Guide to Chapter 13. Chemical Equilibrium
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1 Guide to Chapter 13. Chemical Equilibrium We will spend five lecture days on this chapter. During the first class meeting we will focus on how kinetics makes a segue into equilibrium. We will learn how to write equilibrium constants two different ways, and learn how to interpret equilibrium constants. Next we will do some equilibrium calculations, using equilibrium concentrations to determine equilibrium constants and visa versa. You will become adept at setting up and using ICE tables. We ll discuss mass action expressions that let us understand what needs to happen in order to get to equilibrium. We will spend time with LeChâtelier s principle end the chapter the way we started: tying kinetics and equilibrium together. Read the introductory paragraph to Chapter 13. Read Section 13.1 The Equilibrium State. Learning Objective 1: Know how reaction situations described by kinetics eventually segue into equilibrium situations Do the following end-of-chapter problem: 28 Problem Club Question A (ACS Style): Which is true, by definition, for a system that has reached a state of chemical equilibrium? (A) The substance having the smaller volume has a tendency to form at the expense of the other substances. (B) No further reaction occurs in either direction. (C) The concentrations of reactants and products are necessarily equal. (D) The opposing reactions have equal velocities. (E) The total mass of the products is equal to the total mass of the reactants. Answer: D Read Sections : Section The equilibrium constant, K c ; Section 13.3 The equilibrium constant, K p ; and Section 13.4 Heterogeneous equilibria. Learning Objective 2: Be able to write the equilibrium expressions, K c, for a given balanced chemical reaction involving homogeneous equilibrium. Learning Objective 3: Be able to write the equilibrium expressions, K c, for a given balanced chemical reactions involving heterogeneous equilibrium Learning Objective 4: Be able to write the equilibrium expressions, K p, for a given balanced chemical reaction involving homogeneous equilibrium. Learning Objective 5: Be able to write the equilibrium expressions, K p, for a given balanced chemical reactions involving heterogeneous equilibrium Learning Objective 6: Using given concentrations of reactant(s) and product(s), determine the numerical value of the equilibrium constant K c or K p for a reaction. Learning Objective 7: Know the significance of very large (or small) equilibrium constants. Learning Objective 8: Be able to determine the equilibrium constant expression and the equilibrium constant for situations where chemical equations are reversed, multiplied through by constant coefficients, or added together. Learning Objective 9: Given K c, calculate K p or vice versa. Do Problems 1-4 at the end of Section 13.2, Problems 5 and 6 at the end of Section 13.3 and Problem 7 at the end of section 13.4 Dr. Mattson, General Chemistry, Chm 205, Guide to Chapter 13. Chemical Equilibrium 1
2 Do the following end-of-chapter problems: 30, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58 Problem Club Question B. Write Kc expressions for (a) SnO2(s) + 2 H2(g) (b) PCl5(s) + H2O(l) (c) CO(g) + 2 H2(g) Sn(s) + 2 H2O(g) 2 HCl(g)+ POCl3(g) CH3OH(g) Answer: a. Kc = [H2O] 2 /[H2] 2 b. Kc = [HCl] 2 [POCl3] c. Kc = [CH3OH]/[H2] 2 [CO] Problem Club Question C. When COCl2 is heated to 527 o C in a reaction vessel, the following equilibrium is attained: COCl2(g) CO(g) + Cl2(g) If the equilibrium concentrations are found to be [CO] = , [Cl2] = and [COCl2] = M, what is the equilibrium constant? Answer: Problem Club Question D. Consider the reaction: H2(g) + Cl2(g) 2 HCl(g) Kc = 1.7 x 10 4 Write the equilibrium expressions, K c =, and determine K c for each of the following: (a) 1 /2 H2(g) + 1 /2 Cl2(g) (b) 2 HCl(g) (c) HCl(g) H2(g)+ Cl2(g) HCl(g) 1 / 2 H2(g) + 1 /2 Cl2(g) Answer: a. 130 b. 5.9 x 10-5 c Problem Club Question E. Consider the following equilibria at 127 o C. Calculate Kp for this reaction. 