Thursday - Equilibrium

1. Thursday - Equilibrium • HW and Reading Questions • 1 question quiz – over 16.4 or 16.5 • Le Chatelier’s Principle: • Lecture – take careful notes • Demonstrations / Lab • Problems • Tonight: Read remaining text in chapter 16 • Solve problems 30 - 40 even, 55 • Prepare for quiz over chapter 16 tomorrow • Tomorrow – 1st ½ working problems, 2nd ½ will be a ‘practice quiz’ before real assessment on MONDAY

2. Quiz Answers – 1 pt each (4 total) a) b) [Ag+] = 0.52 M [NH3] = 0.42 M K = .088

3. EQUILIBRIUM AND EXTERNAL EFFECTS • Temperature, catalysts, and changes in concentration affect equilibria. • The outcome is governed by LE CHATELIER’S PRINCIPLE • “...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.”

4. EQUILIBRIUM AND EXTERNAL EFFECTS Henri Le Chatelier 1850-1936 Studied mining engineering. Interested in glass and ceramics.

5. EQUILIBRIUM AND EXTERNAL EFFECTS • Temperature change ---> change in K • Consider the fizz in a soft drink CO2(aq) + HEAT↔ CO2(g) + H2O(liq) • K = P (CO2) / [CO2] • Increase T. What happens to equilibrium position? To value of K? • K increases as T goes up because P(CO2) increases and [CO2] decreases. • Decrease T. Now what? • Equilibrium shifts left and K decreases.

6. Temperature Effects on Equilibrium N2O4 (colorless) + heat↔ 2 NO2 (brown)∆Ho = + 57.2 kJ Kc (273 K) = 0.00077 Kc (298 K) = 0.0059

7. Temperature Effects on Equilibrium Figure 16.6

8. EQUILIBRIUM AND EXTERNAL EFFECTS • Add catalyst ---> no change in K • A catalyst only affects the RATE of approach to equilibrium. Catalytic exhaust system

9. NH3 Production • N2(g) + 3 H2(g) ↔ 2 NH3(g) + heat • K = 3.5 x 108 at 298 K

10. Haber-Bosch Ammonia Synthesis Fritz Haber 1868-1934 Nobel Prize, 1918 Carl Bosch 1874-1940 Nobel Prize, 1931

11. EQUILIBRIUM AND EXTERNAL EFFECTS • Concentration changes • no change in K • only the position of equilibrium changes.

12. Le Chatelier’s Principle Adding a “reactant” to a chemical system.

13. Le Chatelier’s Principle Removing a “reactant” from a chemical system.

14. Le Chatelier’s Principle Adding a “product” to a chemical system.

15. Le Chatelier’s Principle Removing a “product” from a chemical system.

16. Butane-Isobutane Equilibrium butane isobutane

17. butane ButaneIsobutane • At equilibrium with [iso] = 1.25 M and [butane] = 0.50 M. K = 2.5. • Add 1.50 M butane. • When the system comes to equilibrium again, what are [iso] and [butane]? isobutane

18. Butane↔Isobutane Assume you are at equilibrium with [iso] = 1.25 M and [butane] = 0.50 M. Now add 1.50 M butane. When the system comes to equilibrium again, what are [iso] and [butane]? K = 2.5 Solution Calculate Q immediately after adding more butane and compare with K. Q is LESS THAN K. Therefore, the reaction will shift to the ____________.

19. Butane↔Isobutane You are at equilibrium with [iso] = 1.25 M and [butane] = 0.50 M. Now add 1.50 M butane. Solution Q is less than K, so equilibrium shifts right — away from butane and toward isobutane. Set up ICE table [butane] [isobutane] Initial Change Equilibrium 0.50 + 1.50 1.25 - x + x 1.25 + x 2.00 - x

20. Butane↔Isobutane You are at equilibrium with [iso] = 1.25 M and [butane] = 0.50 M. Now add 1.50 M butane. Solution x = 1.07 M At the new equilibrium position, [butane] = 0.93 M and [isobutane] = 2.32 M. Equilibrium has shifted toward isobutane.

21. EQUILIBRIUM AND EXTERNAL EFFECTS • Temperature, catalysts, and changes in concentration affect equilibria. • The outcome is governed by LE CHATELIER’S PRINCIPLE • “...if a system at equilibrium is disturbed, the system tends to shift its equilibrium position to counter the effect of the disturbance.”

22. Le Chatelier’s Principle • Change T • change in K • therefore change in P or concentrations at equilibrium • Use a catalyst: reaction comes more quickly to equilibrium. K not changed. • Add or take away reactant or product: • K does not change • Reaction adjusts to new equilibrium “position”

23. Nitrogen Dioxide EquilibriumN2O4(g) ↔ 2 NO2(g) Increase P in the system by reducing the volume (at constant T). e

24. Nitrogen Dioxide EquilibriumN2O4(g) ↔2 NO2(g) Increase P in the system by reducing the volume. In gaseous system the equilibrium will shift to the side with fewer molecules (in order to reduce the P). Therefore, reaction shifts LEFTand P of NO2 decreases and P of N2O4 increases.

25. Lab Problem – get 5 test tubes and graduated cylinder • In your lab journals • What factors affect chemical equilibrium? Lab • Goals: • Observe what happens when various substances added to equilibrium mixture • Describe chemistry taking place when a chemical causes a change in the equilibrium • Explain how temperature affects the equilibrium position

26. Materials • Iron (III) nitrate solution • Thiocyanate solution • Tin (II) chloride solution • Silver nitrate solution • Sodium hydrogen phosphate solution • Ammonia solution • Heat source (hot plate) • Ice

27. Procedure • Groups of 3 - 4 • Measure 1 mL of the iron (III) nitrate solution into a test tube and add 20 mL water. • Add one drop of the SCN- ion till you get a red-brown color • This will be your ‘stock solution’. You’ll need to transfer a few mL (2-3 mL) of this into 4 separate small test tubes or well plates or fill the well plate half full

28. Fe3+ + SCN- ↔ FeSCN2+ Sn2+ will reduce iron(III) to iron(II). Predict what will happen when SnCl2 is added to the solution. Which way will the reaction shift? What will happen to the color? Add drops of tin (II) chloride to one sample, and see if your prediction comes true . . .

29. Fe3+ + SCN- ↔ FeSCN2+ Silver ions form AgSCN in solution. Predict what will happen when silver nitrate is added to the solution. Which way will the reaction shift? What will happen to the color? Add drops of silver nitrate to one sample, and see if your prediction comes true . . .

30. Fe3+ + SCN- ↔ FeSCN2+ Hydrogen phosphate removes iron(III) as FeHPO4+. Predict what will happen when sodium hydrogen phosphate is added to the solution. Which way will the reaction shift? What will happen to the color? Add drops of sodium hydrogen phosphate to one sample, and see if your prediction comes true . . .

31. Fe3+ + SCN- ↔ FeSCN2+ Ammonia reacts to form solid iron(III)hydroxide. Predict what will happen if ammonia is added. Which way will the reaction shift? What will happen to the color? Test your prediction…

32. Fe3+ + SCN- ↔ FeSCN2+ Determine if the reaction is exothermic or endothermic by heating and cooling the system. Explain your conclusion (example: Our reaction is endothermic because . . . )

33. Wrap up your results… Summarize how LeChatelier’s principle was illustrated by this lab. Summarize all the different ways equilibrium can be shifted according to LeChat’s principle. Turn in by Friday.