The Haber Process: The Haber Process: Making AmmoniaMaking Ammonia

L.O: To understand the L.O: To understand the production of ammonia production of ammonia

through the haber processthrough the haber process

What is a reversible reaction?What is a reversible reaction?

Can you balance this reversible Can you balance this reversible reaction? reaction?

_H2(g) + _N2 (g) _NH3 (g) H=-92kJ/mol

1.1. Why do we need ammonia?Why do we need ammonia?Nitrogen is unreactive so making ammonia is a way of Nitrogen is unreactive so making ammonia is a way of

putting nitrogen in a form that can be absorbed by putting nitrogen in a form that can be absorbed by plants (its most important use). The ammonia once plants (its most important use). The ammonia once made is used in fertilisers. made is used in fertilisers.

2.2. What are the raw materials?What are the raw materials?Nitrogen from air and hydrogen from natural gas (methane)Nitrogen from air and hydrogen from natural gas (methane)

3H2(g) + N2 (g) 2NH3 (g) H=-92kJ/mol

Making Ammonia Invented by Fritz Haber in early 1900s

Fritz Haber, 1868-1934

VideoVideohttp://www.rsc.org/learn-chemistry/resource/res00001027/industrial-process-videos#!

cmpid=CMP00001682

A closed system e.g. the haber A closed system e.g. the haber processprocess

In a closed system no reactants or In a closed system no reactants or products can get in or outproducts can get in or out

Eventually a balance between the Eventually a balance between the amounts of reactants and products is amounts of reactants and products is reached. (not necessarily the same reached. (not necessarily the same amounts of each).amounts of each).

Temperature and Concentration Temperature and Concentration (Pressure) can change the amount of (Pressure) can change the amount of products and reactants.products and reactants.

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Reversible ReactionsReversible ReactionsWhen 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 + B C + D

Endothermic reactions

Increased temperature:

Decreased temperature:

A + B C + D

A + B C + D

More products

Less products

Exothermic reactions

Increased temperature:

Decreased temperature:

A + B C + D

Less products

More products

A + B C + D

In a reversible reaction, If a In a reversible reaction, If a reaction is exothermic will an:reaction is exothermic will an:

Increase in temperature increase the Increase in temperature increase the yield?yield?

Increase in temperature decrease the Increase in temperature decrease the yield?yield?

Decrease in temperature increase the Decrease in temperature increase the yield?yield?

Decrease the temperature decrease the Decrease the temperature decrease the yieldyield

Two answers are correct. Hint: think about Two answers are correct. Hint: think about which direction of the reversible reaction which direction of the reversible reaction needs heat.needs heat.

In a reversible reaction, If a In a reversible reaction, If a reaction is endothermic will an:reaction is endothermic will an:

Increase in temperature increase the Increase in temperature increase the yield?yield?

Increase in temperature decrease the Increase in temperature decrease the yield?yield?

Decrease in temperature increase the Decrease in temperature increase the yield?yield?

Decrease the temperature decrease the Decrease the temperature decrease the yieldyield

Two answers are correct. Hint: think about Two answers are correct. Hint: think about which direction of the reversible reaction which direction of the reversible reaction needs heat.needs heat.

The reaction is reversible so ammonia is removed by The reaction is reversible so ammonia is removed by cooling it as soon as it is madecooling it as soon as it is made

3H2(g) + N2 (g) 2NH3 (g) H=-92kJ/mol

The Haber Process The Haber Process ContinuedContinued

L.O: To understand the L.O: To understand the conditions of the Haber conditions of the Haber

ProcessProcess

The Haber process worksheetThe Haber process worksheet

1.1. What does the graph show about the effect of What does the graph show about the effect of temperature on the Haber process?temperature on the Haber process?

2.2. Suggest why a temperature of 400Suggest why a temperature of 400ooC is chosen when a C is chosen when a lower temperature gives an equilibrium mixture with lower temperature gives an equilibrium mixture with greater % conversion to ammonia. greater % conversion to ammonia.

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150 200 250 300 350 400 450 500 550

Temp (C)

% c

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Production of ammonia at 400 atm pressure

3H2(g) + N2 (g) 2NH3 (g) H=-92kJ/mol

Hint: reaction rates?

Reduces %conversion

The Haber Compromise - TemperatureThe Haber Compromise - Temperature

Haber Process: The economicsHaber Process: The economicsA while ago we looked at reversible reactions:

A + B C + D

Endothermic, increased temperature

A + B C + 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

N2 + 3H2 2NH3

0

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40

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60

70

80

90

100

0 100 200 300 400 500 600 700pressure (atm)

% c

on

ver

sio

n

at a temperature of 400C

1.1. What does the graph show about the effect of pressure What does the graph show about the effect of pressure on the Haber process?on the Haber process?

2.2. Suggest why a pressure of 200 atm is chosen when a Suggest why a pressure of 200 atm is chosen when a higher pressure gives an equilibrium mixture with greater higher pressure gives an equilibrium mixture with greater % conversion to ammonia. % conversion to ammonia.

3H2(g) + N2 (g) 2NH3 (g) H=-92kJ/mol

Hint: costs?

Increases %conversion

The Haber Compromise - PressureThe 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 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)moles means more space is taken up)

In the haber process overall there are 4 moles of reactants and 2 moles of productsIn 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 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.be filled with more ammonia.

If we use If we use veryvery high pressures the cost of the equipment used increases drastically and there are also safety issues. Better 90% 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.conversion at 200atm than 95% conversion at 600 atm.

3H2(g) + N2 (g) 2NH3 (g) H=-92kJ/mol

The Haber Compromise - PressureThe Haber Compromise - Pressure

1.1. Is the forward reaction exothermic or Is the forward reaction exothermic or endothermic?endothermic?

2.2. Will heating the mixture give an equilibrium Will heating the mixture give an equilibrium mixture with more or less ammonia?mixture with more or less ammonia?

3.3. Are there more gas molecules of reactant or Are there more gas molecules of reactant or product?product?

4.4. Will raising the pressure give an equilibrium Will raising the pressure give an equilibrium mixture with more or less ammonia?mixture with more or less ammonia?

3H2(g) + N2 (g) 2NH3 (g) H=-92kJ/mol

exothermic

less

reactant

more

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

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Haber Process SummaryHaber Process Summary

•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.

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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