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Acid-Base Reactions: TITRATION

Acid-Base Reactions: TITRATION. Neutralization Reactions. You can neutralize a base with an acid. The result will form water molecules NaOH + HCl → NaCl + H 2 0. Acid-Base Reactions. Strong acid + strong base HCl + NaOH ----> SALT WATER Strong acid + weak base

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Acid-Base Reactions: TITRATION

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  1. Acid-Base Reactions:TITRATION

  2. Neutralization Reactions • You can neutralize a base with an acid. • The result will form water molecules • NaOH + HCl → NaCl + H20

  3. Acid-Base Reactions • Strong acid + strong base HCl + NaOH ----> SALT WATER • Strong acid + weak base HCl + NH3 ---> ACID • Weak acid + strong base HOAc + NaOH ---> BASIC • Weak acid + weak base HOAc + NH3 ---> Ka / Kb

  4. pH of Aqueous NH3 QUESTION: What is the effect on the pH of adding NH4Cl to 0.25 M NH3(aq)? NH3(aq) + H2O n NH4+(aq) + OH-(aq) Let us first calculate the pH of a 0.25 M NH3 solution. [NH3] [NH4+] [OH-] initial 0.25 0 0 change -x +x +x equilib 0.25 x x

  5. pH of Aqueous NH3 Assuming x is << 0.25, we have [OH-] = x = [Kb(0.25)]1/2 = 0.0021 M This gives pOH = 2.67 and so pH = 14.00 - 2.67 = 11.33 for 0.25 M NH3

  6. pH of NH3/NH4+ Mixture We expect that the pH will decline on adding NH4Cl. Let’s test that! [NH3] [NH4+] [OH-] I 0.25 0.1 0 C -x +x +x E 0.25 - x 0.10 + x x

  7. pH of NH3/NH4+ Mixture [OH-] = x = (0.25 / 0.10)Kb = 4.5 x 10-5 M This gives pOH = 4.35 and pH = 9.65 pH drops from 11.33 to 9.65 on adding a common ion.

  8. Titrations pH Titrant volume, mL

  9. Strong acid titrated with a strong base Figure 18.4

  10. Acetic acid titrated with NaOH Weak acid titrated with a strong base Figure 18.5

  11. Acid-Base Titrations Additional NaOH is added. pH increases and then levels off as NaOH is added beyond the equivalence point. Adding NaOH from the buret to acetic acid in the flask, a weak acid. In the beginning the pH increases very slowly.

  12. C6H5CO2H = HBz Benzoate ion = Bz- Acid-Base TitrationSection 18.3 You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution? HBz + NaOH ---> Na+ + Bz- + H2O Kb = 1.6 x 10-10

  13. pH at half-way point? pH at equivalence point? pH of solution of benzoic acid, a weak acid QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. Benzoic acid + NaOH

  14. Acid-Base Reactions QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution? Strategy —find the conc. of the conjugate base Bz- in the solution AFTER the titration, then calculate pH. This is a two-step problem 1. stoichiometry of acid-base reaction 2. equilibrium calculation

  15. Acid-Base Reactions STOICHIOMETRY PORTION 1. Calc. moles of NaOHreq’d (0.100 L)(0.025 M) = 0.0025 mol HBz This requires 0.0025 mol NaOH 2. Calc. volume of NaOHreq’d 0.0025 mol (1 L / 0.100 mol) = 0.025 L 25 mL of NaOHreq’d

  16. Acid-Base Reactions QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution? STOICHIOMETRY PORTION 25 mL of NaOHreq’d 3. Moles of Bz- produced = moles HBz = 0.0025 mol 4. Calc. conc. of Bz- There are 0.0025 mol of Bz- in a TOTAL SOLUTION VOLUME of 125 mL [Bz-] = 0.0025 mol / 0.125 L = 0.020 M

  17. Acid-Base Reactions Equivalence Point Most important species in solution is benzoate ion, Bz-, the weak conjugate base of benzoic acid, HBz. Bz- + H2ODHBz+ OH- Kb = 1.6 x 10-10 [Bz-][HBz] [OH-] I 0.020 0 0 C - x +x +x E 0.020 - x xx

  18. Acid-Base Reactions x = [OH-] = 1.8 x 10-6 pOH = 5.75 -----> pH = 8.25

  19. pH at half-way point? Equivalence point pH = 8.25 QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH at half-way point?

  20. Acid-Base Reactions You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 M NaOH. What is the pH at the half-way point? Both HBz and Bz- are present. This is a BUFFER! At the half-way point, [HBz] = [Bz-] Therefore, [H3O+] = Ka = 6.3 x 10-5 pH = 4.20 = pKa of the acid

  21. Weak diprotic acid (H2C2O4) titrated with a strong base (NaOH) Figure 18.6

  22. Weak base (NH3) titrated with a strong acid (HCl) Figure 18.7

  23. Indicator • a compound that can reversibly change color depending on the concentration of H3O+ ions. • Used to measure pH

  24. Indicators for Acid-Base Titrations

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