Unit 14 Equilibrium

1. Unit 14 Equilibrium

2. The Concept of Equilibrium • Ex) elevator, football game, moving walkway

3. 1. Reversible Reactions: Chemical reaction in which the products can regenerate the original reactants. A double arrow is used to express a reversible reaction. Ex) 2NO2(g) ↔ N2O4(g) reactants yield products ↔ or or Forward Rxn: 2NO2 → N2O4Reverse Rxn:N2O4 → 2NO2 2. Some reactions can reverse on their own, some under certain conditions, (temp, pressure or a catalyst) and some will not reverse. Ex) Single Replacement Rxns: 3CuCl2(aq) + 2Al(s)→ 3Cu(s) + 2AlCl3(aq)

4. 3. Chemical Equilibrium: State of a product’s and reactant’s concentrations remaining constant, not equal. This is because the rate of the forward and reverse reactions are equal. The symbol “[ ]” denotes concentration.

5. Reaction rates are affected by concentration. If the concentration of the reactant increases, the rate of the forward reaction will increase.

6. B. As the reaction proceeds, the concentration of the reactants decrease and the products increases. This will cause the forward reaction to decrease.

7. 4. Reaching equilibrium does not mean the reaction has stopped. It only means the rate of the forward and reverse reactions are equal. This is not a static equilibrium, but a dynamic equilibrium that stays constant over time.

8. Chem Study Equilibrium Goldfish Video See 2:45 min

9. A ↔ B AMOUNT Concentration becomes ConstantNOT Equal SPEED Rates become Equal

10. Reaction Rates & Equilibrium: A. Collision Theory: (Theoretical Model) 1. In order for reactions to occur between substances, the particles must collide. 2. According to the CollisionTheory, a successful collision occurs when A. the collision is energetic enough, and B. the particles collide with the correct orientation.

11. Ineffective Collision – Insufficient Energy - No Products 3. Effective collisions lead to the formation of products; ineffective collisions do not lead to the formation of products. Effective and Ineffective Collisions Effective Collision – Sufficient Energy – Forms Products

12. Effective collisions have enough energy, and the correct orientation to form products. Ineffective collisions revert to the original reactants.

13. 4. A successful collision results in bond breaking (endothermic) and bond forming (exothermic). 5. The minimum energy needed to produce an effective collision is called the activation energy for the reaction. It’s abbreviation is Ea.

15. Activation Energy

16. 6. A transitional structure results from a successful collision. The structure is present while old bonds are breaking and new bonds are forming. It is called an activated complex and is unstable and short-lived. It is neither reactantnor product. 7. A substance that increases the rate of a chemical reaction by providing a mechanism with a lower energy of activation is called a catalyst. 8. A system that has just one phase is called a homogeneous system, (g→g→g) while a system that has more than one phase is called a heterogeneous (s→l→g) system.

17. Endothermic 9. Energy Diagram: shows changes in energy during a reaction. Exothermic

18. Energy Diagram for a Chemical Rxn. Reaction Pathway Activated Complex Ea forward reaction (80 kJ) Ea reverse reaction (140 kJ) (kJ) Added Catalyst Energy of Reactants ∆H = -60 kJ (for.) +60 kJ (rev.) Potential Energy of Products Forward Reaction Reverse Reaction (Exo) (Endo)

19. Factors Affecting Reaction Rates 1. Chemical Kinetics is concerned with the rate at which a reaction occurs. 2. Reaction rate is a measure of the rate or speed of a chemical reaction. Reaction rate is determined by measuring the change in concentration of reactants and products over a certain amount of time. These reaction rates are determined experimentally. 3. Rate-influencing factors are those factors that affect rate of reactions by altering the frequency, orientation, or energy at which particles collide.

20. 4. According to the Collision Theory, the following 5 factors affect the rate of a reaction: 1. Nature of Reactants: A. Structure: complexity of bonds broken and formed & the orientation.

21. B. State: Homogeneous systems have reactants and products in the same state. Heterogeneous systems have reactants and products in different states. Homogeneous systems usually react faster and heterogeneous systems react slower. 2 liquids would usually react quickly Iron rusts in air very slowly

22. 2. Temperature: (Average kinetic energy!)Rule of thumb is that every 10°Cincreasein temperature doubles the reaction rate. Reaction Rate

23. 3. Concentration: increasedconcentration, increases# collisions. Aqueous solutions can change [conc] or molarity, gases can change pressure. Solids and pure liquids like watercannot change concentration.

24. 4. Surface Area: Increasedsurface area, increasesfrequency of collisions. This is especially true for heterogeneoussystems (s→l→g).

25. 5. Catalyst: Increases the rate of reaction without being consumed in the reaction. Catalysts speed up the reaction rate by lowering the activationenergy needed for the reaction to occur. Uncatalyzed reaction (slow) Catalyzed reaction (fast)

27. 5. Inhibitors:Decrease the rate of the reaction by taking the place of a reactant and stopping the reaction (opposite of a catalyst.)

