1. Approaching
Equilibrium
Lesson 1

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

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

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.
Heat
hydrated copper sulphate
anhydrous copper sulphate
steam
CuSO4.5H  CuSO H2O

5. Simple reversible reactions
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.
ammonium chloride
ammonia
hydrogen chloride
NH4Cl(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. Dynamic equilibrium
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.
This is called a dynamic equilibrium. A B A B
these combine
these decompose

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

12. Approaching Equilibrium
Reactants
Products
Forward Rate
Reverse Rate

13. Approaching Equilibrium
Reactants
Products
Forward Rate
Reverse Rate

14. Approaching Equilibrium
Reactants
Products
Forward Rate
Reverse Rate

15. Approaching Equilibrium
Reactants
Products
Forward Rate
Reverse Rate

16. Approaching Equilibrium
Reactants
Products
Forward Rate
Reverse Rate

17. Approaching Equilibrium
Reactants
Products
Forward Rate
Reverse Rate

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

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

20. Graphing the Approach to Equilibrium
product
 Concentration
reactant
Time

21. Graphing the Approach to Equilibrium
reactant
 Concentration
product
Time

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

24. At dynamic equilibrium: The concentrations of A and B are constant
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. Approaching Equilibrium from Products
Reactants
Products
Forward Rate
Reverse Rate

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

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

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

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

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

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

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

36. Characteristics of equilibrium
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