Lecture 43: Equilibrium 4

Transcription

1 Lecture 43: Equilibrium 4 Read: BLB HW: BLB 15:51,53,70 Sup 15:11 13 Know: LeChatelierʼs principle catalysts math refresher in BLB Appendix A Need help?? Get help!! TAs in CRC (211 Whitmore) and SI hours on Chem 110 website; my office hours (Mon 12:30-2 & Tues 10:30-12 in 324 Chem Bldg [or 326 Chem]). THIS week is last week for SI, CRC and Sheets Office Hours Final Exam: MONDAY, May 4, 12:20 pm. locations under exam schedule ; no pds, ipods, graphing calculators, etc. Only non-text programmable calculators allowed. Bring PSU ID and pencils Concept Final review session with Sheets: Thursday 6pm in 108 Forum. Please work through the Concept Exam (on lecture note page) before the review session & bring it with you, along with any questions you may have. This review is meant to complement to the review sessions that your TAs will be holding in which they will go over the practice exams. good luck & may the Chemistry gods smile upon you! Sheets Page 1 Lecture 43

2 Chem 110 lays the foundation. What we ve covered this semester Atoms Molecules Interactions between the Same Molecules Interactions between Different Molecules Reactions Thermochemistry Equilibria Structure of atoms & molecules controls their properties & functions Energy changes tell us about changes in electronic transitions, molecular or ionic properties, reactions Sheets Page 2 Lecture 43

3 LeChatelierʼs principle: an example to illustrate this principle the following chemical system is in a closed 1.0 L container: 4 HBr(g) + O 2 (g) 2 H 2 O(g) + 2 Br 2 (g) initial conditions: T = 500 K [HBr] = 4.0 M [H 2 O] = 0.0 M [O 2 ] = 1.0 M [Br 2 ] = 0.0 M ΔH rxn = 466 kj at equilibrium: [HBr] = 1.0 M [H 2 O] = 1.5 M [O 2 ] = 0.25 M [Br 2 ] = 1.5 M What is K c? K c = Sheets Page 3 Lecture 43

4 LeChatelierʼs principle: an example (cont.) For the following reaction: 4 HBr(g) + O 2 (g) 2 H 2 O(g) + 2 Br 2 (g) A. What will happen to [Br 2 ] if 4.0 moles of HBr are added to the system which is at equilibrium? System is no longer at equilibrium. Which direction will it go to get to equilibrium? Using LeChatelierʼs principle: reactant or product added to a mixture at equilibrium will cause reaction to shift in the direction that consumes part of the added material Q = Sheets Page 4 Lecture 43

5 Applying LeChatelierʼs principle: an example For the following reaction: 4 HBr(g) + O 2 (g) 2 H 2 O(g) + 2 Br 2 (g) B. What will happen if the volume decreases from 1.0 L to 0.5 L? [] = n V : as V, [ ] so P x = [X] RT (partial P) Using LeChatelierʼs principle: since pressure is increased by disturbance, reaction will shift in direction that will reduce pressure 4 HBr(g) + O 2 (g) 2 H 2 O(g) + 2 Br 2 (g) 5 mol total 4 mol total Sheets Page 5 Lecture 43

6 LeChatelierʼs principle: an example (cont.) C. What will happen if 5.0 moles of inert gas is added to the system? inert gas: gas that does NOT participate in the reaction. How does inert gas effect concentrations of reactants & products? 4 HBr(g) + O 2 (g) 2 H 2 O(g) + 2 Br 2 (g) Using LeChatelierʼs principle: Sheets Page 6 Lecture 43

7 Summarizing LeChatelierʼs principle so far changing concentration, or V so that [ ] changes, or P changes puts a stress on the system stresses do not change K eq! Q changes; system shifts to re-establish equilibrium Q K BUT what if temperature changes??? K eq does depend on temperature change depends on whether the reaction is exothermic (ΔH is ) or endothermic (ΔH is +) Sheets Page 7 Lecture 43

8 LeChatelierʼs principle: an example (cont.) For the following reaction: 4 HBr(g) + O 2 (g) 2 H 2 O(g) + 2 Br 2 (g) D. What will happen if the temperature is increased to 1000 K? 4 HBr(g) + O 2 (g) 2 H 2 O(g) + 2 Br 2 (g) ΔH rxn = 466 kj treat heat as a product (if exothermic) or as a reactant (if endothermic) (review Lecture 38) Using Le Chatelierʼs principle: when heat is added to a system, reaction shifts in the direction that absorbs heat for this particular reaction, which is exothermic (heat is a product), adding heat will cause reaction to shift to left Sheets Page 8 Lecture 43

9 Back to the Haber process (cont.) N 2 (g) + 3H 2 (g) 2NH 3 (g) + heat This system is at equilibrium. What will happen to [NH 3 ] if add N 2 [NH 3 ] remove H 2 [NH 3 ] raise T [NH 3 ] increase P [NH 3 ] decrease V [NH 3 ] add catalyst [NH 3 ] Sheets Page 9 Lecture 43

10 Example from old exam: 5 CO(g) + I 2 O 5 (g) I 2 (g) + 5 CO 2 (g) ΔH = 1175 kj For this reaction at equilibrium, which of the changes will NOT have any effect on the equilibrium concentrations of reactants or products? A. adding more CO(g) to the mixture B. increasing the pressure by decreasing the volume of the container C. removing some of the I 2 (g) D. raising the temperature of the mixture E. adding more I 2 O 5 (g) Sheets Page 10 Lecture 43

11 Example from old exam: 5 CO(g) + I 2 O 5 (g) I 2 (s) + 5 CO 2 (g) ΔH = 1237 kj For this reaction at equilibrium, which of the changes will NOT have any effect on the equilibrium concentrations of reactants or products? A. adding more CO(g) to the mixture B. increasing the pressure by decreasing the volume of the container C. removing some of the I 2 (s) D. raising the temperature of the mixture E. adding more I 2 O 5 (g) Sheets Page 11 Lecture 43

12 Example from old exam: Consider the following reaction: CO(g) + H 2 O (g) CO 2 (g) + H 2 (g) K c = 2.7 If mole CO, mole H 2 O, mole CO 2 and mole H 2 are introduced into a 2.0 L vessel, which of the following statements will be true? A. The system is at equilibrium and no change will occur. B. The system is not at equilibrium and will proceed to the right to achieve equilibrium. C. The system is not at equilibrium and will proceed to the left to achieve equilibrium. D. The system is not at equilibrium, and it cannot achieve equilibrium. E. The system is at equilibrium, but the addition of a catalyst will cause the reaction to proceed to the right. Sheets Page 12 Lecture 43

13 Example: Consider the hypothetical reaction A(g) 2 B(g) A flask is charged with 0.55 atm of pure A, after which it is allowed to reach equilibrium at 0 C. At equilibrium the partial pressure of A is 0.36 atm. What is the total pressure in the flask at equilibrium? What is the value of K p? A B initial 0.55 atm change final (consumed) 0.36 atm (produced) Sheets Page 13 Lecture 43

14 Before next class: Review: entire semester! Please bring any last minute questions you may have. This is for touch-up or clarification questions, NOT a lecture repeat! Print out the question, so I can put up on the document reader, as weʼve done previously before the three midterm exams Donʼt forget the math refresher in BLB Appendix A Good luck studying for finals! Please start now. Answers: p. 3: K c = p. 10: B p. 11: C p. 12: B p. 13: P total = 0.74 atm; K p = 0.40 Sheets Page 14 Lecture 43