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2. TOPIC 8 Review Book Chapter 19 Textbook
EQUILIBRIUM
TOPIC 8 Review Book
Chapter 19 Textbook

3. Objectives Reversible reactions Equilibrium – classification
Le Chatelier’s Principle

4. When the system reaches the equilibrium no net change can be observed.
Reversible Reactions
Some reactions occurs simultaneously, forming and decomposing the product.
The two equations can be combined into one, by using a double arrow, which tells us that it is a reversible reaction: 2SO2(g) + O2(g) ↔ 2SO3(g)
When the system reaches the equilibrium no net change can be observed.

5. the rates of the forward and reverse reactions are equal
Equilibrium is a state in which there are no observable changes as time goes by.
Chemical equilibrium is achieved when:
the rates of the forward and reverse reactions are equal
the concentrations of the reactants and products remain constant
Chemical equilibrium
N2O4 (g)
2NO2 (g)
14.1

6. Physical equilibrium Phase Equilibrium
When a substance is changing phase an equilibrium between phases can be established
H2O (s)   H2O (l)

7. Physical Equilibrium

8. Solution Equilibrium
When a solution is saturated, if there is solid present, the solid solute and the solute in solution are at equilibrium.

9. The Concept of Equilibrium
[A] = concentration of A
[B] = concentration of B
As the reaction progresses
[A] decreases to a constant,
[B] increases from zero to a constant.
When [A] and [B] are constant, equilibrium is achieved.

10. Le Chatelier’s Principle
The French chemist Henri Le Chatelier ( ) studied how the equilibrium position shifts as a result of changing conditions

11. Le Chatelier’s Principle
IF A STRESS (CHANGE) IS APPLIED TO A SYSTEM AT EQUILIBRIUM THE SYSTEM WILL REACT IN THE DIRECTION THAT COUNTERACTS THE STRESS.
In other words : the system is a REBEL!

12. Le Chatelier’s Principle
What items did he consider to be stress on the equilibrium?
Concentration
Temperature
Pressure
Each of these will now be discussed in detail

13. Stress I - CONCENTRATION
Case 1
Increasing the concentration of reactants
Consequence: more products will be formed.
Equilibrium shifts to the right

14. Case 2 – Increasing concentration of products
Consequence
The system will react to decrease the concentration of products by forming more reactants.
Equilibrium will shift to the left or the reactants.

15. Case 3: Decreasing the concentration of reactants
Consequence
The system will react to increase the concentration of reactants by using more products.
Equilibrium will shift to the left of the reactants

16. Case 4: Decreasing the concentration of products
Consequence: more products will be formed.
Equilibrium shifts to the right or the products will be favored.
This is a way to make a reaction go to completion. As products form they are taken away!

17. Stress II - Temperature
If temperature is increased the system will favor the reaction that lowers the temperature (absorbing heat) then the ENDOTHERMIC RECTION will be favored.
A decrease in temperature favors the exothermic reaction
C + O2(g) → CO2(g) kJ

18. Increase in temperature favors endothermic reaction
Decrease in temperature favors exothermic reaction

19. ENDOTHERMIC APPARATUS
Air conditioners absorb heat of a room and they LOWER the temperature of the room they are in.

20. EXOTHERMIC APPARATUS
Heaters are exothermic, they release heat into a room and increase the temperature of the room they are in.

21. Stress III - Pressure
– Changes in pressure will only effect gaseous equilibria.
Increasing the pressure will favor side with the smaller volume
N2(g) + 3H2(g) ↔ 2NH3(g)
this equilibrium shifts to the right with an increase in pressure because the product side occupies a smaller volume
4 mol of gas in reactants vs 2 in product

22. Effect of a Catalyst
The addition of a catalyst changes the rate of both forward and reverse reactions equally. It causes the equilibrium to be established more quickly but it does not change the equilibrium concentrations.

