Chapter 13 Chemical Equilibrium Section 13 6 Solving

Chapter 13 Chemical Equilibrium

Section 13. 6 Solving Equilibrium Problems 1) Write the balanced equation for the reaction. 2) Write the equilibrium expression using the law of mass action. 3) List the initial concentrations. 4) Calculate Q, and determine the direction of the shift to equilibrium. Copyright © Cengage Learning. All rights reserved 2

Section 13. 6 Solving Equilibrium Problems 5) Define the change needed to reach equilibrium, and define the equilibrium concentrations by applying the change to the initial concentrations. 6) Substitute the equilibrium concentrations into the equilibrium expression, and solve for the unknown. 7) Check your calculated equilibrium concentrations by making sure they give the correct value of K. Copyright © Cengage Learning. All rights reserved 3

Section 13. 6 Solving Equilibrium Problems EXERCISE! Consider the reaction represented by the equation: Fe 3+(aq) + SCN-(aq) Fe. SCN 2+(aq) Fe 3+ Trial #1 9. 00 M Trial #2 3. 00 M Trial #3 2. 00 M SCN 5. 00 M 2. 00 M 9. 00 M Fe. SCN 2+ 1. 00 M 5. 00 M 6. 00 M Find the equilibrium concentrations for all species. Copyright © Cengage Learning. All rights reserved 4

Section 13. 6 Solving Equilibrium Problems EXERCISE! Answer Trial #1: [Fe 3+] = 6. 00 M; [SCN-] = 2. 00 M; [Fe. SCN 2+] = 4. 00 M Trial #2: [Fe 3+] = 4. 00 M; [SCN-] = 3. 00 M; [Fe. SCN 2+] = 4. 00 M Trial #3: [Fe 3+] = 2. 00 M; [SCN-] = 9. 00 M; [Fe. SCN 2+] = 6. 00 M Copyright © Cengage Learning. All rights reserved 5

Section 13. 6 Solving Equilibrium Problems CONCEPT CHECK! A 2. 0 mol sample of ammonia is introduced into a 1. 00 L container. At a certain temperature, the ammonia partially dissociates according to the equation: NH 3(g) N 2(g) + H 2(g) At equilibrium 1. 00 mol of ammonia remains. Calculate the value for K. Copyright © Cengage Learning. All rights reserved 6

Section 13. 6 Solving Equilibrium Problems CONCEPT CHECK! A 1. 00 mol sample of N 2 O 4(g) is placed in a 10. 0 L vessel and allowed to reach equilibrium according to the equation: N 2 O 4(g) 2 NO 2(g) K = 4. 00 × 10 -4 Calculate the equilibrium concentrations of: N 2 O 4(g) and NO 2(g). Copyright © Cengage Learning. All rights reserved 7

Section 13. 7 Le Châtelier’s Principle § If a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change. Copyright © Cengage Learning. All rights reserved 8

Section 13. 7 Le Châtelier’s Principle Effects of Changes on the System 1. Concentration: The system will shift away from the added component. If a component is removed, the opposite effect occurs. 2. Temperature: K will change depending upon the temperature (endothermic – energy is a reactant; exothermic – energy is a product). Copyright © Cengage Learning. All rights reserved 9

Section 13. 7 Le Châtelier’s Principle Effects of Changes on the System 3. Pressure: a) The system will shift away from the added gaseous component. If a component is removed, the opposite effect occurs. b) Addition of inert gas does not affect the equilibrium position. c) Decreasing the volume shifts the equilibrium toward the side with fewer moles of gas. Copyright © Cengage Learning. All rights reserved 10

Section 13. 7 Le Châtelier’s Principle To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 11

Section 13. 7 Le Châtelier’s Principle Equilibrium Decomposition of N 2 O 4 To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Copyright © Cengage Learning. All rights reserved 12