2 NH3(g) N2(g) + 3 H2(g) Kc = 2.6 x 10-5 Answer: Problem Club Question F (ACS Style): Answer: D Problem Club Question G (ACS Style): Answer: E Read Sections 13.5 Using the equilibrium constant. Learning Objective 10: Use the reaction quotient (Q) to determine if a reaction mixture is at equilibrium. Predict the direction a reaction will shift by comparing the reaction quotient (Q) with K c or K p. Learning Objective 11: Given sufficient data concerning initial or equilibrium concentrations of reactants and/or products determine the equilibrium concentrations of each substance in the balanced reaction and the value of K c. Learning Objective 12: Given sufficient data concerning initial or equilibrium partial pressures of gaseous reactants and/or products determine the equilibrium partial pressures of each substance in the balanced reaction and the value of K p Dr. Mattson, General Chemistry, Chm 205, Guide to Chapter 13. Chemical Equilibrium 2
3 Learning Objective 13: Given the equilibrium constant, K c, and information concerning the concentration (initial or equilibrium) of reactants/products, calculate the equilibrium concentration of each species in a balanced equilibrium reaction. Do Problems 8-15 embedded within this big section. Do the following end-of-chapter problems: 60, 62, 64, 66, 68, 70, 72, 74, 76, 94, 96, 98, 100, 106 Problem Club Question H. Consider the reaction below along with the equilibrium constant given. N2O4(g) 2 NO2(g) Kc = 0.20 Suppose that [N2O4] = M at equilibrium. Calculate the concentration of NO2 at equilibrium. As in most of these problems, begin by writing the equilibrium expression, Kc. Answer: Problem Club Question I. Consider the equilibrium:. N2(g) + O2(g) 2 NO(g) Kc = at 2200 o C If moles of NO were placed in a 1.0 L flask and allowed to reach equilibrium, what is the equilibrium concentration of each gas? Hint: Make an ICE table. Answer: [N2] = [O2] = [NO] = Problem Club Question J. Consider the equilibrium: H2(g) + I2(g) 2 HI(g) Kc = 9.00 Hydrogen, iodine and HI were placed in a flask in order that the initial concentrations were [H2] = 2 M, [I2] = 4 M, and [HI] = 1.5 M. Determine if the system is at equilibrium. If not, which direction must it shift in order to attain equilibrium? Start with the Qc expression. Answer: Qc = 0.28 so reaction must shift right. Problem Club Question K. For which value for K will the equilibrium mixture consist almost entirely of reactants? A. 1 x B C. 1.5 D. 19 E. 3 x 10 7 Answer: A Problem Club Question L. Consider the reaction: PCl5(g) PCl3(g) + Cl2(g) Kc = 1.8 at 250 o C A 1.5 mol sample of PCl5 is injected into a 0.50-L reaction vessel. Calculate the concentration of each gas at equilibrium. Answer: [PCl5] = 1.41 M; [PCl3] = 1.59 M; [Cl2] = 1.59 M Problem Club Question M. For the all-gas system: 2 X + Y 3 Z K = 0.34 Starting with 0.90 M concentrations of X and Y, it was found that the equilibrium concentration of X was 0.60 M. What is [Z] at equilibrium? Answer: 0.45 Problem Club Question N. Suppose 2.00 mol NO were placed in a 1.0-L flask at 2273 K. At equilibrium, mol N2 and mol O2 are present. What is Kc for the reaction at this temperature? N2(g) + O2(g) 2 NO(g) Dr. Mattson, General Chemistry, Chm 205, Guide to Chapter 13. Chemical Equilibrium 3
4 Answer: 0.10 Problem Club Question O. At 70 o C, Kc = for the equilibrium given below. If at equilibrium, N2O4 = 3.22 M, what is the molar concentration of NO2? Answer: N2O4(g) 2NO2(g) Problem Club Question P. The value of Kc for the equilibrium PCl5(g) PCl3(g) + Cl2(g) is at 250 o C. If 0.20 mol PCl mol Cl2 and 0.50 mol PCl5 were mixed in a 5.0 L container, which direction, if either, will the reaction shift in order to establish equilibrium? Answer: shift right Problem Club Question Q (ACS Style): Answer: C Problem Club Question R (ACS Style): Answer: D Problem Club Question S (ACS Style): Answer: B Problem Club Question T (ACS Style): Answer: