1. Dynamic Equilibrium What does mean?
What happens when a system is in a state of equilibrium?
Name 2 factors that affect the position of equilibrium.
How can we use a treadmill to help understand dynamic equilibrium?

2. Chemical reactions
Most chemical reactions are not reversible. For example the combustion of methane.
CH4(g) + O2(g)   CO2(g) + 2H2O(l)
It is almost impossible to return the carbon dioxide and water to the original methane and oxygen.

3. Reversible
Some chemical reactions, however, will go backwards and forwards depending on the conditions.
CoCl2·6H2O(s)  CoCl2(s) + 6H2O(l)
pink blue

4. How do we write them down?
This is the symbol for used for reversible reactions.
CoCl2·6H2O(s) CoCl2(s) + 6H2O(l)

5. What is equilibrium?
Reversible reactions reach a balance point, where the amount of reactants and the amount of products formed remains constant.
This is called a position of equilibrium.

6. General equation
This is the equation for a general reaction which has reached dynamic equilibrium:
How did it reach this balanced state?

7. At the beginning of the reaction, the concentrations of A and B were at their maximum.
That means that the rate of the reaction was at its fastest.
A B  C D

8. As A and B react, their concentrations fall
As A and B react, their concentrations fall. That means that they are less likely to collide and react, and so the rate of the forward reaction falls as time goes on.

9. In the beginning, there isn't any C and D, so there can't be any reaction between them.
As time goes on, though, their concentrations in the mixture increase and they are more likely to collide and react.
With time, the rate of the reaction between C and D increases

10. Eventually, the rates of the two reactions will become equal.
A and B will be converting into C and D at exactly the same rate as C and D convert back into A and B again

11. At this point there won't be any further change in the amounts of A, B, C and D in the mixture.
As fast as something is being removed, it is being replaced again by the reverse reaction.
We have reached a position of dynamic equilibrium.

12. Dynamic equilibrium is like running up an escalator at the same speed as the escalator is moving down.

13. Dynamic Equilibrium.
In dynamic equilibrium the forward and backwards reactions continue at equal rates so the concentrations of reactants and products do not change.
On a molecular scale there is continuous change.
On the macroscopic scale nothing appears to be happening. The system needs to be closed – isolated from the outside world.

14. Demo

15. Dynamic equilibrium
The concentration of reactants and products remains the same.
The rate of the forward reaction is equal to the rate of the reverse reaction.
Only applies if you have a closed system.

16. Factors affecting the position of equilibrium
In a closed system the following factors can affect the position of equilibrium:
Concentration of reactants or products
Pressure in reactions involving gases
Temperature

17. Le Chatelier’s principle
When a system in dynamic equilibrium is subjected to a change, the position of equilibrium will shift to minimise the change.

18. The effect of concentration
A B C D
What would happen if the concentration of a reactant was increased?

19. The effect of concentration
A B C D
Increasing the concentration of a reactant causes the forward reaction to be favoured (to remove the reactant added)
The position of equilibrium will therefore shift to the right, forming more products.

20. The effect of pressure
If gases are present changing the pressure will alter the position of equilibrium.
Increasing the pressure will cause the position of equilibrium to shift to the side with fewer moles of gas. This will decrease the pressure and allow equilibrium to be restored.
In this equation increasing the pressure will cause the equilibrium to shift to the right, as there are 4 moles of gas on the left and 2 moles on the right.

21. The effect of temperature
A B C D
ΔH = - 80kJmol-1
What would happen if the temperature was increased?

22. The effect of temperature
A B C D
ΔH = - 80kJmol-1
For the reaction above the forward reaction is exothermic and the backward reaction endothermic.
Increasing the temperature causes the position of equilibrium to move to decrease the temperature.
The system opposes the change by taking in heat, the endothermic reaction is favoured and the position of equilibrium moves to the left.

23. Catalysts
Catalysts do not alter the position of equilibrium or alter the composition of an equilibrium system.
Catalysts speed up the rate of the forward and backward reactions equally.
Catalysts speed up the rate at which equilibrium is reached.

24. Equilibrium questions
2A (g) + B (s) C (g) + D (g)
ΔH = +55kJmol-1
What would happen to the position of equilibrium in each of these cases and why?
The concentration of D was increased?
The pressure was decreased?
The temperature was decreased?
A catalyst was added?

25. Practical
[CoCl4]2-(aq) + 6H2O(l) (blue) [Co(H2O)6]2+(aq) + 4Cl- (pink)
Colour in ice
Colour in hot water
Follow the instructions on the sheet and record your observations in a table
Is the forward reaction exothermic or endothermic?
Answer the questions on the sheet