRI is very complex at times, and this isn't as much. I sure learnt something, and I hope you did too. Remember to comment!

As always,

AB