1. Chem 2 Chapter 18 ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

2.  The exact balancing of 2 opposite processes ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

3.  A. Chemical equilibrium – ◦ dynamic state where the concentrations of all products and reactants remain constant. ◦ The forward reaction and the reverse reaction are happening at the same rate. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

4.  1. Theoretically, all reactions can proceed in either direction if the conditions are right.  2. Written with arrows pointing both ways  A + B C + D  3. The system must be closed so that no gases escape. If they escape they won’t be there for the reverse reaction to occur.  4. When the forward and backward reactions are going under the same conditions and at the same rate-this is called chemical equilibrium. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

5.  As the forward reaction occurs more products are formed. As their concentrations increase the reverse reaction speeds up until:  rate reverse rxn=rate forward rxn ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

6.  1. A chemical reaction is in equilibrium when  A. forward and reverse reactions have ceased.  B. the equilibrium constant equals 1.  C. forward and reverse reaction rates are equal.  D. No reactants remain. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

7.  1. K is always the same for the same reaction at the same temperature regardless of the concentrations of the reactants and products.  2. Equilibrium Expression for this reaction: ◦ aA + bB ⇄ cC + dD K eq = [C] c [D] d [A] a [B] b ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

8.  3. Equilibrium positions (which are infinite) are dependent on the initial concentrations, the constant is NOT.  4. K is dependent on temperature and found experimentally or calculated from thermodynamics. It has no units.  5. Only gases and aqueous solutions are included in the eq. expression. Pure substances are assigned an implied value of 1. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

9.  2. What is the equilibrium expression for this reaction?  H 2 + I 2 ⇄ 2HI ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

10.  2. What is the equilibrium expression for this reaction?  H 2 + I 2 ⇄ 2HI  K= [HI] 2 [ H 2 ][I 2 ] Answer this question ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

11.  3. Values for K are found _____________________  4. Values for K vary with _____________________  5. When doing calculations involving K, concentrations should be expressed in _______________________ ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

12.  3. Values for K are found __ experimentally or through calculation __  4. Values for K vary with ___ temperature _____  5. When doing calculations involving K c, concentrations should be expressed in ___ moles/liter __________ Answer this question ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

13.  K eq >1 means it favors the products at eq.  Remember that products are in the numerator of the equilibrium expression. Concentrations of products is greater than concentrations of reactants.  5/1 > 1 K eq <1 means it favors the reactants at eq.  Remember that reactants are in the denominator of the equilibrium expression. Concentrations of reactants is greater than concentrations of products.  1/5 < 1 ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

14.  H 2 + I 2 ⇄ 2HI  Calculate K for this reaction. At equilibrium the following concentrations are found:  [ H 2 ] = 3.32 x 10-3  [ I 2 ] = 6.52 x 10-4  [ HI ] = 1.35 x 10-2 ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

15.  H 2 + I 2 ⇄ 2HI  Calculate K for this reaction.  K = [1.35 x 10 -2 ] 2 [3.32 x 10 -3 ][6.52 x 10 -4 ] K = 84.2 Which side of the reaction is favored? ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

16.  Read p. 560-561 in your text about, “Fixing the Nitrogen Problem”.  Find out who Fritz Haber was and why he won the Nobel Prize in chemistry in 1918. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

17.  Henry Louis Le Chatelier (8 October 1850 - 17 September 1936) was an influential French/Italian chemist of the late 19th and early 20th centuries. He is most famous for devising a principle named after himself used by chemists to predict the effect of a change in conditions on a chemical equilibrium. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

18.  If a stress is placed on a system in equilibrium, the equilibrium will shift in the direction that tends to relieve the stress.  Changes in pressure, concentration, and temperature will cause stress on a system in equilibrium. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

19.  Changes in Pressure ◦ Only affect systems with gases. ◦ Total number of moles on each side must be different. ◦ An increase in pressure will shift equilibrium in the direction that reduces the pressure – the side with fewer molecules will be favored. ◦ Equilibrium position will change but the equilibrium constant will not. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

20.  Consider the Haber Process for the synthesis of ammonia. ◦ N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) ◦ What direction will equilibrium shift with an increase in pressure? ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

21.  Consider the Haber Process for the synthesis of ammonia. ◦ N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) ◦ With an increase in pressure, Eq will shift to the side with fewer molecules. Reactants side has 4 molecules and the Products side has only 2. ◦ So… the products will be favored (shift right) ◦ So what happens to K? It is unchanged. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

22.  Changes in Concentration ◦ An increase in concentration, increases collisions, which increases reaction rate. ◦ Increase Conc. Of reactants – shift right ◦ Increase Conc. Of products – shift left ◦ Equilibrium position will change but the equilibrium constant will not. LeChatelier’s Principle ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

23.  Consider the Haber Process for the synthesis of ammonia. ◦ N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) ◦ What direction will equilibrium shift with an increase in N 2 ? H 2 ? NH 3 ? LeChatelier’s Principle ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

