Making fertilisers industrially

The Ostwald process for making nitric acid

Ammonium nitrate is perhaps the most common compound found in fertilisers. It can be made by reacting an alkaline solution of ammonium hydroxide with nitric acid. The word and symbolic equations for the reaction are given below:

ammonium hydroxide_(aq) + nitric acid_(aq) → ammonium nitrate_(aq) + water_(l)

NH₄OH_(aq) + HNO_3(aq) → NH₄NO_3(aq) + H₂O_(l)

The two reactants in the above equation; the ammonium hydroxide and the nitric acid are both obtained thanks to the Haber process. Now recall that ammonium hydroxide is simply made by dissolving ammonia in water:

ammonia_(g) + water_(l) ⇌ ammonium hydroxide_(aq)

NH_3(s) + H₂O_(g) ⇌ NH₄OH_(aq)

Acidic non-metal oxides

One way to make an acid is to simply dissolve a non-metal oxide in water, for example the word equations below show how some common everyday acids are made by dissolving non-metal oxides in water:

Non-metal oxide + water → acid

Carbon dioxide + water → carbonic acid

Sulfur dioxide + water → sulfurous acid

Sulfur trioxide + water → sulfuric acid

Nitrogen dioxide + water →nitric acid

Making nitric acid

The word equation above shows that to make nitric acid you simply have to dissolve nitrogen dioxide gas in water. However one of the main problems with making nitric acid is actually getting the nitrogen dioxide gas that can then be dissolved in water to make the nitric acid. Nitrogen gas is a very unreactive gas so simply burning nitrogen gas in air/oxygen to make the nitrogen dioxide gas will not work!

nitrogen_(g) + oxygen_(g) → nitrogen dioxide _(g)

Burning ammonia gas

So what is needed is another way of preparing nitrogen dioxide gas. What about burning ammonia? Ammonia burns in oxygen with a yellowish coloured flame; as shown in the image below:

However there is a problem, ammonia burns to produce nitrogen gas and water. No nitrogen dioxide gas is produced as we might have been hoped:

ammonia_(g) + oxygen_(g) → nitrogen_(aq) + water_(l)

This time in the presence of a platinum catalyst the ammonia gas is oxidised to give nitrogen monoxide gas and water vapour:

ammonia_(g) + oxygen_(g) → nitrogen monoxide_(g) + water_(l)

4NH_3(s) + 5O_2(g) → 4NO_(g) + 6H₂O_(l)

nitrogen monoxide_(g) + oxygen_(g) → nitrogen dioxide_(g)

2NO_(g) + O_2(g) → 2NO_2(g)

The Ostwald process for making nitric acid

Now that a method had been found to make nitrogen dioxide gas all that was needed was a method to scale up the process to produce large amounts of nitric acid. The scientist who devised the industrial process for the large scale manufacture of nitric acid was the German Nobel prize winning chemist Friedrich Wilhelm Ostwald. Ostwald dissolved nitrogen dioxide gas in the presence of air/oxygen and water to make nitric acid, an equation for this reaction is shown below:

nitrogen dioxide_(g) + water_(l) + oxygen_(g) → nitric acid_(aq)

4NO_2(g) + 2H₂O_(l) + O_2(g) → 4HNO_3(aq)

The image below shows an outline of the Ostwald process for making nitric acid, at first glance it might look complicated but it is actually very straightforward.

Starting from the left hand-side of the image:

• At point 1 in the image above oxygen gas from the air enters a compressor where it is compressed to between 4-10 atmospheres pressure and then pre-heated before it enters the reactor.


• At point 2 in the image above liquid ammonia from the Haber process enters the vaporiser where it is turned into a gas. Next the oxygen and gaseous ammonia enter the reactor.


• At point 3- Inside the reactor the ammonia is oxidised to nitrogen monoxide gas in a high temperature catalysed reaction. A platinum/rhodium catalyst is used and temperatures are in the range 800-950^OC. This reaction is highly exothermic and releases a large amount of heat energy. This heat can be used to generate electricity or used as a heat source to pre-heat gases elsewhere in the reaction. An equation for the reaction that takes place inside the reactor is shown below:

  ammonia_(g) + oxygen_(g) → nitrogen monoxide_(g) + water_(l)

  4NH_3(s) + 5O_2(g) → 4NO_(g) + 6H₂O_(l)



• At point 4- The nitrogen monoxide gas leaves the reactor and is cooled in the cooler. Here cold water is turned into steam as the hot nitrogen monoxide loses its heat energy. The cool nitrogen monoxide gas now joins with oxygen to form nitrogen dioxide gas:

  nitrogen monoxide_(g) + oxygen_(g) → nitrogen dioxide_(g)

  2NO_(g) + O_2(g) → 2NO_2(g)



• At point 5 - The final vessel in the image is called the absorption tower. Here a shower of water falls from the top of the tall tower and meets the nitrogen dioxide gas as it rises up from the bottom of the tower. The nitrogen dioxide gas dissolves in the shower of water to form nitric acid. The nitric acid then leaves at the base of the absorption tower and is collected in a large tank. The nitric acid produced can then be reacted with ammonium hydroxide solution, made by dissolving ammonia in water. This neutralisation reaction will then form ammonium nitrate:

  ammonium hydroxide_(aq) + nitric acid_(aq) → ammonium nitrate_(aq) + water_(l)

  NH₄OH _(aq) + HNO_3(aq) → NH₄NO_3(aq) + H₂O_(l)

Key points

• Ammonia is a basic gas that is very soluble in water. Ammonia gas dissolves in water to form an alkaline solution of ammonium hydroxide.


• Non-metal oxides are acidic. This means that they dissolve in water to form acids.


• To make nitric acid the brown gas nitrogen dioxide is dissolved in water.


• The simplest way to make nitrogen dioxide gas is by burning ammonia in the presence of a platinum catalyst. This forms the colourless nitrogen monoxide which immediately oxidises in air to form the brown gas nitrogen dioxide.

Check your understanding - Questions on the Ostwald process.

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