Introductory Chemistry , 3 rd Edition Nivaldo Tro

1. Introductory Chemistry, 3rd EditionNivaldo Tro Chapter 15 Chemical Equilibrium Basic Principles of Chemistry Online Southeast Missouri State University Cape Girardeau, MO 2009, Prentice Hall

2. Equilibrium vs. Disequilibrium • When systems are at equilibrium with their surroundings, their conditions are the same as the surroundings and they stay that way. • When systems are in disequilibrium with their surroundings, their conditions are not the same as the surroundings. • Systems that are in disequilibrium tend to change until they reach equilibrium with their surroundings. • Living things are in controlled disequilibrium with their environment—they are not at the same conditions as the environment and do not tend to change toward those conditions. Tro's Introductory Chemistry, Chapter 15

3. Reaction Rates • Some chemical reactions proceed rapidly. • Like the precipitation reactions studied in Chapter 7 where the products form practically the instant the two solutions are mixed. • Other reactions proceed slowly. • Like the decomposition of dye molecules of a sofa placed in front of a window. • The rate of a reaction is measured in the amount of reactant that changes into product in a given period of time. • Generally moles of reactant used per second. • Like miles per hour. • Chemists study ways of controlling reaction rates. Tro's Introductory Chemistry, Chapter 15

4. Reaction Rates, Continued Initially, only reactants are present After 15 seconds, the left reaction is 60% complete, but the right reaction is only 20% complete After 30 seconds, the left reaction is complete, whereas the right reaction is only 40% done. After 45 seconds, the right reaction is still not complete Tro's Introductory Chemistry, Chapter 15

5. 2 N2O5 (g) → 4 NO2(g) + O2(g) Over time, the concentrations of reactants decrease as products increase. Tro's Introductory Chemistry, Chapter 15

6. 2 N2O5 (g) → 4 NO2(g) + O2(g):Rate vs. Time Because reactant concentrations decrease, the rates of reactions slow down over time. Tro's Introductory Chemistry, Chapter 15

7. Collision Theory • In order for a reaction to take place, the reacting molecules must collide with each other. • Once molecules collide they may react together or they may not, depending on two factors: • Whether the collision has enough energy to “start to break the bonds holding reactant molecules together." • Whether the reacting molecules collide in the proper orientation for new bonds to form. Tro's Introductory Chemistry, Chapter 15

8. Effective Collisions • Collisions in which these two conditions are met (and therefore the reaction occurs) are called effective collisions. • The higher the frequency of effective collisions, the faster the reaction rate. • There is a minimum energy needed for a collision to be effective. We call this the activation energy. • The lower the activation energy, the faster the reaction will be. Tro's Introductory Chemistry, Chapter 15

9. Effective Collisions:Kinetic Energy Factor For a collision to lead to overcoming the energy barrier, the reacting molecules must have sufficient kinetic energy so that when they collide, it can form the activated complex. Tro's Introductory Chemistry, Chapter 15

10. Effective Collisions:Orientation Effect Tro's Introductory Chemistry, Chapter 15

11. Reaction Energy Diagram Tro's Introductory Chemistry, Chapter 15

12. Factors Effecting Reaction Rate:Reactant Concentration • The higher the concentration of reactant molecules, the faster the reaction will generally go. • Increases the frequency of reactant molecule collisions. • Since reactants are consumed as the reaction proceeds, the speed of a reaction generally slows over time. Tro's Introductory Chemistry, Chapter 15

13. Effect of Concentration on Rate Low concentrations of reactant molecules leads to fewer effective collisions, therefore a slower reaction rate. High concentrations of reactant molecules lead to more effective collisions, therefore a faster reaction rate. Tro's Introductory Chemistry, Chapter 15

14. Factors Effecting Reaction Rate:Temperature • Increasing the temperature increases the number of molecules in the sample with enough energy so that their collisions can overcome the activation energy. • Increasing the temperature also increases the frequency of collisions. • So the rate increases because the frequency of effective collisions increases. • Both these mean that increasing temperature increases the reaction rate. Tro's Introductory Chemistry, Chapter 15

15. Effect of Temperature on Rate Low temperatures lead to fewer molecules with enough energy to overcome the activation energy, and less frequent reactant collisions, therefore a slower reaction rate High temperatures lead to more molecules with enough energy to overcome the activation energy, and more frequent reactant collisions, therefore, a faster reaction rate.

