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Chemistry 20

Chemistry 20. Chapter 8. PowerPoint presentation by R. Schultz. robert.schultz@ei.educ.ab.ca. 8.1 Limiting and Excess Reagents. Recall the illustration from Chapter 7:. 3 slices toast + 2 slices turkey + 4 strips bacon 1 sandwich.

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Chemistry 20

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  1. Chemistry 20 Chapter 8 PowerPoint presentation by R. Schultz robert.schultz@ei.educ.ab.ca

  2. 8.1 Limiting and Excess Reagents Recall the illustration from Chapter 7: 3 slices toast + 2 slices turkey + 4 strips bacon 1 sandwich 6 slices toast + 4 slices turkey + 8 strips bacon 2 sandwiches

  3. 8.1 Limiting and Excess Reagents • What would happen in the following situation? figure 8.1, page 296 2 sandwiches 6 sandwiches 5 sandwiches Only 2 sandwiches be made because …..

  4. 8.1 Limiting and Excess Reagents toast is the limiting reagent Do Thought Lab 8.1, page 296

  5. 8.1 Limiting and Excess Reagents limiting reagent is completely consumed in in a particular chemical reaction excess reagent is partially consumed in a particular chemical reaction even the identity of products of a chemical reaction are sometimes determined by whether a given reactant is limiting or excess

  6. 8.1 Limiting and Excess Reagents how to identify limiting reagent: easiest way – find moles of each reactant, use to find which produces the least number of moles of product – any product!

  7. 8.1 Limiting and Excess Reagents Example: Practice Problem 6, page 299 limiting C3H6(g) + 2 NH3(g) + 2 O2(g) C3H3N(g) + HCN(g) + 4 H2O(g) n2600 g n11.0 kg n3 pick a product – it doesn’t matter which, and find out which makes least number of moles of productI’ll use C3H3N and call it n3 limiting reagent is not necessarily the one with smaller mass

  8. 8.1 Limiting and Excess Reagents Once you’ve identified the limiting reagent you can do stoichiometry to calculate expected yields Example: Practice Problem 7&8, page 303 7. identify the limiting reagent – find which makes least moles of Mg3(PO4)(s) limiting 3 Mg(NO3)2(aq) + 2 Na3PO4(aq) Mg3(PO4)2(s) + 6 NaNO3(aq) n2125.0 mL1.2 mol/L n3 n1100.0 mL0.5 mol/L

  9. 8.1 Limiting and Excess Reagents 8. Calculate the mass of Mg3(PO4)2(s) formed n = 0.017 mol x 262.87 g/mol = 4 g What would happen if you used the wrong substance as limiting reagent? You would calculate a larger mass of Mg3(PO4)2(s)

  10. 8.1 Limiting and Excess Reagents Worksheet BLM 8.1.3 Worksheet BLM 8.1.5, questions 1-3 only

  11. 8.2 Predicted and Experimental Yields Predicted or theoretical yield – determined by stoichiometry Experimental or actual yield – what you end up getting Lab 8A, page 300

  12. 8.2 Predicted and Experimental Yields Factors limiting experimental yield: • competing reactions • incomplete reaction (because it’s slow) • incomplete reaction (because it reaches equilibrium)• reactant purity • mechanical losses (details page 306)

  13. 8.2 Predicted and Experimental Yields Example: question 4 page 311

  14. 8.2 Predicted and Experimental Yields a) 2 NaCl(aq) + 1 Pb(NO3)2(aq) 1 PbCl2(s) + 2 NaNO3(aq) n23.50 g n10.58 g precipitaten3m=? limiting Worksheet BLM 8.2.1

  15. 8.2 Predicted and Experimental Yields b) Worksheet BLM 8.2.1

  16. 8.3 Acid-Base Titration Titration Set-up: fig 8.5, page 312 Titration talk: “titration ofwith sample titrant” Point where erlenmeyer flask contains stoichiometrically equivalent moles of acid and base: equivalence point if indicator is properly chosen, endpoint occurs at equivalence point Point where indicator changes colour: endpoint

  17. 8.3 Acid-Base Titration standardizing: doing a titration to find the concentration of a titrant solution to be used in further analyses HCl(aq) needs to be standardized since pure HCl is a gas and escapes from solution NaOH(aq) needs to be standardized since its solutions absorb CO2(g) from the air causing its pH to drop popular titrants

  18. 8.3 Acid-Base Titration endpoints observed using acid-base indicators indicators are weak acid/base pairs where the 2 members have different colours chart page 10 of Data Booklet shows indicator acid/base pairs HIn(aq)  H+(aq) + In‾(aq) colour 1 colour 2

  19. 8.3 Acid-Base Titration HIn(aq) In‾(aq) HIn(aq) HIn(aq) In‾(aq) In‾(aq) green

  20. 8.3 Acid-Base Titration Indicators used to show endpoint Discuss questions 4-6, page 314

  21. 8.3 Acid-Base Titration Titration calculations – solution stoichiometry Example: Practice Problem 21, page 315 Questions states that “a student titrates HCl(aq) with NaOH(aq)” Which is the titrant? NaOH(aq) HCl(aq)+ NaOH(aq) → H2O(l) + NaCl(aq) n1v=(29.51-1.50) mLc=0.150 mol/L n2v=20.00 mLc=? to be continued …….

  22. 8.3 Acid-Base Titration Practice Problem 21, page 315, continued Practice Problem 22, page 315 states that “a student uses NaOH(aq) to titrate HNO3(aq)” Which is the titrant? NaOH(aq) Note that the base isn’t always the titrant Worksheet BLM 8.3.3, omit 1a, b

  23. 8.3 Acid-Base Titration Investigation 8.C, page 316

  24. 8.3 Acid-Base Titration Titration curves: Titration of a strong acid with a strong base: Titration of a strong base with a strong acid: figures 8.8, 8.9, page 318

  25. 8.3 Acid-Base Titration Discuss questions 8, 9, 10 page 319 Thought Lab 8.2 page 319 – Plotting a Titration Curve

  26. 8.3 Acid-Base Titration

  27. 8.3 Acid-Base Titration Chapter Review

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