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2. EQUILIBRIUM TOPIC 8 Review Book Chapter 19 Textbook

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

4. 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. N2O4(g) 2NO2(g) 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 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 (1850-1936) 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) + 393.5 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. 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 nowCopy reaction in your notes and predict what would be the consequence of each change.Then record your observations Co(H2O)62+ + 4 Cl- <--> CoCl4 2-+ 6H2O Pink Blue Change 1 – Increase [Cl-] by adding HCl Change 2- Decrease [Cl-] by adding AgNO3

30. Co(H2O)62+ + 4 Cl- <--> CoCl4 2-+ 6H2OPinkBlue DH > 0 Endothermic Reaction! Change 3 – Increasing temperature Consequence Change 4 – Decreasing temperature Consequence

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

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 • 2 • 4 • 2 • 3 • 4 • 3 • 4 • 1 • 3 • 1 • 1 • 3 • 4 • 1 • 3 • 3 • 4

37. Review Book p 149 - Entropy • 1 • 1 • 2 • 4 • 2 • 2 • 2 • 2 • 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+]