Exothermic vs endothermic: 09/05/11 EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration) ENDOTHERMIC – energy is taken in but not necessarily given out (e.g. photosynthesis)
Reversible Reactions 09/05/11 Some chemical reactions are reversible. In other words, they can go in either direction: If a reaction is EXOTHERMIC in one direction what must it be in the opposite direction? A + B C + D NH 4 Cl NH 3 + HCl e.g. Ammonium chloride Ammonia + hydrogen chloride For example, consider copper sulphate: Hydrated copper sulphate (blue) Anhydrous copper sulphate (white) + Heat + Water CuSO 4 + H 2 O CuSO 4 .5H 2 O
Reversible Reactions 09/05/11 When a reversible reaction occurs in a CLOSED SYSTEM (i.e. no reactants are added or taken away) an EQUILIBRIUM is achieved – in other words, the reaction goes at the same rate in both directions: More products Less products Less products More products A + B C + D Endothermic reactions Increased temperature: Decreased temperature: A + B C + D A + B C + D Exothermic reactions Increased temperature: Decreased temperature: A + B C + D A + B C + D
Making Ammonia 09/05/11
450 O C
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. To produce ammonia from nitrogen and hydrogen you have to use three conditions: Nitrogen + hydrogen Ammonia N 2 + 3H 2 2NH 3 Recycled H 2 and N 2 Nitrogen Hydrogen Mixture of NH 3 , H 2 and N 2 . This is cooled causing NH 3 to liquefy. Fritz Haber, 1868-1934
The HABER process is not part of the curriculum however it provides a very good example of how equilibrium can impact an industrial process
Which way will the balance lie?
Le Chatelier did cry.
“ It will move to cancel out
Whichever change you bring about!”
Uses of Ammonia 09/05/11 Nitrogen monoxide Ammonia is a very important chemical as it can be used to make plant fertilisers and nitric acid: More ammonia can then be used to neutralise the nitric acid to produce AMMONIUM NITRATE (a fertiliser rich in nitrogen). The trouble with nitrogen based fertilisers is that they can also create problems – they could contaminate our drinking water. Hot platinum catalyst Ammonia gas Oxygen Cooled Water and oxygen Nitrogen monoxide Nitric acid Ammonia + nitric acid Ammonium nitrate NH 3 + HNO 3 NH 4 NO 3
Haber Process: The economics 09/05/11 A while ago we looked at reversible reactions:
1) If temperature was DECREASED 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
A + B C + D Endothermic, increased temperature A + B C + D Exothermic, increase temperature Exothermic Endothermic Nitrogen + hydrogen Ammonia N 2 + 3H 2 2NH 3
Haber Process Summary 09/05/11
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
200 atm pressure
450 O C
Recycled H 2 and N 2 Nitrogen Hydrogen Mixture of NH 3 , H 2 and N 2 . This is cooled causing NH 3 to liquefy. To compromise all of these factors, these conditions are used: