University:
Boston University
Course:
CAS CH 101 | General Chemistry 1
Academic year:

2020
Views:

412

Pages:

8
Author:

John Ferguson

Discussion #6 1. What is true if a forward reaction is spontaneous? (Chose all that apply) K≪1 K≫1 𝑄 >1 K 𝑄 =1 K 𝑄>1 𝑄<1 𝑄 <1 K ln 𝑄0 K 𝑄>K ln 𝑄 <0 K 𝑄=K ln 𝑄 =0 K 2. If K=10-13 and Q=102 what must be true? Circle all that apply. a. b. c. d. e. The reaction will be spontaneous The reaction will be nonspontaneous The reaction is at equilibrium Reactants are favored at equilibrium Products are favored at equilibrium f. Reactants and products are equally favored at equilibrium g. Currently, the reaction has more products than reactants h. Currently, the reaction has more reactants than products 3. The following reaction is at equilibrium and has an equilibrium constant K: A(g) + B(g) ⇌ 2C(g) If we double the volume of the container, circle all that must be true immediately after the volume is changed below. Hint: write an expression for K and Q and compare them. a. b. c. d. e. Q increases f. Q decreases g. Q will stay the same K increases K decreases K will stay the same K will double 4. The following reaction is at equilibrium has an equilibrium constant K: A(g) ⇌ B(g) + C(g) If we double the volume of the container, circle all that must be true immediately after the volume is changed below. a. b. c. d. e. Q will stay the same f. Q =2K K g. Q = 2 K increases K decreases K will stay the same K will double 5. The following reaction is at equilibrium has an equilibrium constant K: A(g) ⇌ B(aq) + C(g) If we double the volume of the container, circle all that must be true immediately after the volume is changed below. a. b. c. d. e. K increases K decreases K will stay the same K will double Q increases f. g. h. i. 1 Q decreases Q will stay the same Q =2K K Q=2 6. If an equilibrium constant of the reaction decreases with increasing temperature what must be true. a. b. c. d. e. Equilibrium constant always increases with temperature Equilibrium constant never changes Reaction is Endothermic Reaction is Exothermic Need more information 7. Consider the following reaction at equilibrium: A(aq) + 2B(aq) ⇌ C(aq) and K = 3 × 102. For this reaction, ∆rH° = –40. kJ/mol. In each of the following changes (a – d) circle the choice that indicates how the system will respond to return to equilibrium: a. Put a flame under the container: More products will form || More reactants will form || No change b. Adding 100 mL of water: c. More products will form || More reactants will form || No change The volume of the container is doubled: More products will form || More reactants will form || No change d. Remove 0.5 mol of A: More products will form || More reactants will form || No change 8. For the reaction A(aq) + 2B(aq) ⇌ C(aq), K = 1×10-13 at 25°C. 4 moles of A are added to 6 moles of C (at 25°C) in 1 L of water. Answer the following questions: a. Based on the equilibrium constant, are the reactants or product favorable? b. Calculate the Q for the initial conditions (Q1). c. Based on the initial conditions which of the following will form: A B C 9. Consider the exothermic reaction: 2 NH3(aq) + H2CO3(aq) ⇌ (NH2)2CO(aq) + 2 H2O(l) a. 1.0 moles of NH3 and 3.0 moles of (NH2)2CO are added to 1.0 L of water. Calculate the initial value of Q before any reaction takes place. 