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Equilibrium

Equilibrium. Chapter 14 p. 496. Standard. HS-PS1-6. Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium PET sim: reaction rates Mission to Mars. Equilibrium. Many chemical reactions ‘go to completion’

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Equilibrium

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  1. Equilibrium Chapter 14 p. 496

  2. Standard HS-PS1-6. Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium PET sim: reaction rates Mission to Mars

  3. Equilibrium • Many chemical reactions ‘go to completion’ • But some chemical reactions are ‘reversible’: • forward reaction () where reactants become products • reverse reaction () where products become reactants Teacher notes: equilibrium arrows The chemical reaction that feeds the world - Daniel D. Dulek

  4. Equilibrium Model: Toothpick Lab Graph (Part A): Make a graph of Number of Toothpicks (final) in containers R and P against transfer round. Use different colors for the R and P lines. Remember: Title Independent (x) vs dependent variable (y) Design scale appropriately (and use ruler) axis labels Include legend

  5. Equilibrium Model: Toothpick Lab Think about it…. What changes can be observed in the number of toothpicks in R and P during the ten transfer rounds? Now write about it….. Formal writing, using academic vocab Suggested ~ 2 sentences

  6. Equilibrium Model: Toothpick Lab 1. What changes can be observed in the number of toothpicks in R and P during the ten transfer rounds? 2.What would happen if you continued doing the transfer for the entire class period (100+ transfer rounds!!). How many toothpicks would there be in R and P at the end of the period? Why? Explain.

  7. Equilibrium Model: Toothpick Lab 2.What would happen if you continued doing the transfer for the entire class period (100+ transfer rounds!!). How many toothpicks would there be in R and P at the end of the period? Why? Explain. 3a. What do the toothpicks represent? What do the cups represent? 3b. What would happen if we changed the parameters of our experiment to 1/4 forward and 1/3 back? 3c. A model always has limitations (it cannot be taken literally). What are some of the limitations of the toothpick model?

  8. Equilibrium model:Toothpick lab 3a. What do the toothpicks represent? What do the cups represent? 3b. What would happen if we changed the parameters of our experiment to 1/4 forward and 1/3 back? 3c. A model always has limitations (it cannot be taken literally). What are some of the limitations of the toothpick model? 4. Compare your new graph to the original graph we made (where there was a forward reaction only). How are they similar/different? Why?

  9. Graph 4. Compare your new graph A to the original graph we made (where there was a forward reaction only). How are they similar/different? Why? 5. Draw a tangent on your new graph P line at: • transfer round 2 • transfer round 6 What is the slope at transfer round 6? • What does this indicate?

  10. Equilibrium Model:Think about it….. 6. In this activity, the reactions between R and P appear to stop when no further changes were observed. Do chemical reactions actually stop when they reach equilibrium? Explain. PhET: Salts and Solubility

  11. Equilibrium model Think about it…. Part B “Does the position of equilibrium depend on the initial number of reactants?” 1. Calculate the P/R ratio from Number of Toothpicks (final) transfer round 6 (2 sig figs) for both Parts A and B. 2. Does the P/R ratio depend on the initial number of reactants? Yes or No. ANS = NO

  12. Frayer Summarizer: Equilibrium definition/facts/diagram/examples On the basis of this experiment, write your own definition of the word ‘equilibrium’ (closed book)

  13. Frayer Equilibrium definition = “A state of balance… in which the rate….. of the forward reaction equals….. the rate of the reverse reaction...and….. the concentrations…… of products and reactants…… remain unchanged.”

  14. Frayer: Equilibrium Facts • a dynamiccondition • Forward and reverse reactions continuously occur, but there is no overall change. • Does equilibrium mean there are equal amounts in R and P? No, equilibrium does not have to be a 50:50 mix Diagram: draw a representation of equilibrium (be creative)

  15. Frayer – Facts : Closed System What is the difference between a CLOSED SYSTEM and an open system? Draw it!

  16. Equilibrium Model:Think about it….. Part C: ‘Does the position of equilibrium depend upon the starting point?’ Hypothesis = ?? One of the toothpicks in your experiment is marked with black ink. Keep an eye on that particular toothpick during your experiment. Where was the labelled toothpick at transfer round 6? “Radioisotope labeling” https://www.youtube.com/watch?v=k16VeDo_jMo

  17. Equilibrium modelThink about it….. Part C ‘Does the position of equilibrium depend upon the starting point?’ Calculate the P/R ratio. How does the data for Part A and C differ? How is the data similar? What does this data tell us about equilibrium?

