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Who could not be moved by the deadly accident that occurred at the start of August in Beirut. An estimated 2750 tonnes of ammonium nitrate (NH4NO3) exploded in spectacular fashion after the warehouse storing the fertilizer caught fire and caused the explosion.

The accident which killed at least 220 people, injured more than 5,000 and left an estimated 300,000 people homeless[1] was so loud the explosion could be heard over 200 miles away in Cyprus.

I knew from my teaching that ammonium nitrate is used as both a fertilizer and explosive but the devastating effects of the blast got me looking at the chemical in more detail.

Firstly, why is ammonium nitrate used as a fertilizer? Well, living organisms need nitrogen (they are a key component of amino acids). The problem most living things have with nitrogen is assimilating it. Even though 80% of the air is nitrogen, the triple bond has too high a bond enthalpy for most organisms to break (there are a few species of bacteria that can break it) so nitrogen is needed in another form. Ammonium nitrate is ionic so it is soluble, allowing it to be easily taken up by plants to make amino acids.

Secondly, why a fertilizer? Well, when ammonium nitrate is heated, it will thermally decompose. This is good as the reactant does need to react with something else. You just need one reactant. And when it does decompose, it produces gases. No liquids or other solids, just gases. The faster it decomposes, the more explosively these gases are released.

It will decompose at around 230oC and at temperatures between 260oC – 300oC it will explode. This sort of temperature is relatively low and would easily be reached in a fire.

The equation for the reaction is:

2NH4NO3(s) → 2N2(g)+ 4H2O(g) +O2(g)

1 mole of reactant produces 4.5 moles of gas – this is quite a difference.

If you saw the TV images of the explosion, you may recall seeing orange smoke just after the huge shockwave. This was actually nitrogen dioxide (NO2) and was formed in a side reaction:

4NH4NO3(s) → 3N2(g) + 2NO2 (g) + 8H2O(g)                             [That’s a tricky one to balance]

Note that all of these products are also gases.

Some ammonia (NH3) and nitrogen dioxide (N2O) are also produced in other side reactions (note again, all the products are gases):

NH4NO3 (s) → N2O(g) + 2H2O(g)

NH4NO(s) → NH3(g) + HNO3(g)

Like most things in chemistry, when the reaction is broken down it is quite simple and easy to understand but what tragic and deadly results such simplicity can bring.

[1] https://www.nature.com/articles/d41586-020-02361-x