Chemical Equilibrium: Reversible Reactions and Equilibrium Expression

1. Chapter 18 Preview Lesson Starter Objectives Reversible Reactions Equilibrium, a Dynamic State The Equilibrium Expression

2. Section 1 The Nature of Chemical Equilibrium Chapter 18 Lesson Starter • List two everyday processes that can easily be reversed and two that cannot. • The freezing of water and the melting of ice can be reversed • The cooking of an egg or the lighting of a match cannot be reversed. • For the reversible processes, describe the conditions that favor the process going in a particular direction. • Low temperature favors freezing, and high temperature favors melting.

3. Section 1 The Nature of Chemical Equilibrium Chapter 18 Objectives Define chemical equilibrium. Explain the nature of the equilibrium constant. Write chemical equilibrium expressions and carry out calculations involving them.

4. Section 1 The Nature of Chemical Equilibrium Chapter 18 Reversible Reactions Theoretically, every reaction can proceed in two directions, forward and reverse. Essentially all chemical reactions are considered to be reversible under suitable conditions. A chemical reaction in which the products can react to re-form the reactants is called areversible reaction.

5. Section 1 The Nature of Chemical Equilibrium Chapter 18 Reversible Reactions, continued • A reversible chemical reaction is in chemical equilibriumwhen the rate of its forward reaction equals the rate of its reverse reaction and the concentrations of its products and reactants remain unchanged. • A a state of dynamic equilibrium has been reached when the amounts of products and reactants remain constant. • Both reactions continue, but there is no net change in the composition of the system.

6. Section 1 The Nature of Chemical Equilibrium Chapter 18 Equilibrium and Vapor Pressure Click below to watch the Visual Concept. Visual Concept

7. Section 1 The Nature of Chemical Equilibrium Chapter 18 Reversible Reactions, continued The chemical equation for the reaction at equilibrium is written using double arrows to indicate the overall reversibility of the reaction.

8. Section 1 The Nature of Chemical Equilibrium Chapter 18 Chemical Equilibrium Click below to watch the Visual Concept. Visual Concept

9. Section 1 The Nature of Chemical Equilibrium Chapter 18 Equilibrium, a Dynamic State Many chemical reactions are reversible under ordinary conditions of temperature and concentration. They will reach a state of equilibrium unless at least one of the substances involved escapes or is removed from the reaction system. When the products of the forward reaction are favored, there is a higher concentration of products than of reactants at equilibrium. • The equilibrium “lies to the right”

10. Section 1 The Nature of Chemical Equilibrium Chapter 18 Equilibrium, a Dynamic State, continued When the products of the reverse reaction are favored, there is a higher concentration of reactants than of products at equilibrium. • the equilibrium “lies to the left” • In other cases, both forward and reverse reactions occur to nearly the same extent before chemical equilibrium is established. • Neither reaction is favored, and considerable concentrations of both reactants and products are present at equilibrium.

11. Section 1 The Nature of Chemical Equilibrium Chapter 18 Equilibrium, a Dynamic State, continued products of the forward reaction favored, lies to the right products of the reverse reaction favored,lies to the left Neither reaction is favored

12. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression Initially, the concentrations of C and D are zero and those of A and B are maximum. Over time the rate of the forward reaction decreases as A and B are used up. The rate of the reverse reaction increases as C and D are formed. When these two reaction rates become equal, equilibrium is established.

13. Reaction Rate Over Time for an Equilibrium System Section 1 The Nature of Chemical Equilibrium Chapter 18

14. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued • After equilibrium is reached, the individual concentrations of A, B, C, and D undergo no further change if conditions remain the same. • A ratio of their concentrations should also remain constant. • The equilibrium constant is designated by the letter K.

15. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued • The constant K is independent of the initial concentrations. • K is dependent on the temperature of the system. The Equilibrium Constant • The numerical value of K for a particular equilibrium system is obtained experimentally. • If K is equal to 1 at equilibrium, there are roughly equal concentrations of reactants and products.

16. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued The Equilibrium Constant, continued • If the value of K is small, the reactants are favored. • A large value of K indicates that the products are favored. • Only the concentrations of substances that can actually change are included in K. • Pure solids and liquids are omitted because their concentrations cannot change.

