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KS4 Chemistry

KS4 Chemistry. Reversible Reactions. Contents. Reversible reactions. Comparing reactions. Dynamic equilibrium. Changing conditions. The Haber process. Summary activities.

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KS4 Chemistry

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  1. KS4 Chemistry Reversible Reactions

  2. Contents Reversible reactions Comparing reactions Dynamic equilibrium Changing conditions The Haber process Summary activities

  3. Most chemical reactions are considered irreversible in that products are not readily changed back into reactants. Mg + 2HCl  MgCl2 + H2 Wood reacting with oxygen Irreversible reactions • When magnesium reacts with acid it is not easy to unreact it and get back the magnesium. • When wood burns it is pretty difficult to un-burn it back into wood again!

  4. Although most chemical reactions are difficult to reverse, it is possible to find reactions ranging from irreversible to the fully reversible. Many biochemical reactions are reversible Reversible reactions • Indeed, many of the biochemical reactions that take place in living things are reversible. • There are also some very important industrial reactions, like the Haber process, that are reversible.

  5. The change from blue hydrated copper sulphate to white anhydrous copper sulphate is one of the most commonly known reversible reactions. Heating copper sulfate heat hydrated copper sulphate anhydrous copper sulphate steam CuSO4.5H20 CuSO4 + 5H2O

  6. Ammonium salts are made by reacting ammonia with an acid but some of these salts will decompose back into reactants when heated. Heat makes the solid disappear as it changes into gases. Solid reappears as it changes back again in the cool part of the tube. hydrogen chloride ammonium chloride ammonia heat NH4Cl(s) NH3(g) + HCl(g) Heating ammonium chloride

  7. Reversible or irreversible?

  8. Contents Reversible reactions Comparing reactions Dynamic equilibrium Changing conditions The Haber process Summary activities

  9. A reversible reaction is where products can, under appropriate conditions, turn back into reactants. There will be a range of conditions over which both the forward and backward reaction will take place, and this can lead to a state of balance with reactants and products present in unchanging amounts. This is called a dynamic equilibrium. B A Dynamic equilibrium A B these decompose these combine

  10. Dynamic equilibrium What is special about the forward and backward reactions during dynamic equilibrium?

  11. It is rather like the situation where a man is walking the wrong way along a moving pavement or escalator. Neither have stopped but the man could remain in the same place for ever! The symbol is used to mean dynamic equilibrium. Dynamic equilibrium Equilibrium – becauseof the unchanging amounts Dynamic – because the reaction is still occurring The man stays in the same place! Dynamic equilibrium can only take place in a closed system, otherwise products would be able to escape.

  12. In reversible reactions, equilibrium means balance but this balance does not have to be at the half-way point. We may have mainly reactants with just a little product, or vice versa. There are two factors that we can change that influence the position of an equilibrium: Temperature Concentration (or pressure in gas reactions) Changing the equilibrium Adding a catalyst speeds up the time it takes to reach equilibrium, but does not change the position of equilibrium. • Finding the conditions that gives the most product is really important in industrial chemical reactions.

  13. True or false?

  14. Contents Reversible reactions Comparing reactions Dynamic equilibrium Changing conditions The Haber process Summary activities

  15. All reactions are exothermic (give out heat) in one direction and endothermic (take in heat) in the other. E.g. nitrogen dioxide (NO2) joins to form dinitrogen tetroxide (N2O4) exothermically. Temperature and equilibrium Gets cold going backward (endothermic) 2NO2 N2O4 Gets hot going forward (exothermic) The hotter a reaction is, the more likely it is to go in the endothermic direction. • Heating will give more NO2 in the equilibrium mixture • Cooling will give more N2O4in the equilibrium mixture..