16. Reaction Dynamics • If the products of a reaction are removed from the system as they are made, then a chemical reaction will proceed until the limiting reactants are used up. • However, if the products are allowed to accumulate; they will start reacting together to form the original reactants. This is called the reverse reaction. • Reactions that can proceed in both the forward and reverse directions are called reversible reactions. Tro's Introductory Chemistry, Chapter 15

17. Reaction Dynamics, Continued • The forward reaction slows down as the amounts of reactants decreases. • At the same time, the reverse reaction speeds up as the concentration of the products increases. • Eventually, the forward reaction is using reactants and making products as fast as the reverse reaction is using products and making reactants. This is called chemical equilibrium. • Dynamic equilibrium is reached when the rates of two opposite processes are the same. Tro's Introductory Chemistry, Chapter 15

18. Chemical Equilibrium • When a reaction reaches equilibrium, the amounts of reactants and products in the system stay constant. • The forward and reverse reactions still continue. • Because they go at the same rate, the amounts of materials do not change. Tro's Introductory Chemistry, Chapter 15

19. Equilibrium As the reaction proceeds, the forward reaction slows down as the reactants get used up. At the same time, the reverse reaction speeds up as product concentration increases. Initially, we only have reactant molecules in the mixture. The reaction can only proceed in the forward direction, making products. Once equilibrium is established, the concentrations of the reactants and products in the final mixture do not change, (unless conditions are changed). Eventually, the forward and reverse rates are equal. At this time equilibrium is established.

20. Rate forward Rate reverse Equilibrium, Continued Initially, only the forward reaction takes place. As the forward reaction proceeds it makes products and uses reactants. Because the reactant concentration decreases, the forward reaction slows. As the products accumulate, the reverse reaction speeds up. Once equilibrium is established, the forward and reverse reactions proceed at the same rate, so the concentrations of all materials stay constant. Eventually, the reaction proceeds in the reverse direction as fast as it proceeds in the forward direction. At this time equilibrium is established. Rate Time Tro's Introductory Chemistry, Chapter 15

21. Hypothetical Reaction2 RedBlue The reaction slows over time, but the red molecules never run out! At some time between 100 and 110 sec, the concentrations of both the red and the blue molecules no longer change— equilibrium has been established. Notice that equilibrium does not mean that the concentrations are equal! Once equilibrium is established, the rate of red molecules turning into blue is the same as the rate of blue molecules turning into red.

22. Hypothetical Reaction2 RedBlue, Continued Tro, Chemistry: A Molecular Approach

23. Equilibrium  Equal • The rates of the forward and reverse reactions are equal at equilibrium. • But that does not mean the concentrations of reactants and products are equal. • Some reactions reach equilibrium only after almost all the reactant molecules are consumed—we say the position of equilibrium favors the products. • Other reactions reach equilibrium when only a small percentage of the reactant molecules are consumed—we say the position of equilibrium favors the reactants. Tro's Introductory Chemistry, Chapter 15

24. An Analogy: Population Changes When Narnians feel overcrowded, some will emigrate to Middle Earth. However, as time passes, emigration will occur in both directions at the same rate, leading to populations in Narnia and Middle Earth that are constant, though not necessarily equal. Tro's Introductory Chemistry, Chapter 15

25. Disturbing and Re-EstablishingEquilibrium • Once a reaction is at equilibrium, the concentrations of all the reactants and products remain the same. • However, if the conditions are changed, the concentrations of all the chemicals will change until equilibrium is re-established. • The new concentrations will be different, but the equilibrium constant will be the same. • Unless you change the temperature. Tro's Introductory Chemistry, Chapter 15

26. Le Châtelier’s Principle • Le Châtelier’s principle guides us in predicting the effect on the position of equilibrium when conditions change. • “When a chemical system at equilibrium is disturbed, the system shifts in a direction that will minimize the disturbance.” Tro's Introductory Chemistry, Chapter 15

27. An Analogy: Population Changes When an influx of population enters Middle Earth from somewhere outside Narnia, it disturbs the equilibrium established between Narnia and Middle Earth. When the populations of Narnia and Middle Earth are in equilibrium, the emigration rates between the two states are equal so the populations stay constant. The result will be people moving from Middle Earth into Narnia faster than people moving from Narnia into Middle Earth. This will continue until a new equilibrium between the populations is established, However, the new populations will have different numbers of people than the old ones.

28. The Effect of Concentration Changes on Equilibrium • Adding a reactant will decrease the amounts of the other reactants and increase the amount of the products until a new position of equilibrium is found. • That has the same Keq. • Removing a product will increase the amounts of the other products and decrease the amounts of the reactants. • You can use to this to drive a reaction to completion! • Remember: Adding more of a solid or liquid does not change its concentration and, therefore, has no effect on the equilibrium. Tro's Introductory Chemistry, Chapter 15

29. The Effect of Concentration Changes on Equilibrium, Continued When NO2 is added, some of it combines to make more N2O4. Tro's Introductory Chemistry, Chapter 15

30. The Effect of Concentration Changes on Equilibrium, Continued When N2O4 is added, some of it decomposes to make more NO2. Tro's Introductory Chemistry, Chapter 15

31. Practice—Predict the Effect on the Equilibrium When the Underlined Substance Is Added to the Following Systems: • 2 CO2(g) Û 2 CO(g) + O2(g) • BaSO4(s) ÛBa2+(aq) + SO42-(aq) • CH4(g) + 2 O2(g) Û CO2(g) + 2 H2O(l) Tro's Introductory Chemistry, Chapter 15

32. Practice—Predict the Effect on the Equilibrium When the Underlined Substance Is Added to the Following Systems, Continued: • 2 CO2(g) Û 2 CO(g) + O2(g) • BaSO4(s) ÛBa2+(aq) + SO42-(aq) • CH4(g) + 2 O2(g) Û CO2(g) + 2 H2O(l) Shift right, removing some of the added CO2 and increasing the concentrations of CO and O2. Shift left, removing some of the added Ba2+ and reducing the concentration of SO42-. Shift right, removing some of the added CO2 and decreasing the O2, while increasing the concentration of CO2.

