Today I will take a break from my usual posting style of summarizing new research publications (besides who could compete with the post that user @irime put out this week! If you haven't checked that post out, I recommend it!). Instead, let us take a step back in time to the late 1890's to early 1900's and discuss the development of an important chemical reaction known as the Haber process (or Haber-Bosch process if you want to credit its further scale up).

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The Haber Process

Quite simply the Haber process is a chemical reaction which allows for the production of Ammonia (NH₃) from Hydrogen and Nitrogen from our atmosphere:

3 H₂ + N₂ <--> 2NH₃ ΔG = -92 kJ/mol

From looking at it you might be thinking, okay.. so? These elements are in the air so the ammonia must be able to form easily (after all you might note that the Gibbs free energy is negative! [4] That means the reaction is spontaneous!). Unfortunately this was not the case and the discovery of how to appropriately perform this reaction revolutionized the world as we know it.

Wait... why doesn't the reaction proceed? First principles says it should damn it!

Nitrogen is not a particularly reactive element, as the elemental form (N₂ is held together very very tightly by a triple bond, see image to the right). In order for this reaction to proceed we have to break a part that triple bond of nitrogen. To do this, one can heat the nitrogen up to about 3000 °C!

Unfortunately for us, the nitrogen fixation reaction above is exothermic meaning that heat is one of the products. Doubly unfortunate for us the reaction is an equilibrium reaction, this means that if we add more heat, it will reduce the yield (because we are shifting the equilibrium back toward the reactants side as we are adding in more of a product).

This leaves us with a problem, if we want the reaction to work we must raise the temperature up to break that nitrogen bond. But if we raise the temperature up and break the nitrogen bond then the reaction wants to go backwards and we don't get any product! This was where a man named Fritz Haber came in!

Fritz Haber

Fritz Haber was a German scientist who was working at the University of Karlsruhe in Karlsruhe, Germany (I am sure you figured the location out, but sometimes its fun to state the obvious). He was a great physical chemist and realized the only way to circumvent the issues with the equilibrium of this reaction would be to find a catalyst that would lower the energy required to split the nitrogen triple bond.

And identify a catalyst he did! Haber identified that the metal osmium could serve as a sort of scaffold to allow the underlying steps of the ammonia production reaction to proceed. [6]

This scaffold reduced the activation energy necessary to break apart the Nitrogen drastically reducing the required temperature and allowed for the reaction to proceed through several steps:

N₂ + H₂ --> N₂H₂
N₂H₂ + H₂ --> N₂H₄
N₂H₄ + H₂ --> 2NH₃

The actual detailed inorganic mechanism by which the Osmium works, is a bit complicated but if you are interested you can see a bit more here.

This catalyst was later optimized from Osmium to Iron and Potassium Hydroxide (as an activator of the iron surface). [8]

Now thanks to the identification of a chemical catalyst, Haber was able to perform the reaction under high pressure and only mildly high temperatures (~500 - 600 K or 227 - 327 °C).

But How To Separate The Gasses?

Okay so we have performed this reaction and made Ammonia gas (hooray?)... but there is still some left over hydrogen and of course atmospheric nitrogen to contend with, how can we separate the ammonia away? The answer to this is simple. Cool the gasses down, the ammonia will precipitate out into a liquid form at a much much higher temperature (-33 °C) then either Nitrogen (-195 °C) or Hydrogen (-252.9 °C).So cool the gas and the ammonia will turn into a liquid first!

Why Was A Reaction To Produce Ammonia Such A Game Changer?

Fertilizer! The worlds population was growing in the late 1800's and it was becoming increasingly difficult to produce enough food to feed everyone. To really ramp up food production farmers were turning to the use of a lot of fertilizer! This fertilizer required a nitrogen source like say... ammonium nitrate.

So the discovery that Haber made in being able to chemically produce ammonia under relatively gentle conditions using a catalyst was a HUGE HUGE deal for the world.

His discovery here was one of the major contributors to the great success that we humans have had on earth! It allowed our populations to grow and people to remain fed! Granted as humanity has continued to expand we have found new challenges in feeding people, but we would not have gotten to where we are with out the discovery that Haber made.

Coincidentally, Haber won a Nobel Prize for this work (and he should have!).

You may not spend much time thinking about chemical reactions like this. However I hope that this short post has illustrated to you how complicated something as simple as producing ammonia can be, and also how much of our quality of life is reliant on simple chemistry such as this!

Sources

Text Sources

1. https://en.wikipedia.org/wiki/Haber_process
2. http://www.ias.ac.in/article/fulltext/reso/007/09/0069-0077
3. https://chem.libretexts.org/Core/Physical_and_Theoretical_Chemistry/Equilibria/Case_Studies/Haber_Process
4. http://surfguppy.com/thermodynamics/caluclate-gibbs-free-energy-nitrogen-and-hydrogen-to-create-ammonia/
5. https://en.wikipedia.org/wiki/Fritz_Haber
6. http://www.spaceflight.esa.int/impress/text/education/Catalysis/Commercial.html
7. Nitrogen Fixation
8. https://www.researchgate.net/publication/256293116_AN_IRON_CATALYST_FOR_THE_SYNTHESIS_OF_AMMONIA_IMPREGNATED_WITH_POTASSIUM_HYDROXIDE
9. https://passel.unl.edu/pages/informationmodule.php?idinformationmodule=1130447045
10. https://www.nobelprize.org/nobel_prizes/chemistry/laureates/1918/

Image Sources

1. Image 1
2. Image 3

All Non Cited Images Are From Pixabay.com, Flickr.com, Pexels.com, or Wikipedia.com And Are Available For Reuse Under Creative Commons Licenses

Any Gifs Are From Giphy.com and Are Also Available for Use Under Creative Commons Licences

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