17. Determining Keq for Reaction at Chemical Equilibrium Section 1 The Nature of Chemical Equilibrium Chapter 18

18. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued The Equilibrium Constant, continued The equilibrium constant, K, is the ratio of the mathematical product of the concentrations of substances formed at equilibrium to the mathematical product of the concentrations of reacting substances. Each concentration is raised to a power equal to the coefficient of that substance in the chemical equation. • The equation for K is sometimes referred to as thechemical equilibrium expression.

19. Equilibrium Constants Section 1 The Nature of Chemical Equilibrium Chapter 18

20. Equilibrium Constants Section 1 The Nature of Chemical Equilibrium Chapter 18

21. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued The H2, I2, HI Equilibrium System • The rate of the reaction between H2 and I2 vapor in a sealed flask at an elevated temperature can be followed by observing the rate at which the violet color of the iodine vapor diminishes. • The color fades to a constant intensity but does not disappear completely because the reaction is reversible. • Hydrogen iodide decomposes to re-form hydrogen and iodine. • The constant color achieved indicates that equilibrium exists among hydrogen, iodine, and hydrogen iodide.

22. Rate Comparison for H2(g) + I2(g) 2HI(g) Section 1 The Nature of Chemical Equilibrium Chapter 18

23. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued The H2, I2, HI Equilibrium System, continued The net chemical equation for the reaction is The following chemical equilibrium expression is The value for K is constant for any system of H2, I2, and HI at equilibrium at a given temperature.

24. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued The H2, I2, HI Equilibrium System, continued At 425°C, the equilibrium constant for this equilibrium reaction system has the average value of 54.34.

25. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued The H2, I2, HI Equilibrium System, continued The balanced chemical equation for an equilibrium system is necessary to write the expression for the equilibrium constant. Once the value of the equilibrium constant is known, the equilibrium constant expression can be used to calculate concentrations of reactants or products at equilibrium.

26. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued Sample Problem A An equilibrium mixture of N2, O2 , and NO gases at 1500 K is determined to consist of 6.4  10–3 mol/L of N2, 1.7  10–3 mol/L of O2, and 1.1  10–5 mol/L of NO. What is the equilibrium constant for the system at this temperature?

27. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued Sample Problem A Solution Given:[N2] = 6.4  10–3 mol/L [O2] = 1.7  10–3 mol/L [NO] = 1.1  10–5 mol/L Unknown:K Solution: The balanced chemical equation is The chemical equilibrium expression is

28. Section 1 The Nature of Chemical Equilibrium Chapter 18 The Equilibrium Expression, continued Sample Problem A Solution, continued

29. Section 2 Shifting Equilibrium Chapter 18 Preview Lesson Starter Objectives Predicting the Direction of Shift Reactions That Go to Completion Common-Ion Effect

30. Section 2 Shifting Equilibrium Chapter 18 Lesson Starter Imagine children playing on a seesaw. Five boys are sitting on one side and five girls on the other, and the seesaw is just balanced. Then, one girl gets off, and the system is no longer at equilibrium. One way to get the seesaw in balance again is for one of the boys to move toward the girls’ side.

31. Section 2 Shifting Equilibrium Chapter 18 Lesson Starter, continued When he gets to the middle, the seesaw is again at equilibrium. The stress of one girl getting off is relieved by having one of the boys move his position. How would a chemical system in equilibrium respond to removing one of the products?

32. Section 2 Shifting Equilibrium Chapter 18 Objectives Discuss the factors that disturb equilibrium. Discuss conditions under which reactions go to completion. Describe the common-ion effect.

33. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift Changes in pressure, concentration, or temperature can alter the equilibrium position and thereby change the relative amounts of reactants and products. Le Châtelier’s principle states that if a system at equilibrium is subjected to a stress, the equilibrium is shifted in the direction that tends to relieve the stress. • This principle is true for all dynamic equilibria, chemical as well as physical. • Changes in pressure, concentration, and temperature illustrate Le Châtelier’s principle.

34. Section 2 Shifting Equilibrium Chapter 18 Le Chatelier's Principal Click below to watch the Visual Concept. Visual Concept

35. Section 2 Shifting Equilibrium Chapter 18 Factors Affecting Equilibrium Click below to watch the Visual Concept. Visual Concept

36. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Pressure A change in pressure affects only equilibrium systems in which gases are involved. For changes in pressure to affect the system, the total number of moles of gas on the left side of the equation must be different from the total number of moles of gas on the right side of the equation. An increase in pressure is an applied stress. • It causes an increase in the concentrations of all species. • The system can reduce the total pressure by reducing the number of molecules.

37. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Pressure, continued the Haber process for the synthesis of ammonia 4 molecules of gas 2 molecules of gas When pressure is applied, the equilibrium will shift to the right, and produce more NH3. • By shifting to the right, the system can reduce the total number of molecules. This leads to a decrease in pressure.

38. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Pressure, continued Even though changes in pressure may shift the equilibrium position, they do not affect the value of the equilibrium constant. The introduction of an inert gas, such as helium, into the reaction vessel increases the total pressure in the vessel. But it does not change the partial pressures of the reaction gases present. • Increasing pressure by adding a gas that is not a reactant or a product cannot affect the equilibrium position of the reaction system.

39. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Concentration An increase in the concentration of a reactant is a stress on the equilibrium system. An increase in the concentration of A creates a stress. To relieve the stress, some of the added A reacts with B to form products C and D. The equilibrium is reestablished with a higher concentration of A than before the addition and a lower concentration of B.

40. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Concentration, continued Changes in concentration have no effect on the value of the equilibrium constant. • Such changes have an equal effect on the numerator and the denominator of the chemical equilibrium expression. The concentrations of pure solids and liquids do not change, and are not written in the equilibrium expression. • When a solvent, such as water, in a system involving acids and bases, is in an equilibrium equation, it is not included in the equilibrium expression.

41. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Concentration, continued High pressure favors the reverse reaction. Low pressure favors the formation of CO2. Because both CaO and CaCO3 are solids, changing their amounts will not change the equilibrium concentration of CO2.

42. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Temperature • Reversible reactions are exothermic in one direction and endothermic in the other. • The effect of changing the temperature of an equilibrium mixture depends on which of the opposing reactions is endothermic and which is exothermic. • The addition of energy in the form of heat shifts the equilibrium so that energy is absorbed. This favors the endothermic reaction. • The removal of energy favors the exothermic reaction.

43. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Temperature, continued A rise in temperature increases the rate of any reaction. In an equilibrium system, the rates of the opposing reactions are raised unequally. The value of the equilibrium constant for a given system is affected by the temperature.

44. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Temperature, continued The synthesis of ammonia by the Haber process is exothermic. A high temperature favors the decomposition of ammonia, the endothermic reaction. • At low temperatures, the forward reaction is too slow to be commercially useful. • The temperature used represents a compromise between kinetic and equilibrium requirements.

45. Temperature Changes Affect an Equilibrium System Section 2 Shifting Equilibrium Chapter 18

46. Section 2 Shifting Equilibrium Chapter 18 Predicting the Direction of Shift, continued Changes in Temperature, continued • Catalysts have no effect on relative equilibrium amounts. • They only affect the rates at which equilibrium is reached. • Catalysts increase the rates of forward and reverse reactions in a system by equal factors. Therefore, they do not affect K.

47. Section 2 Shifting Equilibrium Chapter 18 Reactions That Go to Completion Some reactions involving compounds formed by the chemical interaction of ions in solutions appear to go to completion in the sense that the ions are almost completely removed from solution. The extent to which reacting ions are removed from solution depends on the solubility of the compound formed and, if the compound is soluble, on the degree of ionization.

48. Section 2 Shifting Equilibrium Chapter 18 Reactions That Go to Completion, continued Formation of a Gas H2CO3(aq) H2O(l) + CO2(g) This reaction goes practically to completion because one of the products, CO2, escapes as a gas if the container is open to the air.

49. Section 2 Shifting Equilibrium Chapter 18 Reactions That Go to Completion, continued Formation of a Precipitate If chemically equivalent amounts of the two solutes are mixed, almost all of the Ag+ ions and Cl− ions combine and separate from the solution as a precipitate of AgCl. • AgCl is only very sparingly soluble in water. • The reaction thus effectively goes to completion because an essentially insoluble product is formed.

50. Section 2 Shifting Equilibrium Chapter 18 Reactions That Go to Completion, continued Formation of a Slightly Ionized Product • Neutralization reactions between H3O+ ions from aqueous acids and OH− ions from aqueous bases result in the formation of water molecules, which are only slightly ionized. Hydronium ions and hydroxide ions are almost entirely removed from the solution. The reaction effectively runs to completion because the product is only slightly ionized.