The Haber Process: Making Ammonia L.O: To understand the production of ammonia through the haber process

What is a reversible reaction? Can you balance this reversible reaction? _H 2 (g) + _N 2 (g)  _NH 3 (g) H=-92kJ/mol

1. Why do we need ammonia? Nitrogen is unreactive so making ammonia is a way of putting nitrogen in a form that can be absorbed by plants (its most important use). The ammonia once made is used in fertilisers. 2. What are the raw materials? Nitrogen from air and hydrogen from natural gas (methane) 3H 2 (g) + N 2 (g)  2NH 3 (g) H=-92kJ/mol Making Ammonia Invented by Fritz Haber in early 1900s Fritz Haber,

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A closed system e.g. the haber process  In a closed system no reactants or products can get in or out  Eventually a balance between the amounts of reactants and products is reached. (not necessarily the same amounts of each).  Temperature and Concentration (Pressure) can change the amount of products and reactants.

17/12/2015 Reversible Reactions 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: A+BC+D Endothermic reactions Increased temperature: Decreased temperature: A+BC+D A+BC+D More products Less products Exothermic reactions Increased temperature: Decreased temperature: A+BC+D Less products More products A+BC+D

In a reversible reaction, If a reaction is exothermic will an:  Increase in temperature increase the yield?  Increase in temperature decrease the yield?  Decrease in temperature increase the yield?  Decrease the temperature decrease the yield Two answers are correct. Hint: think about which direction of the reversible reaction needs heat.

In a reversible reaction, If a reaction is endothermic will an:  Increase in temperature increase the yield?  Increase in temperature decrease the yield?  Decrease in temperature increase the yield?  Decrease the temperature decrease the yield Two answers are correct. Hint: think about which direction of the reversible reaction needs heat.

The reaction is reversible so ammonia is removed by cooling it as soon as it is made 3H 2 (g) + N 2 (g)  2NH 3 (g) H=-92kJ/mol

The Haber Process Continued L.O: To understand the conditions of the Haber Process

 The Haber process worksheet

1. What does the graph show about the effect of temperature on the Haber process? 2. Suggest why a temperature of 400 o C is chosen when a lower temperature gives an equilibrium mixture with greater % conversion to ammonia. 3H 2 (g) + N 2 (g)  2NH 3 (g)  H=-92kJ/mol Hint: reaction rates? Reduces %conversion The Haber Compromise - Temperature

Haber Process: The economics A while ago we looked at reversible reactions: A+BC+D Endothermic, increased temperature A+BC+D Exothermic, increase temperature ExothermicEndothermic 1) If temperature was DECREASED the amount of ammonia formed would __________... 2)However, if temperature was INCREASED the rate of reaction in both directions would ________ causing the ammonia to form faster 3)A compromise is met at 45O °C, the reaction is mostly forward. The rate is still fairly quick but an iron catalyst helps the speed. If we use too low temperatures it takes ages to reach equilibrium. It’s better to get a 40% yield in 2 minutes than an 80% yield in 2 hours! Nitrogen + hydrogen Ammonia N 2 + 3H 2 2NH 3

1. What does the graph show about the effect of pressure on the Haber process? 2. Suggest why a pressure of 200 atm is chosen when a higher pressure gives an equilibrium mixture with greater % conversion to ammonia. 3H 2 (g) + N 2 (g)  2NH 3 (g)  H=-92kJ/mol Hint: costs? Increases %conversion The Haber Compromise - Pressure

 More gas molecules means that a gas will take up more space, because there are more gaps between more particles. (So more moles means more space is taken up)  In the haber process overall there are 4 moles of reactants and 2 moles of products  As the reaction produces a product which takes up less space an increase in pressure increases yield because the extra space can be filled with more ammonia.  If we use very high pressures the cost of the equipment used increases drastically and there are also safety issues. Better 90% conversion at 200atm than 95% conversion at 600 atm. 3H 2 (g) + N 2 (g)  2NH 3 (g)  H=-92kJ/mol The Haber Compromise - Pressure

1. Is the forward reaction exothermic or endothermic? 2. Will heating the mixture give an equilibrium mixture with more or less ammonia? 3. Are there more gas molecules of reactant or product? 4. Will raising the pressure give an equilibrium mixture with more or less ammonia? 3H 2 (g) + N 2 (g)  2NH 3 (g)  H=-92kJ/mol exothermic less reactant more

 Explain why a temperature of 450°C and a pressure of 200 atmospheres is used – remember to explain why we have to compromise.

17/12/2015 Haber Process Summary 200 atm pressure 450 O C Iron catalyst 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: 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

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