  16. Opposing changes in temperature The reaction of nitrogen and hydrogen to form ammonia (NH3) is exothermic. How will temperature affect the composition of the equilibrium mixture? Gets cold going backward (endothermic) 3H2 + N2 2NH3 Gets hot going forward (exothermic) Which direction is endothermic? In which direction do reactions move whenheated? Will heating give more or less NH3 in theequilibrium mixture? backward backward less

  17. This applies to gas reactions. Here the rule depends upon the number of gas molecules on each side of the equation Pressure and equilibrium Get more gas molecules in backward direction 2NO2(g) N2O4 (g) Get fewer gas molecules in forward direction The higher the pressure, the more the reaction moves in the direction with fewer gas molecules. • Increasing the pressure will give more N2O4 • Decreasing pressure gives more NO2at equilibrium.

  18. Look at the reaction of nitrogen and hydrogen to form ammonia. Opposing changes in pressure Get more gas molecules in backward direction 3H2(g) + N2 (g)  2NH3 (g) Get fewer gas molecules in forward direction Which direction produces less gas molecules. In which direction do reactions move when compressed? Will high pressure give more or less NH3 in the equilbrium mixture? forward The side that has fewer gas molecules • Increasing the pressure will give more NH3 • Decreasing the pressure give less NH3 at equilibrium.. more

  19. This applies to reactions in solution. Concentration and equilibrium Increasing the concentration of a substance changes the equilibrium in the direction that uses up (decreases) the concentration of the substance added. • E.g. Bismuth chloride reacts with water to give a white precipitate of bismuth oxychloride. BiCl3 (aq) + H2O (l)  BiOCl (s)+ 2HCl (aq) 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.

  20. Chlorine gas reacts with iodine chloride (a brown liquid) converting it to iodine trichloride (a yellow solid). Opposing changes in concentration ICl (l) + Cl2 (g)  ICl3 (s) brown pale green yellow What effect will adding more chlorine have on the colour of the mixture in the U-tube? If the U-tube is turned on its side, chlorine gas pours out of the tube. In which way will this change the equilibrium? Produce more ICl3 and so more yellow solid. Produce less ICl and so more brown liquid.

  21. Dynamic equilibrium and change

  22. Contents Reversible reactions Comparing reactions Dynamic equilibrium Changing conditions The Haber process Summary activities

  23. The Haber process

  24. Is the forward reaction exothermic or endothermic? Will heating the mixture give an equilibrium mixture with more or less ammonia? Are there more gas molecules of reactant or product? Will raising the pressure give an equilibrium mixture with more or less ammonia? Making ammonia 3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol exothermic less reactant more

  25. What does the graph show about the effect of temperature on the Haber process? Suggest why a temperature of 400°C is chosen when a lower temperature gives an equilibrium mixture with greater % conversion to ammonia. The Haber process and temperature Reduces % conversion Hint: reaction rates? 3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol

  26. What does the graph show about the effect of pressure on the Haber process? Suggest why a pressure of 200 atm is chosen when a higher pressure gives an equilibrium mixture with greater % conversion to ammonia. The Haber process and pressure Increases % conversion Hint: costs? 3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol

  27. The Haber compromise 3H2(g) + N2 (g)  2NH3 (g) H=-92kJ/mol • The aim of the chemical industry is not to make chemicals. It is to make money! • If we use 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! • If we use very high pressures the cost of the equipment used increases drastically and there are also safety issues. Better 90% conversion at 200 atm than 95% conversion at 600 atm. • Unchanged reactants can always be recycled. • An iron catalyst is used to speed up the reaction, but it speeds up the reaction in both directions.

  28. The Haber process

  29. Stages of the Haber process

  30. Contents Reversible reactions Comparing reactions Dynamic equilibrium Changing conditions The Haber process Summary activities

  31. Glossary • closed system – A system in which reactants and products cannot be added or removed once the reaction has begun. • dynamic –An equilibrium in which the forward and backward reactions take place at the same rate, so no overall change takes place. • Haber process – The industrial-scale process for making ammonia from nitrogen and hydrogen. • irreversible – A reaction that is impossible or very difficult to reverse. • reversible – A reaction in which the product(s) can be turned back into the reactants.

  32. Anagrams

  33. Multiple-choice quiz

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