33. Effect of Volume Changeon Equilibrium • For solids, liquids, or solutions, changing the size of the container has no effect on the concentration. • Changing the volume of a container changes the concentration of a gas. • Same number of moles, but different number of liters, resulting in a different molarity. Tro's Introductory Chemistry, Chapter 15

34. Effect of Volume Changeon Equilibrium, Continued • Decreasing the size of the container increases the concentration of all the gases in the container. • This increases their partial pressures. • If their partial pressures increase, then the total pressure in the container will increase. • According to Le Châtelier’s principle, the equilibrium should shift to remove that pressure. • The way to reduce the pressure is to reduce the number of molecules in the container. • When the volume decreases, the equilibrium shifts to the side with fewer molecules. Tro's Introductory Chemistry, Chapter 15

35. When the pressure is decreased by increasing the volume, the position of equilibrium shifts toward the side with the greater number of molecules—the reactant side. Since there are more gas molecules on the reactants side of the reaction, when the pressure is increased the position of equilibrium shifts toward the products. The Effect of Volume Change on Equilibrium, Continued Tro's Introductory Chemistry, Chapter 15

36. Practice—Predict the Effect on the Equilibrium When the Volume Is Reduced. • 2 CO2(g) Û 2 CO(g) + O2(g) • BaSO4(s) Û Ba2+(aq) + SO42-(aq) • CH4(g) + 2 O2(g) Û CO2(g) + 2 H2O(l) Tro's Introductory Chemistry, Chapter 15

37. Practice—Predict the Effect on the Equilibrium When the Volume Is Reduced, Continued. • 2 CO2(g) Û 2 CO(g) + O2(g) • BaSO4(s) Û Ba2+(aq) + SO42-(aq) • CH4(g) + 2 O2(g) Û CO2(g) + 2 H2O(l) Shift left because there are fewer gas molecules on the reactant side than on the product side. No effect because none of the substances are gases. Shift right because there are fewer gas molecules on the product side than on the reactant side.

38. The Effect of Temperature Changes on Equilibrium • Exothermic reactions release energy and endothermic reactions absorb energy. • If we write heat as a product in an exothermic reaction or as a reactant in an endothermic reaction, it will help us use Le Châtelier’s principle to predict the effect of temperature changes. • However, heat is not matter and not written in a proper equation. Tro's Introductory Chemistry, Chapter 15

39. The Effect of Temperature Changes on Equilibrium for Exothermic Reactions • For an exothermic reaction, heat is a product. • Increasing the temperature is like adding heat. • According to Le Châtelier’s principle, the equilibrium will shift away from the added heat. • The concentrations of C and D will decrease and the concentrations of A and B will increase. • The value of Keqwill decrease. • How will decreasing the temperature effect the system? aA + bB  cC + dD + heat Tro's Introductory Chemistry, Chapter 15

40. The Effect of Temperature Changes on Equilibrium for Endothermic Reactions • For an endothermic reaction, heat is a reactant. • Increasing the temperature is like adding heat. • According to Le Châtelier’s principle, the equilibrium will shift away from the added heat. • The concentrations of C and D will increase and the concentrations of A and B will decrease. • The value of Keqwill increase. • How will decreasing the temperature effect the system? Heat + aA + bB  cC + dD Tro's Introductory Chemistry, Chapter 15

41. The Effect of Temperature Changes on Equilibrium Tro's Introductory Chemistry, Chapter 15

42. Practice—Predict the Effect on the Equilibrium When the Temperature Is Reduced. • Heat + 2 CO2(g) Û 2 CO(g) + O2(g) • BaSO4(s) Û Ba2+(aq) + SO42-(aq) (endothermic) • CH4(g) + 2 O2(g) Û CO2(g) + 2 H2O(l) (exothermic) Tro's Introductory Chemistry, Chapter 15

43. Practice—Predict the Effect on the Equilibrium When the Temperature Is Reduced, Continued. • Heat + 2 CO2(g) Û 2 CO(g) + O2(g) • Heat + BaSO4(s) Û Ba2+(aq) + SO42-(aq) • CH4(g) + 2 O2(g) Û CO2(g) + 2 H2O(l) + Heat Shift left, reducing the value of Keq. Shift left, reducing the value of Keq. Shift right, increasing the value of Keq.