23. Heat + 2NH3(g)  N2(g) + 3H2(g)
1.- Change : increase in [N2]
What is the effect on the concentration of
a. [ NH3 ]
b. [ H2 ]

24. Heat + 2NH3(g)  N2(g) + 3H2(g)
2 Change: increase in temperature
What is the effect on the concentration of
a. [ N2 ]
b. [ NH3]

25. Heat + 2NH3(g)  N2(g) + 3H2(g)
3: Increase in pressure
What is the effect on the
a. number of moles of N2
B. number of NH3

26. Is the forward reaction endo or exo?
The Concept of Equilibrium
Consider colorless frozen N2O4. At room temperature, it decomposes to brown NO2:
N2O4(g) <- > 2NO2(g).
Clear brown
At some time, the color stops changing and we have a mixture of N2O4 and NO2.
Challenge question
Is the forward reaction endo or exo?

27. The Concept of Equilibrium
As the substance warms it begins to decompose:
N2O4(g)  2NO2(g)
When enough NO2 is formed, it can react to form N2O4:
2NO2(g)  N2O4(g).
At equilibrium, as much N2O4 reacts to form NO2 as NO2 reacts to re-form N2O4
The double arrow implies the process is dynamic.

28. Smog…

29. Demonstration- Do now Copy reaction in your notes and predict what would be the consequence of each change. Then record your observations
Co(H2O) Cl- <--> CoCl H2O
Pink Blue
Change 1 – Increase [Cl-] by adding HCl
Change 2- Decrease [Cl-] by adding AgNO3

30. Co(H2O)6 2+ + 4 Cl- <--> CoCl4 2-+ 6H2O Pink Blue
DH > 0 Endothermic Reaction!
Change 3 – Increasing temperature
Consequence
Change 4 – Decreasing temperature

31. Answers to MC questions handout Le Chatelier (8741)
1 B
2 B
3 D
4 B
5 B
6 C
7 A
8 D
9 B
10 B

32. SAMPLE ANSWERS TO LE CHATELIER’S PRINCIPLE
1 Equilibrium shifts toward the fewer number of moles of gas . Or
The reaction shifts to the side with the smaller volume.
2. Removing CO2 shifts the equilibrium towards the right lowering [ H2CO3 ]

33. 3.-An increase in Temperature favors the endothermic reaction, which produces SO2
4.- A higher [02] causes more collisions, more effective collisions form more product, decreasing [SO2]
6.- As T increases the solubility of the gas decreases, the gas comes out the soda.

34. 7. As the pressure decreases the solubility decrease, and the gas comes out of the soda.
8.- Increasing temperature favors the endothermic reaction.
9.- The rate of dissolving KNO3 is equal to the rate of recrystallizing KNO3

35. 10. a) If T increases N2 increases because the endothermic reaction will be favored.
b) If pressure is increased H2 will decreased, because the equilibrium moves to the side with the smaller volume.
C) No effect because the catalyst does not affect the equilibrium position. It makes both (forward and reverse reaction ) faster.

36. Review Book Answers P146 2 4 3 1 3 1 4

37. Review Book p Entropy 1 2 4 3

38. ENDOTHERMIC REACTION
NH4SCN(s) + Ba(OH)2 (s) NH3(g)+ H2O(l) Ba2+(aq)+NH4 +(aq)
From solid to liquid entropy increases

39. Why does any change occur?
Factors that determine if a physical or a chemical change would take place:
Tendency to lower the energy of the system.
Changes tend to occur if they result in a system that is more stable. Stability implies low energy.

40. 1.-Energy factor - ENTHALPY
All exothermic reactions are favorable in terms of energy, because their products have less energy than the reactants and therefore are more stable.
Then if D H < 0 the change is favorable in terms of energy
Delta H = Heat of reaction = Enthalpy
D H

41. 2.- Tendency to greater disorder. ENTROPY
There is a tendency in nature to change to a state of greater randomness or disorder.
Entropy is a measure of the disorder or randomness in a system.
A system with high entropy is “messy” and has a lack of structure.

42. Entropy = Disorganization, chaos, randomness
High entropy
Randomness, messiness
Low Entropy
Organized, neat

43. Gas = high entropy
Liquid = less entropy
Solid = little entropy

44. Spontaneous change A change that occurs naturally.
When a change results in an stable and more disorganized product the reaction will be spontaneous.
The tendencies that favor a spontaneous reaction are
Low enthalpy and high entropy.

45. Demonstration- write your observations in your notes
Fe3+ (aq) + SCN- (aq) <--> FeSCN2+ (aq)
Pale yellow dark red
In your notebook predict what would happen to the equilibrium if we Decrease [Fe3+]