Reversible Reactions and Dynamic Equilibrium The Haber Process Reversible Reactions and Dynamic Equilibrium

Reversible Reactions N2(g) + 3H2(g) 2NH3(g) (+ heat) Remember this? The Haber process is a REVERSIBLE reaction N2(g) + 3H2(g) 2NH3(g) (+ heat) nitrogen + hydrogen ammonia A reversible reaction is one where the products of the reaction can themselves react to produce the original reactants.

Reversible Reactions If you don’t let any reactants or products escape, both forward and backward reactions can happen at the same time. Reactants make products, and, at the same time, products make reactants. When the forward and backward reactions go on at the same rate a state of dynamic equilibrium exists.

Reversible Reactions In the Haber process we want the dynamic equilibrium to mover to the right – so that lots of ammonia is made (and not much N and H is left around)

Reversible Reactions The french chemist Le Chatelier worked all this lot out!!! In a dynamic equilibrium the position of the equilibrium will shift in order to reverse any changes you introduce.

Reversible Reactions Factors that can affect a reversible reaction include: Changing temperature Changing concentration Changing pressure

Improving the yield of ammonia in the Haber process Effect of pressure On the left hand side there are 4 moles of gas, whilst on the right hand side there are 2 moles Any increase in pressure will favour the forward reaction to produce more ammonia. This is because the forward reaction will tend to decrease the pressure in the system.

Improving the yield of ammonia in the Haber process In the Haber process the pressure is set as high as possible to give the best % yield. High pressure containers are VERY expensive. It could be possible to carry out the reaction at 1000 atmospheres – but this would not be economical (it would cost more than the product is worth). The typical pressure used is 200 to 350 atmospheres.

Improving the yield of ammonia in the Haber process The reaction produces heat when it moves to the right. ( exothermic ) This means that a LOWER temperature would favour the forward reaction, BUT…. Reactions go slower at lower temperatures! In operating the Haber process you have to decide what is more important, the higher YIELD you can get at lower temperatures or the higher RATE at higher temperatures.

Improving the yield of ammonia in the Haber process In order to get ammonia produced at a quicker RATE the reaction is carried out at a high temperature (450oC). It is better to get just a 10% yield in 20 seconds (at a high temperature) that a 20% in 60 seconds (at a lower temperature)

It took over 6500 experiments at different temperatures and pressures carried out by the German Carl Bosch to work all this lot out. He got a Nobel Prize for it in 1931! (Haber got his Nobel Prize in 1918)

Other ways of increasing the yield in the Haber process An IRON catalyst makes the reaction occur more quickly, (it does not affect the % yield i.e. the position of the dynamic equilibrium). Without the catalyst the temperature would have to be much higher (this would lower the yield).

Other ways of increasing the yield in the Haber process Removing the ammonia from the system also pushes the reaction to the right so more ammonia is produced to replace it.

Making Ammonia Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 Fritz Haber, 1868-1934 Guten Tag. My name is Fritz Haber and I won the Nobel Prize for chemistry. I am going to tell you how to use a reversible reaction to produce ammonia, a very important chemical. This is called the Haber Process. Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 To produce ammonia from nitrogen and hydrogen you have to use three conditions: High pressure 450O C Iron catalyst Mixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy. Nitrogen Hydrogen Recycled H2 and N2

Ammonia + nitric acid Ammonium nitrate Uses of Ammonia Ammonia is a very important chemical as it can be used to make plant fertilisers and nitric acid: Water and oxygen Nitrogen monoxide Ammonia gas Oxygen Hot platinum catalyst Nitrogen monoxide Nitric acid Cooled More ammonia can then be used to neutralise the nitric acid to produce AMMONIUM NITRATE (a fertiliser rich in nitrogen). Ammonia + nitric acid Ammonium nitrate NH3 + HNO3 NH4NO3 The trouble with nitrogen based fertilisers is that they can also create problems – they could contaminate our drinking water.

Haber Process: The economics A while ago we looked at reversible reactions: A + B C D Endothermic, increased temperature A + B C D Exothermic, increase temperature More products More reactants + heat Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 Endothermic Exothermic + heat 1) If temperature was INCREASED the amount of ammonia formed would __________... However, if temperature was INCREASED the rate of reaction in both directions would ________ causing the ammonia to form faster If pressure was INCREASED the amount of ammonia formed would INCREASE because there are less molecules on the right hand side of the equation ( decrease, increase )

Haber Process Summary A low temperature increases the yield of ammonia but is too slow A high temperature improves the rate of reaction but decreases the yield too much A high pressure increases the yield of ammonia but costs a lot of money To compromise all of these factors, these conditions are used: 200 atm pressure 450O C Iron catalyst Recycled H2 and N2 Nitrogen Hydrogen Mixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.