24.  Changes in Temperature ◦ Reversible reactions are exothermic in one direction and endothermic in the other. Whether the equilibrium constant increases or decreases because of a temperature change is dependent on what type of reaction it is. ◦ Increase Heat – shifts so that the heat is absorbed (endothermic reaction is favored) ◦ Decrease Heat – shifts so heat is released (exothermic reaction is favored) LeChatelier’s Principle ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

25.  Consider the Haber Process for the synthesis of ammonia. This is an exothermic reaction. ◦ N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) + 92 kJ ◦ What direction will equilibrium shift with an increase in heat? LeChatelier’s Principle ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

26.  Consider the Haber Process for the synthesis of ammonia. This is an exothermic reaction. ◦ N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) + 92 kJ ◦ What direction will equilibrium shift with an increase in heat? Toward the reactants The value of K will decrease. LeChatelier’s Principle ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

28. Which of the following is true about chemical equilibrium? a)It is microscopically and macroscopically static. b)It is microscopically and macroscopically dynamic. c)It is microscopically static and macroscopically dynamic. d)It is microscopically dynamic and macroscopically static. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

29. Which of the following is true about chemical equilibrium? a)It is microscopically and macroscopically static. b)It is microscopically and macroscopically dynamic. c)It is microscopically static and macroscopically dynamic. d)It is microscopically dynamic and macroscopically static. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

30. The value of the equilibrium constant, K, is dependent on I.The temperature of the system. II.The nature of the reactants and products. III.The concentration of the reactants. IV.The concentration of the products. a)I, II b)II, III c)III, IV d)I, II, III e)I, II, III, IV ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

31. The value of the equilibrium constant, K, is dependent on I.The temperature of the system. II.The nature of the reactants and products. III.The concentration of the reactants. IV.The concentration of the products. a)I, II b)II, III c)III, IV d)I, II, III e)I, II, III, IV ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

32. For a certain reaction at 25.0  C, the value of K is 1.2  10 –3. At 50.0  C the value of K is 3.4  10 –1. This means that the reaction is a) exothermic b) endothermic c) More information is needed. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

33. For a certain reaction at 25.0  C, the value of K is 1.2  10 –3. At 50.0  C the value of K is 3.4  10 –1. This means that the reaction is a) exothermic b) endothermic c) More information is needed. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

34. Consider a chemical system (all gases) at equilibrium. The total pressure of the system is increased. Given only this information, which of the following is true? a)Equilibrium shifts to the left and the value of K decreases. b)Equilibrium shifts to the right and the value of K decreases. c) Equilibrium must shift, but we do not know the direction, and the value of K remains constant. d)Equilibrium must shift, but we do not know the direction, and the value of K changes. e)We cannot tell if equilibrium shifts (nor the direction if it does), but the value of K remains constant. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

35. Consider a chemical system (all gases) at equilibrium. The total pressure of the system is increased. Given only this information, which of the following is true? a)Equilibrium shifts to the left and the value of K decreases. b)Equilibrium shifts to the right and the value of K decreases. c) Equilibrium must shift, but we do not know the direction, and the value of K remains constant. d)Equilibrium must shift, but we do not know the direction, and the value of K changes. e)We cannot tell if equilibrium shifts (nor the direction if it does), but the value of K remains constant. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

36. Consider a chemical system (all gases) at equilibrium. The pressure of one of the reactants is increased at constant volume. Which of the following is true? a)Equilibrium shifts to the left and the value of K increases. b)Equilibrium shifts to the right and the value of K increases. c)Equilibrium shifts to the right and the value of K decreases. d)Equilibrium shifts to the left and the value of K decreases. e)Equilibrium shifts, but value of K remains constant. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

37. Consider a chemical system (all gases) at equilibrium. The pressure of one of the reactants is increased at constant volume. Which of the following is true? a)Equilibrium shifts to the left and the value of K increases. b)Equilibrium shifts to the right and the value of K increases. c)Equilibrium shifts to the right and the value of K decreases. d)Equilibrium shifts to the left and the value of K decreases. e)Equilibrium shifts, but value of K remains constant. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

38. Consider a chemical system at equilibrium. The reaction is exothermic (releases energy as heat) as written and the temperature of the system is raised at constant volume. Which of the following is true? a)Equilibrium shifts to the left and the value of K increases. b)Equilibrium shifts to the right and the value of K increases. c)Equilibrium shifts to the right and the value of K decreases. d)Equilibrium shifts to the left and the value of K decreases. e)Equilibrium shifts, but value of K stays constant. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright

39. Consider a chemical system at equilibrium. The reaction is exothermic (releases energy as heat) as written and the temperature of the system is raised at constant volume. Which of the following is true? a)Equilibrium shifts to the left and the value of K increases. b)Equilibrium shifts to the right and the value of K increases. c)Equilibrium shifts to the right and the value of K decreases. d)Equilibrium shifts to the left and the value of K decreases. e)Equilibrium shifts, but value of K stays constant. ©2011 University of Illinois Board of Trustees http://islcs.ncsa.illinois.edu/copyright