  18. Equilibrium Model: Toothpick Lab HONS: Using algebra, calculate the number of molecules (toothpicks) that would be present in R and P at equilibrium if we started with 100 in R and zero in P.*

  19. 14.2 Equilibrium Constant Frayer Look up the definition if equilibrium constant(Keq) in your textbook and copy it onto your Frayer (definition section). See Skills Toolkit p. 503 Read/notes p. 505 Keq shows if the reaction is favorable see Fig 7 and Table 2

  20. Calculating Equilibrium Constant** p. 504 # 1,2 Calculating Keq from [R][P] P. 506 # 1,2 Calculating [P] from Keq & [R] Practice: p. 522-523 # 26-34 evens only (5)

  21. Review: Solubility Consider the dissolving of calcium fluoride ‘Sparingly Soluble Salt’ CaF2 (s) ↔ Ca2+(aq) + 2F-(aq) ‘Dissociation’ Solubility = 3.4 x 10-4mol/L Demo: sparingly soluble salt PET sim: salts&solubility

  22. Solubility Product Constant, Ksp See p. 507-508 Solubility can also be quantified by the ‘Solubility Product Constant’, Ksp Sample Problem C Calculate the Kspof CaF2 Procedure: read Skills Toolkit p. 508 ANS: Ksp= 1.6 x 10-10

  23. Solubility Product Constant, KspSee p. 507-508 What do the numbers in Table 3 p.508 tell us about solubility? Which salt listed has the lowest solubility? Ag2S Ksp= 1.1 x 10-49 Practice Write the equilibrium expression for the Ksp of Ag2S Ag2S(s) ↔ 2Ag+(aq) + S2-(aq)

  24. Solubility Product Constant, Ksp Handout: Ion Sheet p. 509 #1,2,3 Calculating Ksp from Solubility Practice p. 523 # 35-38 Per 2 h’wk p. 523 # 35,36 (practice)

  25. Solubility Product Constant, Ksp p. 510 #1,2,3,4 Calcing Ion Conc using Ksp Practice p. 523-524 # 39-43 odds only (3)

  26. Equilibrium Model:Think about it…… Part D “What happens when more reactants are added to a system at equilibrium” Hypothesis = ? Compare the results obtained in Parts A,B&C with the results from part D. “Does the P/R ratio depend upon the initial number of reactants? Yes/No. Explain.” NO b/c……

  27. Equilibrium Water GamesPart 1 Prediction I predict that at equilibrium, the amount of water will be: • All of the water in P • Most of the water in P, a small amount in R • Half the water in P and the other half in R • Most of the water in R, a small amount in P • All of the water in R • There will not be an equilibrium with this system

  28. Equilibrium Water Games Part 1 Think about it…… “Why was most of the water in P and a smaller amount in R? How can this model be applied to a chemical system at equilibrium?”

  29. Closed System What is the difference between a CLOSED SYSTEM and an open system? Draw it!

  30. Equilibrium Water GamesPart 2 Prediction: Think about it….. Predict what you think will happen to the equilibrium water levels if we add extra water to the Reactant side and then continue the transfer. ‘Stress’

  31. Equilibrium Water GamePart 2 3. Explain: Le Chatelier’s Principle: “If we have a system at equilibrium...and… we apply a stress to it….. it will shift by undoing the stress.” read p. 512 paragraph 3

  32. Chemistry Standard 9a “Students know how to use Le Chatelier’s Principle to predict the effect of changes in: • concentration • temperature • pressure.” Homework read Sec 14.2 p. 512-516

  33. Le Chatelier: concentration Try process of elimination (POE)

  34. NGSS Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium. * [Clarification Statement: Emphasis is on the application of Le Chatelier’s Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products.]

  35. Equilibrium

  36. Le Chatelier’s Principle “If we have a system at equilibrium...and… we apply a stress to it….. it will shift by undoing the change.”

  37. Le Chatelier’s Principle Notes: Worksheet - Equilibrium and Le Chat’s Principle p. 4,5,6 p.6 Problems What are the five things that a chemical engineer could do to make more ammonia (shift equilibrium to the right)?

  38. Typical Le Chatelier’s Principle Problem • Start with a system at equilibrium • Stress applied: • Change in concentration • Change in pressure • Change in temperature • System shifts to relieve stress. Predict: • Does reaction rate increases in forward direction? OR • Does reaction rate increases in reverse direction? • New equilibrium established

  39. Practice question For choice B, count the total number of moles of gas on both sides of the equation.

  40. Pressure change? • How many moles of gas are on the left side of the eqn? • Left = 2 moles of gas • How many moles of gas are on the right side of the equation? • Right = 2 moles of gas • Does changing pressure have any effect on the equilibrium? • No, so the answer CANNOT be B

  41. Practice question ANS = C

  42. Practice

  43. Like cycling from the Valley to the beach energy released by system to surroundings Temp of surroundings increases Reactants have higher energy than products ‘downhill’ E.g. combustion Reminder: Exothermic reaction

  44. Temperature? • Is this reaction endothermic or exothermic? • Exothermic, because the system produces energy. • Choice A: If we add heat to the rxn (STRESS) it will try to remove the added energy (per Le Chat). • How? By moving to the left  (SHIFT) • So choice A cannot be correct.

  45. Pressure? • How many moles of gas are on the left side of the eqn? • Left = 5 moles of gas • How many moles of gas are on the right side of the equation? • Right = 2 moles of gas (careful) • Does changing pressure (STRESS) have any effect on the equilibrium? Yes • Choice D: system SHIFTS to less gas side (right)

  46. Practice question For choice A or C, count the total number of moles of gas on both sides of the equation. ANS = A, greater number of moles of gas side

  47. Practice question: pressure ANS = A, smaller number of moles of gas side

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