2 b. The equilibrium constant of this reaction (K) is 1·103. Which of the following must be true for the reaction mixture specified in part (a)? Circle all that apply.  The reaction will be spontaneous  Products will predominate at equilibrium  The reaction will be nonspontaneous  Reactants and products are in equal proportion at equilibrium  The reaction is at equilibrium   Reactants will predominate at equilibrium Before any reaction occurs, products predominate  Before any reaction occurs, reactants predominate c. A reaction mixture is prepared with the following concentrations: 1.00 M NH3, 0.10 M H2CO3, and 𝑄 0.10M (NH2)2CO. Which of the following will increase the value of the ratio mixture ? Circle all that K apply.      Add products Add reactants Heat the mixture Cool the mixture  The reaction is already at equilibrium. Do nothing Add water 10. At a certain temperature, the equilibrium constant for 2H2(g) + O2(g) ⇌ 2H2O(g) is K= 2500 a. Calculate the numerical value of the equilibrium constant for H2O(g) ⇌ H2(g) + ½ O2(g) b. Calculate the numerical value of the equilibrium constant for 2H2O(g) ⇌ 2H2(g) + O2(g) 11. Given the two reactions below, find the relationships between K1 and K2 A(aq) + B(g) ⇌ C(s) + D(aq) where the K for this reaction = K1 1 1 1 1 A(aq) + 3 B(g) ⇌ 3 C(s) + 3 D(aq) where 3 the K for this reaction = K2 12. Given the two reactions below, find the relationships between K1 and K2 A(aq) + B(g) ⇌ C(s) + D(aq) where the K for this reaction = K1 C(s) + D(aq) ⇌ A(aq) + B(g) where the K for this reaction = K2 3 13. Given the reactions below: A(aq) + B(g) ⇌ C(aq) + D(aq) where the K for this reaction = K1 D(aq) ⇌ ½ E(aq) + F(g) where the K for this reaction = K2 2C(aq) ⇌ E(aq) where the K for this reaction = K3 What is K in terms of K1, K2, and K3 for the following reactions? a. A(aq)+ B(aq)+ D(aq) ⇌ 3C(aq) + 2F(g) b. D(aq) + E(aq) ⇌ A(aq)+ B(aq) + C(aq) In preparation for next week discussion and lectures Mahaffy, 2e chapter 14:  Measure of the acidity of a solution o calculations of pH and [H3O+]:  pH= ‒log[H3O+] [H3O+]=10-pH pOH= ‒log[OH-] [OH-]= 10-pOH Measure of the basicity of a solution o calculations of pOH and [OH-]:  Water ionization constant:  For the reaction of acid and water, equilibrium constant is expressed as Ka  For the reaction of base and water, equilibrium constant is expressed as Kb  Kw = Ka · Kb  pKa = ‒logKa  Weak acids and weak bases: Kw=[H3O+(aq)][OH-(aq)] pKb = ‒logKb pKw = ‒logKw Kw<, or =). If you need more information in order to answer, circle nothing. a. What is true for an acidic solution? [H3O+(aq)] < > = [OH-(aq)] pH < > = pOH b. What is true for a basic solution? [H3O+(aq)] < > = [OH-(aq)] pH < 4 > = pOH 3. If a solution has a pH < 7 but has equal concentrations of H3O+ and OH- what must be true? (circle one) acidic basic neutral 4. The Kw for the autoionization of water is 1∙10-14 at 25°C and at 60°C, Kw is 1∙10-13. Circle all below that must be true: a. b. c. d. e. Pure water is acidic at 60 °C Autoionization is an endothermic process Autoionization is an exothermic process Pure water at 60°C has a pH = 7 Pure water at 60°C has a pH < 7 5. You have a solution with the pH of 7.3 choose what applies: a. Solution is basic b. Solution is acidic c. I need more information  If you need more information, what information do you need? 6. Calculate the pHs of the following aqueous solutions assuming that all the following are strong acids and bases