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Chemistry of Ammonium Nitrate

How the chemistry of ammonium nitrate caused a disastrous explosion in Beirut

Welcome

Hi, I’m Chris… Founder of Plymouth Tutors. I moved from Oxford to Plymouth in 1995 and now I run a small select Tuition Agency. We offer the finest academic and music tuition in the Plymouth area. I’ll be writing about what’s new in the world of those academic subject areas, but also interesting articles on music related to the instruments we tutor. For example, in this first post we’ll take a look at the chemistry of ammonium nitrate.

The explosion in Beirut

A massive explosion shook Beirut on Thursday evening, 4th August 2020 at around 6.07pm. This horrific event wounded thousands of people. The devastating blast was felt more than 150 miles away in Cyprus and flattened an area of the city near the port of Beirut. Moreover, it capsized a docked passenger ship, was felt as strongly as a 3.3-magnitude earthquake and tragically, the explosion killed at least 135 people. This Guardian video illustrates what a disaster this was for the people living nearby:

What happened in Beirut?

A warehouse storing thousands of tons of unsecured, high-explosive ammonium nitrate was at the centre of the explosion. In fact, the unsecured warehouse had been storing nearly 2,750-tons of ammonium nitrate for more than 6 years! 

Ammonia at GCSE and A-Level

Ammonium nitrate is made from ammonia. Ammonia is manufactured in the Haber Process. Ammonium nitrate is most commonly used as a fertiliser to improve crop yields. This is because ammonium nitrate contains a high percentage of nitrogen which is an essential element in amino acids. Amino acids are the building blocks of proteins and are therefore essential for growth and repair of damaged cells. Fertilisers contain nitrogen to improve the growth of plants and crops, because the plants incorporate the nitrogen into their protein molecules.

chemistry of ammonium nitrate

Making ammonia

The Haber process converts nitrogen gas and hydrogen gas into ammonia gas. A dynamic equilibrium exists, because we have a reversible reaction in a closed system. Moreover, the rate of the forward reaction and back reaction will be the same. Additionally, the concentrations of the reactants and products will remain constant.

The equation is …

chemistry of ammonium nitrate - making ammonia

Changing the Pressure

1 mole of nitrogen reacts with 3 moles of hydrogen to make 2 moles of ammonia, as seen in the above equation.

The products, therefore take up less space than the reactants.

Le Chetalier’s Principle shows that …

  • Increasing the pressure would shift the equilibrium to the right hand side.
  • The reaction will therefore try to reduce the pressure by making more ammonia which takes up less space.
  • The electricity becomes expensive and the process is potentially dangerous, if the pressure is too high.

A pressure of 250 atmospheres is therefore used as a compromise.

Changing the Temperature

The forward reaction is exothermic and the back reaction is endothermic.

Le Chetalier’s Principle shows that …

  • Decreasing the temperature shifts the equilibrium to the right hand side. The reaction will therefore give out more heat to warm itself up again.
  • The rate of reaction will be too slow, if the temperature is too low.

For this reason, a temperature of 400 C is used as a compromise. An iron catalyst helps to speed up the rate.

Chemistry of ammonium nitrate at GCSE and A-Level

Ammonia reacts with nitric acid in a neutralisation reaction to make ammonium nitrate. Importantly, ammonium nitrate is an effective fertiliser, but it also readily forms explosive mixtures.

Disasters

The following information is taken from the wikipedia article: Ammonium nitrate disasters

Ammonium nitrate decomposes, non-explosively, into the gases nitrous oxide and water vapour when heated. However, it can be induced to decompose explosively by detonation. Large stockpiles of the material can also be a major fire risk due to their supporting oxidation. Fire can easily escalate to detonation. Explosions are not rare: relatively minor incidents occur most years, and several large and devastating explosions have also occurred. The Oppau explosion of 1921 (one of the largest artificial non-nuclear explosions), the Texas City disaster of 1947 and the 2015 Tianjin explosions in China are further examples of ammonium nitrate disasters. The 2020 Beirut explosions were certainly amongst the most terrible.

Ammonium nitrate can explode through two mechanisms:

  • Shock-to-detonation transition. An explosive charge within or in contact with a mass of ammonium nitrate causes the ammonium nitrate to detonate. Examples of such disasters are Kriewald, Morgan (present-day Sayreville, New Jersey), Oppau, and Tessenderlo.
  • Deflagration to detonation transition. A fire that spreads into the ammonium nitrate can result in an explosion (Texas City, TX; Brest; West, TX; Tianjin; Beirut), Ammonium nitrate mixing with a combustible material during the fire can cause even more destruction (Repauno, Cherokee, Nadadores). The fire must be confined at least to a degree for successful transition from a fire to an explosion. This appears to be what happened in Beirut.

Conclusion

Ammonium nitrate as a fertiliser has been essential to help feed an expanding world population. However, it can be extremely dangerous. A knowledge of the chemistry of ammonium nitrate and close adherence to Health & Safety regulations are therefore both essential. The people of Beirut were already struggling with an economic crisis and the effects of the corona virus. In light of this, the recent disastrous explosion leaves the people of Beirut in a desperate position. Most importantly, international aid will be essential if they are going to be able to rebuild their port, their city and their lives.

If you are in a position to help, please follow the link below because it takes you directly to the Red Cross Beirut Appeal website:

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