1 TitrationsTitrations pHpH Titrant volume, mL At what point in a reaction does neutralization occur?

19.4 Neutralization Reactions > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. You can use a neutralization reaction to determine the concentration of an acid or base. The process of adding a measured amount of a solution of known concentration to a solution of unknown concentration is called a titration.Titration

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

19.4 Neutralization Reactions > 4 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A reaction between an acid and a base will go to completion when the solutions contain equal moles of hydrogen ions and hydroxide ions. Acid-Base Reactions The balanced equation provides the correct ratio of acid to base.

5 In strong acids, are all ionizable hydrogens completely ionized? In weak acids? In strong acids, all ionizable hydrogens are completely ionized. In weak acids, all ionizable hydrogens are partially ionized. Some hydrogens in these acids (those with larger K a values) have a greater degree of ionization.

6 Acetic acid titrated with NaOH Figure 18.5 Weak acid titrated with a strong base

19.5 Salts in Solution > 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Interpret Graphs One curve is for the addition of sodium hydroxide, a strong base, to ethanoic acid, a weak acid. An aqueous solution of sodium ethanoate exists at the equivalence point. CH 3 COOH(aq) + NaOH(aq) → CH 3 COONa(aq) + H 2 O(l)

19.5 Salts in Solution > 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Interpret Graphs This difference in pH exists because hydrolysis occurs with some salts in solution.

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

10 Acid-Base Titration Section 18.3 You titrate 100. mL of a M solution of benzoic acid with M NaOH to the equivalence point. What is the pH of the final solution? HBz + NaOH ---> Na + + Bz - + H 2 O You titrate 100. mL of a M solution of benzoic acid with M NaOH to the equivalence point. What is the pH of the final solution? HBz + NaOH ---> Na + + Bz - + H 2 O C 6 H 5 CO 2 H = HBz Benzoate ion = Bz - K b = 1.6 x

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

12 Acid-Base Reactions 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 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 QUESTION: You titrate 100. mL of a M solution of benzoic acid with M NaOH to the equivalence point. What is the pH of the final solution?

13 STOICHIOMETRY PORTION M * V = mol 1.Calc. moles of NaOH req’d (0.100 L)(0.025 M) = mol HBz This requires mol NaOH 2.Calc. volume of NaOH req’d mol (1 L / mol) = L 25 mL of NaOH req’d M * V = mol 1.Calc. moles of NaOH req’d (0.100 L)(0.025 M) = mol HBz This requires mol NaOH 2.Calc. volume of NaOH req’d mol (1 L / mol) = L 25 mL of NaOH req’d

14 STOICHIOMETRY PORTION 3. Moles Bz - produced = moles HBz = mol 4. Calc. conc. of Bz - There are mol of Bz - in a TOTAL SOLUTION VOLUME of 3. Moles Bz - produced = moles HBz = mol 4. Calc. conc. of Bz - There are mol of Bz - in a TOTAL SOLUTION VOLUME of 125 mL [Bz - ] = mol / L = M = M

15 Equivalence Point Most important species in solution is benzoate ion, Bz -. It will react to form the weak conjugate base, benzoic acid, HBz. Bz - + H 2 O  HBz + OH - K b = 1.6 x [Bz - ] [HBz] [OH - ] I C - x +x+x E x x x Most important species in solution is benzoate ion, Bz -. It will react to form the weak conjugate base, benzoic acid, HBz. Bz - + H 2 O  HBz + OH - K b = 1.6 x [Bz - ] [HBz] [OH - ] I C - x +x+x E x x x

16 Acid-Base Reactions x = [OH - ] = 1.8 x pOH = > pH = 8.25

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

18 Acid-Base Reactions You titrate 100. mL of a M solution of benzoic acid with M NaOH. What is the pH at the half-way point? At the half-way point, [HBz] = [Bz - ] Therefore, [H 3 O + ] = K a = 6.3 x pH = 4.20 = pK a of the acid Both HBz and Bz - are present. This is a BUFFER! Both HBz and Bz - are present. This is a BUFFER!

19 Figure 18.7 Weak base (NH 3 ) titrated with a strong acid (HCl)

19.4 Neutralization Reactions > 20 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Key Concepts In general, acids and bases react to produce a salt and water. Neutralization occurs when the number of moles of hydrogen ions is equal to the number of moles of hydroxide ions.

19.4 Neutralization Reactions > 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chemists use acid-base reactions to determine the concentration of an acid or a base in solution. BIG IDEA Reactions