28. Energy diagram involving a catalyst:

29. 11. What is in the reaction vessel at time = 0? H2 + N2 12. Write the forward reaction: 3H2 + N2 → 2NH3 13. What kind of reaction is the forward reaction? synthesis it slows down 14. Write the reverse reaction: 2NH3 → 3H2 + N2 15. What kind of reaction is the reverse reaction? decomposition it speeds up 16. Over time the concentration of which substance(s) decreases? H2 + N2 17. Over time the concentration of which substance(s) increases? NH3 18. Mark on the graph with a dashed line when equilibrium is reached. 19. At equilibrium which substance(s) is(are) present in the greater concentration? H2 + N2 20. Is the forward or reverse reaction favored? reverse (min)

31. Equilibrium Systems and Stress – Le Chatlier’s Principle 1. Le Chatlier’s Principle states that if a stress is imposed on a system at equilibrium, the equilibrium position will shift toward the direction that tends to minimize the stress. (Pure liquids and solids are not affected by changes in equilibrium.) 2. In chemistry, Le Chatlier’s Principle is used to manipulate the outcome of reversible reactions to maximize the amount of product produced by altering the temperature, pressure, or removing a reactant or product from a reaction. Crash Course in Chemistry – Equilibrium Video Khan Academy – LeChatlier’s Principle

32. The Haber Process Fritz Haber 1868-1934 Nobel Prize in Chemistry 1918 Known as “the Father of chemical warfare for weaponizing chlorine gas during WWI 3. An application of Le Chatlier’s Principle is The Haber Process. This was used by Germanyin WWI to produce ammonia(NH3) for nitrogen containing explosives. Today this process is used to create ammonia (NH3) for household cleaners and especially fertilizersallowing for a four-fold increase in food production from 1900 to 2000.

33. 4. There are 4 factors which determine whether a reaction favors making reactants “lies to the left”or favors making products “lies to the right”. A. Changes in Concentration -Adding/Removing Reactants -Adding/Removing Products B. Changes in Pressure C. Changes in Temperature

34. Changes in Concentration - (Reactants and Products) • Adding a substance to a system at equilibrium drives the system AWAY from that substance and makes more of the substance(s) on the opposite side. 2. Removing a substance from a system at equilibrium drives the system toward REPLACING that substance. Ex) N2O4(g) ↔ 2NO2(g) ↑ N2O4(g) shift right making more NO2 ↑ NO2(g) shift left making more N2O4 ↓ N2O4(g) shift left making more N2O4 ↓ NO2(g) shift right making more NO2

35. B. Changes in Pressure • Avogadro’s Law – Equal volumes of gases at the same temperature and pressure have the same number of molecules. (1 mole of any gas at STP = 22.4L = 6.02 x1023 particles) 2. According to Boyle’s Law an increase in pressure means a decrease in volume. So if the pressure is increased on a system at equilibrium, the side which occupies the lower volume will be favored. (inverse relationship)  3. If pressure is increased, the reaction will shift in the direction that produces fewer moles.

36. Ex) 2NO2(g) ↔ N2O4(g) ↑ pressure shift to right making more N2O4 2H2O2(g) ↔ 2H2O(g) + O2(g) ↑ pressure shift to left making more H2O2 H2(g) + Cl2(g) ↔ 2HCl(g) ↑ pressure shift to neither! making more same # of moles each side • Pressure changes will only affect gases!.

37. C. Changes in Temperature • Exothermic Reactionsrelease heat. If temperature is increased on this system then the reaction which absorbs or uses heat will increase. If temperature is decreased on an exothermic reaction, then the reaction which releases heat will decrease. Ex) H2(g) + I2(g) ↔ 2HI(g) + Heat Increasing the temperature will produce a higher yield of H2 + I2. Lowering the temperature will produce a higher yield of HI.

38. C. Changes in Temperature 2. Endothermic Reactionsabsorb heat. If temperature is increased on this system then the reaction which absorbs or uses heat will increase. If temperature is decreased on an endothermic reaction, then the reaction which absorbs heat will decrease. Ex) HEAT + NH4Cl(s) ↔ NH3(g) + HCl(g) Increasing the temperature will produce a higher yield of NH3 + HCl. Lowering the temperature will produce a higher yield of NH4Cl.

39. Conclusion: These examples have been illustrations of Le Chatlier’s Principle which states that a system at equilibrium, when subjected to a stress, will temporarily adjust itself to relieve the stress. This means that the shift to the right or left, or the increasedforward or reverse reaction, will be temporary and a new equilibrium will be reestablished.

40. Practice: • 2SO2(g) + O2(g) 2SO3(g) + heat What conditions of temperature and pressure favor high equilibrium concentrations of SO3? (high / low) pressure; (high / low) temperature

41. 2. 3H2(g) + N2(g) 2NH3(g) + heat The commercial production of ammonia uses the Haber Process which is expressed by the above equation. What condition of temperature and pressure will provide a maximum yield of NH3? (high / low) pressure: (high / low) temperature

42. 3. 4HCl(g) + O2(g) 2H2O(g) + 2Cl2(g) + heat Increasing the temperature of the reaction will (increase / decrease) the forward reaction. Decreasing the pressure on the system will (increase / decrease) the forward reaction.