Nitrogen and Ammonia

Almost 80% of the air is nitrogen.

Nitrogen is used to make several important chemicals, including nitrogen-based fertilisers.

Farmers often add ammonium nitrate fertiliser to soil to increase the yield of crops.

One disadvantage of artificial fertilisers is that the nitrates can be washed out of the soil into streams and rivers, so they can contaminate our drinking water.

Large amounts of nitrogen gas are used to make fertilisers. The first step is to make ammonia.

The Haber Process

Ammonia is manufactured in the Haber process.

The raw materials are nitrogen from the air and hydrogen obtained from natural gas (methane).

	nitrogen	+	hydrogen		ammonia
	N2(g)	+	3H2(g)		2NH3(g)

The symbol  means the reaction is reversible; it can go in both directions. Some of the ammonia formed immediately breaks down into hydrogen and nitrogen again.

When the rate of the forward reaction is the same as that of the backward reaction, it is said to be in equilibrium. For every two molecules of ammonia formed, two others break down.

In order to produce more ammonia, the reaction conditions must be altered to move the equilibrium to the right. However, it is optimal for the rate of reaction to be fast.

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Increasing Temperature

The forward reaction is exothermic, it gives out heat. The backward reaction is therefore endothermic and takes in heat.

Increasing the temperature will therefore increase the rate of reaction, because the particles will collide more frequently. However, increasing the temperature of an exothermic reaction decreases the yield, whereas decreasing the temperature increases the yield. The opposite is true of an endothermic reaction.

In the Haber process, a moderately high temperature of around 450 ºC is used, so the rate is fairly fast, and the yield is reasonable. A higher temperature would produce the ammonia faster, but in smaller quantities.

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Increasing Pressure

In gaseous equations, increasing the pressure will move the equilibrium in the direction which produces the least number of molecules.

In the forward reaction, four molecules of gas react to produce two molecules of ammonia. Therefore, increasing the pressure moves the equilibrium to the right, and so increases the yield of ammonia.

A high pressure of around 200 atmospheres is used in the Haber process.

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Using a Catalyst

A catalyst increases the rate of both reactions equally, so it doesn't affect the yield, but does make it faster. This saves time and money. The purified gases are passed over a hot iron catalyst in this reaction.

To stop the ammonia breaking down, it is immediately cooled and liquified, so it can be collected.

The remaining nitrogen and hydrogen are recycled to react again.

The next step in the production of fertiliser is to convert the ammonia to nitric acid by oxidising it. It is reacted with oxygen in the presence of a hot platinum catalyst, producing nitrogen monoxide.

The nitrogen monoxide reacts with more oxygen from the air to form nitrogen dioxide.

Nitrogen dioxide is then dissolved in water to form nitric acid.

The ammonia produced in the Haber process is reacted with nitric acid to form ammonium nitrate fertiliser. This is a neutralisation reaction , because ammonia is an alkali.

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