At 25°C. a. 0.50M H2SO4 Reaction: b. 0.75 M Ba(OH)2 Reaction: 7. What is the concentration of a barium hydroxide solution if the pH is 11.48 at 25°C? 8. Write down an equilibrium reaction where equilibrium constant is Ka. 9. Write down an equilibrium reaction where equilibrium constant is Kb. Unless told otherwise assume all reactions are run at room temperature (25°C). Do your calculations to the right number of significant figures. Review rules of significant figures for logarithms and antilogarithms from Packet 5. 10. A solution of 0.05 M of HOClO3 in water has a pH=1.3 at room temperature. a. Based on the information given do you have a reaction of an acid with water or base with water? Why? b. Write the chemical equation for the acid-base reaction that occurs in this solution. What are the conjugate acid and base pairs in the solution? 5 c. Write the expression for equilibrium constant K. (Does your expression represent Ka or Kb?) d. Using information given calculate equilibrium concentration of hydronium. e. What percent of the acid reacted in solution? Is this a strong or weak acid? f. Based on the information given will you expect the equilibrium constant to be: (choose one) Ka<<1 or Ka >>1 11. Calculate the pH of the following aqueous solutions assuming that all the following are strong acids and bases. Remember log(1) = 0; log(10) = 1 a. 3.2M HCl b. 1.5M HCl c. 0.0024M HCl d. 0.039M HCl e. 0.0090 M NaOH (hint: calculate pOH first and knowing pH + pOH =14 at room temperature) f. 5.0M NaOH 12. What is the concentration of a sodium hydroxide solution if the pH is 11.48? 13. What is the concentration of a sodium hydroxide solution if the pH is 11.90? 14. What is the concentration of a sodium hydroxide solution if the pH is 14? 15. What is the concentration of HCl solution if the pH is 2.48? 16. What is the concentration of HCl solution if the pH is 3.40? 17. What is the concentration of HCl solution if the pH is 5.16? 18. What is the concentration of HCl solution if the pH is 1.04? 6 1. 2. 3. 4. 5. 6. 7. 8. 9. Q Answers Q Q < K, ln( ) < 0, < 1 K K B, D, G C, G C, G C, G D a. b. c. d. more reactants will form more reactants will form no change more reactants will form a. Q = ∞ b. A, B a. Q = ∞ b. ii, v, vii c. i, iii, vi 10. a. Knew = 0.020 b. Knew =0.00040 11. K 2 = (K1 )1⁄3 12. K 2 = (K1 )−1 13. a. K= b. K= K1 ∙ (K2 )2 K3 1 K1 ∙ K3 Answers in preparation for Chapter 14 1. 2. [OH-] = [H3O+] c. 𝐾𝑎 = 9. 10. 11. a. -0.5 b. -0.2 c. 2.62 d. 1.41 e. 11.95 f. 14.7 12. 0.0030 M 13. 0.0079 M 14. 1 M 15. 0.0033 M 16. 3.98 x 10-4 M 17. 6.9 x 10-6 M 18. 9.1·10-2 [H3 O+ ]∙[A− ] [HA] A−(aq) + H2O(l) ⇌ HA(aq) + OH−(aq); Kb= [HClO4 ] d. 0.05 M e. 100%; strong f. Ka >> 1 a. [H3O+] > [OH-] and pH < pOH b. [H3O+] < [OH-] and pH > pOH 3. neutral 4. b, e 5. c; need to know temp 6. H2SO4 pH = 0.3, Ba(OH)2 pH = 14.18 7. 1.5× 10−3 𝑀 8. HA(aq) + H2O(l) ⇌ A-(aq) + H3O+(aq); Ka= [H3 O+ ][ClO4 − ] [OH−]∙[HA] [A− ] a. pH < 7; acid + water b. HClO4(aq) + H2O(l) ⇌ ClO4¯(aq) + H3O+(aq); acid base conj. base conj. acid 7 Exam 1 answers 1. 2. a. b. c. d. e. 4 18 8/3 0 0 a. 2 or 4 b. 0 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. a. 4060kPa b. 2.77*103 kPa 29g/mol 538g/mol 59.2kPa 0.31Pa C3H8>C2H6>CH4 A>d>b>s 281K or 282K PA=PB=PC a. 430m/s b. Ar c. N2 13. 3K+ (g) + PO43-(g) -> 3K+ (aq) + PO43-(aq) 14. (NH4)2CO3(s) -> 2 NH4+(aq) + CO32- (aq) 15. 4.08bar 8