1. Approaching Equilibrium Lesson 1

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

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

4. Simple reversible reactions Heating copper sulphate The change from blue hydrated copper sulphate to white anhydrous copper sulphate is one of the most commonly known reversible reactions. hydrated copper sulphate Heat anhydrous copper sulphate steam CuSO 4.5H20  CuSO 4 + 5H 2 O

5. Heating ammonium chloride Ammonium salts are made by reacting ammonia with an acid but some of these salts will decompose back into reactants when heated. Heat Heat makes the solid disappear as it changes into gases. Solid reappears as it changes back again in the cool part of the tube. Simple reversible reactions ammonium chloride ammonia hydrogen chloride NH 4 Cl(s)NH3(g) + HCl(g)

6. Approaching Equilibrium Many chemical reactions are reversible if the activation energy is low and the system is closed. Reactants ⇌ Products

8. Approaching Equilibrium Many chemical reactions are reversible if the activation energy is low. Reactants ⇌ Products Forward Reaction is left to right Reverse Reaction is right to left

9. Dynamic Equilibrium Recall that we proposed that reverse reactions could occur with separate activation energies.

10. 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 both reactants and products present in unchanging amounts. dynamic equilibrium.This is called a dynamic equilibrium. A B A B these combine these decompose Dynamic equilibrium

11. Lets see what happens to some reactants if they are placed in a reaction vessel and allowed to react.

12. Reactants Products Approaching Equilibrium Forward Rate Reverse Rate

13. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

14. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

15. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

16. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

17. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium

18. Reactants Products Reverse Rate Forward Rate Are Equal Approaching Equilibrium

19. Reactants Products Reverse Rate Forward Rate Are Equal Are Constant Approaching Equilibrium

20. Graphing the Approach to Equilibrium Concentration Time reactant product

21. Graphing the Approach to Equilibrium Concentration Time reactant product

22. Graphing the Approach to Equilibrium Concentration Time Forward rate Reverse rate Overall rate

24. In general terms: At dynamic equilibrium: –The concentrations of A and B are constant –The forward and reverse reaction rates are equal

25. [ ] of A decreases while [ ] of B increases till equilibrium is reached. Equilibrium is reached when rate of forward reaction is the same as the reverse reaction.

26. Approaching Equilibrium Reactant concentrations start high and decrease as the reaction proceeds. The forward rate, which depends on collisions of the reactants, also decreases. Product concentrations start at zero and increase as the reaction proceeds. The reverse rate, which depends on collisions of the products, also increases. Eventually the forward rate is equal to the reverse rate and the concentrations are constant. This is equilibrium.

27. Characteristics of a System at Equilibrium The Forward rate = The Reverse rate The Reactant and Product concentrations are constant The Macroscopic (observable) properties are constant The system is Dynamic as the forward and reverse reactions continue. The equilibrium can be approached from starting with reactants or starting with products.

28. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

29. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

30. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

31. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

32. Reactants Products Reverse Rate Forward Rate Approaching Equilibrium from Products

33. Reactants Products Reverse Rate Forward Rate Are Equal Are Constant Same as before Approaching Equilibrium from Products

34. If you start with products all aspects of the approach to equilibrium are reversed Reactants and Forward Rate increases Products and Reverse Rate decrease

35. Conditions Necessary for Equilibrium Ea is low so the reaction is reversible Constant temperature Closed system

36. Discussion Time Characteristics of equilibrium A tell B ---------------------------, B rephrase A says: That’s exactly what I mean, Or rephrase his/her statement Then continue with next statement

37. Summary of characteristics of Equilibrium Closed system Ea is low so the reaction is reversible Constant temperature Constant Concentration Forward rate = reverse rate No visible/measurable/Macroscopic Change like colour and mass Dynamic i.e Reactions continue to take place Can be approached from reactants or products