C Problem Club Question U (ACS Style): Answer: D Problem Club Question V (ACS Style): Answer: B Read Sections 13.6 Factors that alter the composition of an equilibrium mixture and Section Altering an equilibrium mixture: Changes in concentration equilibrium will shift (right or left) when a change in concentration has been introduced. Do Problem 16 at the end of this section. Do the following end-of-chapter problem: 78 Problem Club Question W. Consider the equilibrium: H2(g) + CO2(g) H2O(g) + CO(g) Predict the qualitative effect on the equilibrium concentration of each gas by an increase in the pressure of water. Answer: H2 and CO2 increases; CO decreases Read Sections Altering an equilibrium mixture: Changes in pressure and volume equilibrium will shift (right or left) when a change in pressure has been introduced. equilibrium will shift (right or left) when a change in volume has been introduced. Do Problems 17 and 18 at the end of this section. Do the following end-of-chapter problems: 80 Problem Club Question X. For the system A(g) following statements is possible? B(g) + 2 C(g), the Kc is 8.0 x Which of the Dr. Mattson, General Chemistry, Chm 205, Guide to Chapter 13. Chemical Equilibrium 4
5 A. The equilibrium [B] and [C] is much greater than [A]. B. Increasing the pressure will decrease Kc. C. Adding A will increase Kc D. Increasing the volume container will increase Kc Answer: A Problem Club Question Y. Which of the following would shift left if the container volume were decreased? Shift right? Not shift? (a) N2(g) + O2(g) (b) 2 NO(g) + O2(g) (c) 2 HI(g) (d) 2 HI(g) 2 NO(g) H2(g) + I2(g) H2(g) + I2(s) 2 NO2(g) Answer: a. no shift b. shift right c. no shift Problem Club Question Z. For which of the equilibria in the previous question is Kc = Kp? Answer: a and c Problem Club Question AA (ACS Style): Answer: D Read Sections Altering an equilibrium mixture: Changes in temperature. equilibrium will shift (right or left) when a change in temperature has been introduced. Learning Objective 26: Qualitatively predict how K c or K p will change (increase or decrease ) when the temperature is changed. Do Problems at the end of this section. Do the following end-of-chapter problems: 34, 36, 82 Problem Club Question BB. Consider the reaction: Which way will the equilibrium shift by: C(s) + 2 H2(g) CH4(g) DH = -75 kj: (a) an increase in temperature (b) an increase in the size of the reaction container's volume (c) an increase in the pressure of hydrogen (d) a decrease in the pressure of methane (e) addition of more carbon Answer: a. L b. L c. R d. R d. right e. no effect Problem Club Question CC. Consider the equilibrium: 2 NH3(g) N2(g) + 3 H2(g) DH = -92 kj The concentration of NH3 may be increased by: (possibly more than one of the following!) A. increasing the pressure in the container. B. decreasing the pressure in the container C. adding an inert gas such as argon D. increasing the temperature E. adding hydrogen Answer: a. Y b. N c. N d. Y e. Y Problem Club Question DD. (ACS-Style) Answer: C Dr. Mattson, General Chemistry, Chm 205, Guide to Chapter 13. Chemical Equilibrium 5
6 Problem Club Question EE (ACS Style): Answer: A Problem Club Question FF (ACS Style): Answer: C Read Sections The effect of a catalyst on equilibrium. equilibrium will shift (right or left) when a catalyst has been added. Do Problem 22 at the end of this section. Do the following end-of-chapter problems: 86 Problem Club Question GG. Which of the following will be affected by the addition of a catalyst:: (a) Kc (b) Kp (c) the rate constant Answer: C Problem Club Question HH. The only stress to an equilibrium resulting in a change to the value of the equilibrium constant is: A. a change in temperature B. a change in the pressure C. an addition of a reactant D. the addition of a catalyst E. the removal of a product Answer: A Read Sections The link between chemical equilibrium and chemical kinetics. Learning Objective 25: Given a concentration vs. time plot, identify