The Haber Process

1. The Haber Process Noadswood Science, 2013

2. The Haber Process • To be able to describe the Haber Process

3. B A Reversible Reactions • Many reactions, such as burning fuel, are irreversible - they go to completion and cannot be reversed easily • Reversible reactions are different – in a reversible reaction, the products can react to produce the original reactants again… • If the reaction releases energy in one direction (exothermic), it will take in same amount of energy in the other (endothermic)… A B These decompose These combine

4. Reversible • The animation below shows a reversible reaction involving white anhydrous copper(II) sulfate and blue hydrated copper(II) sulfate copper(II) sulfate + water hydrated copper(II) sulfate • The reaction between anhydrous copper(II) sulfate and water is used as a test for water – the white solid turns blue in the presence of water…

5. Equilibrium • In reversible reactions equilibrium means balance but this balance does not have to be at the half-way point – it may be mostly reactants with just a little product or vice versa • There are 2 factors that we can change that influence the position of an equilibrium: - • Temperature • Concentration (or pressure in gas reactions) • Finding the conditions that gives the most product is really important in industrial chemical reactions

6. Temperature • All reactions are exothermic (give out heat) in one direction and endothermic (take in heat) in the other • For example nitrogen dioxide (NO2) joins to form dinitrogen tetroxide (N2O4) exothermically… 2NO2 N2O4 • The hotter a reaction is, the more likely it is to go in the endothermic direction (in this example heating gives more NO2, cooling give more N2O4) Cold going backwards (endothermic) Hot going forwards (exothermic)

7. Pressure • Pressure applies to gas reactions – it depends upon the number of gas molecules on each side of the equation 2NO2 N2O4 • The higher the pressure the more the reaction moves in the direction with less gas molecules (in this increasing the pressure gives more N2O4, decreasing the pressure gives more NO2) Increase molecule number in backward direction Decrease molecule number in forward direction

8. Concentration • Increasing the concentration of a substance tips the equilibrium in the direction that uses up (decreases) the concentration of the substance added BiCl3 + H2O BiOCl + 2HCl • Bismuth chloride reacts with water to give a white precipitate of bismuth oxychloride: - • Adding water will produce more BiOCl solid (to use up the H2O) • Adding acid (HCl) will result in less BiOCl solid to use up the HCl

9. Catalysts • Why are catalysts useful? • If we remove the products from an equilibrium mixture, more reactants are converted into products • If a catalyst is used, the reaction reaches equilibrium much sooner, because the catalyst speeds up the forward and reverse reactions by the same amount • The concentration of reactants and products is nevertheless the same at equilibrium as it would be without the catalyst

10. The Haber Process • Ammonia (NH3) is a compound of nitrogen and hydrogen – it is a colourless gas with a choking smell, and a weak alkali which is very soluble in water • Ammonia is used to make fertilisers, explosives, dyes, household cleaners and nylon – it is also the most important raw material in the manufacture of nitric acid • Ammonia is manufactured by combining nitrogen and hydrogen in an important industrial process called the Haber process…

11. The Haber Process • The raw materials for this process are hydrogen and nitrogen – hydrogen is obtained by reacting natural gas (methane) with steam, or through the cracking of oil; and nitrogen is obtained by burning hydrogen in air (air is ~80% nitrogen –when hydrogen is burned in air, the oxygen combines with the hydrogen, leaving nitrogen behind) • Nitrogen and hydrogen will react together under these conditions: - • A high temperature - about 450ºC • A high pressure - about 200 atmospheres (200 times normal pressure) • An iron catalyst

12. The Haber Process 3H2 + N2 2NH3

13. The Haber Process

14. Haber Process - Temperature • The reaction of nitrogen and hydrogen to form ammonia (NH3) is exothermic • How will temperature affect the composition of the equilibrium mixture? 3H2 + N2 2NH3 Cold going backwards (endothermic) Hot going forwards (exothermic)

15. Haber Process - Temperature 3H2 + N2 2NH3 • Which direction is endothermic? • Which direction do reactions move when heated? • Will heating give more or less NH3 in the equilibrium mixture? Cold going backwards (endothermic) Hot going forwards (exothermic)

16. Haber Process - Temperature 3H2 + N2 2NH3 • Which direction is endothermic? Backwards • Which direction do reactions move when heated? Backwards • Will heating give more or less NH3 in the equilibrium mixture? Less Cold going backwards (endothermic) Hot going forwards (exothermic)

17. Haber Process - Temperature • The forward reaction in the Haber process is exothermic – this means that if the temperature is increased, the position of equilibrium moves in the direction of the reverse reaction, and less ammonia is formed

18. Haber Process – Temperature • You might think that a low temperature would be a good choice for the Haber process: if the forward reaction is exothermic, the yield of product at equilibrium is increased at lower temperatures • However, if the temperature is too low the rate of reaction will be too low – this would make the process uneconomical • A compromise temperature is chosen: low enough to get a good yield of ammonia but high enough to obtain a reasonable rate of reaction

19. Haber Process - Pressure • The reaction of nitrogen and hydrogen to form ammonia (NH3) is affected by pressure 3H2 + N2 2NH3 • Which direction produces less gas molecules? • Which direction do reactions move when compressed? • Will high pressure give more or less NH3 in the equilibrium? Increase molecule number in backward direction Decrease molecule number in forward direction

20. Haber Process - Pressure Increase molecule number in backward direction 3H2 + N2 2NH3 • Which direction produces less gas molecules? Forward • Which direction do reactions move when compressed? The side which has less gas molecules • Will high pressure give more or less NH3 in the equilibrium? More Decrease molecule number in forward direction

21. Haber Process – Pressure • There are 1 + 3 = 4 molecules of gas on the left of the equation, only two molecules of gas on the right • In an equilibrium involving gases, an increase in pressure favours the reaction which produces the smallest number of molecules – in this case, an increase in pressure favours the forward reaction, and more ammonia is produced

22. Haber Process - Pressure • There is a limit to the pressure that can be used industrially, because very high pressures require very strong and expensive equipment • This means a compromise pressure is chosen - high enough to get a good yield of ammonia, but not so high that it would add too much to the costs of the process • The pressure chosen is usually about 200 atmospheres - equivalent to about half the pressure of the water around the wreck of the RMS Titanic…