The Harber Process - GCSE AQA C3

These are some quick revision notes that might help you understand the Harber Process more clearly. Hope it helps :)

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  • Created by: Iswariya
  • Created on: 19-05-13 18:12

Ammonia

 

The formula for ammonia is NH3. 

The main use for ammonia is as fertilisers.

 Fertilisers increase the yield of crops and it is important as it makes sure we can grow enouhg food to feed everyone on the planet.

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Haber Process Diagram

(http://www.chemguide.co.uk/physical/equilibria/haberflow.gif)

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The Haber Process (Summary)

The nitrigen is obtained easily from the air, which is 78% nitrogen.

The hydrogen comes from natural gas or from other sources like crude oil.

Some of the nitrogen and hydrogen reacts to form ammonia. because the reaction is reversible (it occurs in both directions) ammonia breaks down again into hydrogen and nitrogen.

The reaction reaches an equilibrium.

 

 

(http://scienceaid.co.uk/chemistry/applied/images/habereq.png)

 

 

 

 

(http://www.blobs.org/science/chemistry/1b2.gif)

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Industrial Condition For The Haber Process

Industrial Conditions:

Pressure: 200 atmospheres

Temperature: 450 oC

Catalyst: Iron

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Comprimises That Is Made

Higher pressures favour the forward reaction ( since there are 4 molecules of gas on the left-hand side, for every 2 molecules on the right).

So the pressure is set as high as possible to give the best %yield, without making the plant too expensive to build so, only 200 atmospheres operating pressure is used.

The forward reaction is exothermic, which means that increasing the temperature will actually move the equilibrium the wrong way (away from ammonia and toward N2 and H2. So the yield of ammonia would be greatter at lower temperatures.

The trouble is, lower temperatures mean a lower rate of reaction. So what they do is increase the temperature anyway, to get a much faster rate of reaction.

The 450 oC  is a compromise between maximum yield and speed of reaction. It's better to wait just 20 seconds for a 10% yield than to wait 60 seconds for a 20% yield.

The ammonia is formed as a gas but, as it cools in the condenser it liquefies and is removerd.

The unused hydrogen (H2) and nitrogen (N2) are recycled so nothing is wasted.

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The Iron Catalyst

The iron catalyst makes the reaction go faster, which gets it to the equilibrium proportions more quickly. But remember, the catalyst doesn't affect the position of the equilibrium ( i.e the % yield).

Without the catalyst the temperature would have to be raised even further to get a quick enough reaction, and that would reduce the % yield even further. So the catalyst is very important.

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How Pressure and Temperature Effects the % Yield

(http://www.ausetute.com.au/images/rorampre.gif)

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