regions where kinetic measurements and equilibrium calculations can be obtained. Learning Objective 26: Determine K c or K p from given data concerning the forward and reverse reaction rates. Do Problem 23 at the end of this section. Do the following end-of-chapter problems: 88, 90, 92 Problem Club Questions II. Consider the following equilibrium in order to answer the next five questions: 2SO2(g) + O2(g) 2SO3(g) DH = -99 kj Kp = 17 atm -1. Problem Club Question JJ. General Questions. a. What will happen to Kp if the temperature is increased? b. Suppose a sample of SO3 is placed in an empty flask and allowed to come to equilibrium. If the equilibrium pressure of SO3 is 1 atm, then are the pressures of SO2 and O2 greater than or less than 1 atm? c. Complete the sentences a it pertains to the above equilibrium: If the container is expanded from 10-L to 20-L, the pressures of all the gases will temporarily (increase/decrease). Immediately after expanding the container volume, the equilibrium constant Kp (increases/decreases/remains unchanged). LeChatelier's Principle predicts that the reaction will (shift left/shift right/not shift) in order to re-establish equilibrium. The change in pressure for SO2 is (twice/half/same) as that for O2. Answer: a. Kp will decrease; b. both less than 1 atm; c. decrease; remains unchanged; shift left; twice Problem Club Question KK. Suppose a sample of SO3 is placed in an empty flask and allowed to come to equilibrium as described in Part 1b above. If the equilibrium pressure of SO3 is 1 atm, then determine Dr. Mattson, General Chemistry, Chm 205, Guide to Chapter 13. Chemical Equilibrium 6
7 the equilibrium pressures of SO2 and O2 (Answer will have units of atm). Hint: Start by setting up an initial/change/equilibrium table. The initial concentration of SO3 is not given but you do know the initial concentrations of SO2 and O2 from the equilibrium expression. Next, write the equilibrium expression for Kp in terms of the pressure of each gas, PSO2 etc. Also substitute in the values from the table above. OK, ready for something new? Your answer in the previous step should have looked like: Kp = PSO3/PSO2PO2 = 1 2 /(2x) 2 (x) = 17. Solve for x by taking the cubed root on your calculator. (If you have never done this, try taking the cubed root of the number: Enter 343, then the y x button and followed by "=". The answer should be 7. You will be doing many of these in the weeks ahead.) So back to the problem at hand, solve for x and complete the answer to the original question. Answer: PSO2 = atm; PO2 = atm; PSO3 = 1.00 atm Problem Club Question LL. Use the table to determine the original pressure of SO3 Answer: 1.49 atm Problem Club Question MM. On graph paper, plot Pressure (atm) on the y-axis and Reaction Time (in minutes) on the x-axis. Using the data and the results that you have determined, plot the concentration of all three gases for this reaction. Assume that it took 2.0 minutes to reach equilibrium. Answer: The plot should have PSO3 starting at 1.49 atm and decreasing to 1.00 atm at two minutes and horizontal thereafter. The other two gases will start at 0.0 atm and level out at two minutes. The horizontal regions for SO2 and O2 reflects their equilibrium concentrations. Problem Club Question NN. Answer the following questions about the plot in the previous problem. (a) Label the kinetics region and the equilibrium region. (b) If the temperature were increased, what would happen to the kinetics region? (c) Consider DH in order to predict which way each equilibrium concentration will change if the temperature is increased. Answer: a 0-2 minutes is kinetics region where all slopes are non-zero. b. it would decrease from 2 minutes to some smaller value c. an increase in temperature will always shift exothermic reactions to the left Problem Club Question OO (ACS Style): Answer: E Dr. Mattson, General Chemistry, Chm 205, Guide to Chapter 13. Chemical Equilibrium 7
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