Presentation on theme: "Reversible Reactions and Dynamic Equilibrium"— Presentation transcript:
1Reversible Reactions and Dynamic Equilibrium The Haber ProcessReversible Reactions and Dynamic Equilibrium
2Reversible Reactions N2(g) + 3H2(g) 2NH3(g) (+ heat) Remember this?The Haber process is a REVERSIBLE reactionN2(g) H2(g) NH3(g) (+ heat)nitrogen + hydrogen ammoniaA reversible reaction is one where the products of the reaction can themselves react to produce the original reactants.
3Reversible ReactionsIf 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.
4Reversible ReactionsIn 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)
5Reversible ReactionsThe 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.
6Reversible ReactionsFactors that can affect a reversible reaction include:Changing temperatureChanging concentrationChanging pressure
7Improving the yield of ammonia in the Haber process Effect of pressureOn the left hand side there are 4 moles of gas, whilst on the right hand side there are 2 molesAny 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.
8Improving 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.
9Improving 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.
10Improving 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)
11It 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)
12Other 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).
13Other 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.
15Making Ammonia Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3 Fritz Haber,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 AmmoniaN2 + 3H NH3To produce ammonia from nitrogen and hydrogen you have to use three conditions:High pressure450O CIron catalystMixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.NitrogenHydrogenRecycled H2 and N2
16Ammonia + nitric acid Ammonium nitrate Uses of AmmoniaAmmonia is a very important chemical as it can be used to make plant fertilisers and nitric acid:Water and oxygenNitrogen monoxideAmmonia gasOxygenHot platinum catalystNitrogen monoxideNitric acidCooledMore ammonia can then be used to neutralise the nitric acid to produce AMMONIUM NITRATE (a fertiliser rich in nitrogen).Ammonia + nitric acid Ammonium nitrateNH3 + HNO NH4NO3The trouble with nitrogen based fertilisers is that they can also create problems – they could contaminate our drinking water.
17Haber Process: The economics A while ago we looked at reversible reactions:A+BCDEndothermic, increased temperatureA+BCDExothermic, increase temperatureMore productsMore reactants+ heatNitrogen + hydrogen AmmoniaN2 + 3H NH3EndothermicExothermic+ heat1) 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 fasterIf 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 )
18Haber Process SummaryA low temperature increases the yield of ammonia but is too slowA high temperature improves the rate of reaction but decreases the yield too muchA high pressure increases the yield of ammonia but costs a lot of moneyTo compromise all of these factors, these conditions are used:200 atm pressure450O CIron catalystRecycled H2 and N2NitrogenHydrogenMixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.