1. 19.4 Neutralization Reactions > 1 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

2. 19.4 Neutralization Reactions > 2 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Acid-Base Reactions What products form when an acid and a base react ?

3. 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

4. 4 Acid-Base Reactions: NEUTRALIZATION

5. Neutralization Reactions NaOH + HCl → NaCl + H 2 0 NaOH + HCl → NaCl + H 2 0 REACT a base with an acid. REACT a base with an acid. forms salt and water forms salt and water

6. 19.4 Neutralization Reactions > 6 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.

7. 19.4 Neutralization Reactions > 7 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Acid-Base Reactions For hydrochloric acid and sodium hydroxide, the mole ratio is 1:1. HCl(aq) + NaOH(aq) → NaCl(aq) + H 2 O(l) 1 mol

8. 19.4 Neutralization Reactions > 8 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. For sulfuric acid and sodium hydroxide, the ratio is 1:2. Acid-Base Reactions 1 mol2 mol1 mol2 mol Two moles of the base are required to neutralize one mole of the acid. H 2 SO 4 (aq) + 2NaOH(aq) → Na 2 SO 4 (aq) + 2H 2 O(l)

9. 19.4 Neutralization Reactions > 9 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Acid-Base Reactions Similarly, hydrochloric acid and calcium hydroxide react in a 2:1 ratio. 2HCl(aq) + Ca(OH) 2 (aq) → CaCl 2 (aq) + 2H 2 O(l) 2 mol1 mol 2 mol

10. 19.4 Neutralization Reactions > 10 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 19.7 Finding the Moles Needed for Neutralization How many moles of sulfuric acid are required to neutralize 0.50 mol of sodium hydroxide? The equation for the reaction is H 2 SO 4 (aq) + 2NaOH(aq) → Na 2 SO 4 (aq) + 2H 2 O.

11. 19.4 Neutralization Reactions > 11 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. KNOWNS mol NaOH = 0.50 mol 1 mol H 2 SO 4 /2 mol NaOH (from balanced equation) Analyze List the knowns and the unknown. 1 UNKNOWN mol H 2 SO 4 = ? mol To determine the number of moles of acid, you need to know the number of moles of base and the mole ratio of acid to base. Sample Problem 19.7

12. 19.4 Neutralization Reactions > 12 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculate Solve for the unknown. 2 Use the mole ratio of acid to base to determine the number of moles of acid. Sample Problem 19.7 0.50 mol NaOH × 1 mol H 2 SO 4 2 mol NaOH = 0.25 mol H 2 SO 4

13. 19.4 Neutralization Reactions > 13 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Evaluate Does the result make sense? 3 Because the mole ratio of H 2 SO 4 to NaOH is 1:2, the number of moles of H 2 SO 4 should be half the number of the moles of NaOH. Sample Problem 19.7

14. 19.4 Neutralization Reactions > 14 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Why are acid-base reactions called neutralization reactions? The complete reaction of an acid with a base creates a solution of a salt in water. This solution has a neutral pH. It is neither acidic nor basic.

15. 15 TitrationsTitrations pHpH Titrant volume, mL At what point in a reaction does neutralization occur?

16. 19.4 Neutralization Reactions > 16 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

17. 19.4 Neutralization Reactions > 17 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The steps in an acid-base titration are as follows: unknown 1.A measured volume of an acid solution of unknown concentration is added to a flask. 2.Several drops of an indicator are added to the solution while the flask is gently swirled. known 3.Measured volumes of a base of known concentrations are mixed into the acid until the indicator just barely changes color.Titration

18. 19.4 Neutralization Reactions > 18 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.Titration A flask with a known volume of acids (and an indicator) is placed beneath a buret that is filled with a base of known concentration. The base is slowly added from the buret to the acid. A change in the color of the solution is the signal that neutralization has occurred.

19. 19.4 Neutralization Reactions > 19 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The solution of known concentration is the standard solution.Titration You can use a similar procedure to find the concentration of a base using a standard acid.

20. 19.4 Neutralization Reactions > 20 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Neutralization occurs when the number of moles of hydrogen ions is equal to the number of moles of hydroxide ions.Titration

21. 19.4 Neutralization Reactions > 21 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Neutralization occurs when the number of moles of hydrogen ions is equal to the number of moles of hydroxide ions.Titration The point at which neutralization occurs is called the equivalence point.

22. 19.4 Neutralization Reactions > 22 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The indicator that is chosen for a titration must change color at or near the pH of the equivalence point.Titration The point at which the indicator changes color is the end point of the titration.

23. 19.4 Neutralization Reactions > 23 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. This graph shows how the pH of a solution changes during the titration of a strong acid (HCl) with a strong base (NaOH). The initial acid solution has a low pH (about 1). As NaOH is added, the pH increases because some of the acid reacts with the base.

24. 19.4 Neutralization Reactions > 24 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. The pH change of a solution during the titration of a strong acid (HCl) with a strong base (NaOH). The equivalence point for this reaction occurs at a pH of 7. As the titration nears the equivalence point, the pH rises dramatically because hydrogen ions are being used up.

25. 19.4 Neutralization Reactions > 25 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. This graph shows how the pH of a solution changes during the titration of a strong acid (HCl) with a strong base (NaOH). Interpret Graphs Extending the titration beyond the point of neutralization produces a further increase of pH.

26. 19.4 Neutralization Reactions > 26 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. This graph shows how the pH of a solution changes during the titration of a strong acid (HCl) with a strong base (NaOH). Interpret Graphs If the titration of HCl and NaOH could be stopped right at the equivalence point, the solution in the beaker would consist of only H 2 O and NaCl, plus a small amount of indicator.

27. 19.4 Neutralization Reactions > 27 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Sample Problem 19.8 Determining Concentration by Titration A 25-mL solution of H 2 SO 4 is neutralized by 18 mL of 1.0M NaOH. What is the concentration of the H 2 SO 4 solution? H 2 SO 4 (aq) + 2NaOH(aq) → Na 2 SO 4 (aq) + 2H 2 O(l).

28. 19.4 Neutralization Reactions > 28 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. KNOWNS [NaOH] = 1.0M V NaOH = 18 mL = 0.018 L V H 2 SO 4 = 18 mL = 0.018 L Analyze List the knowns and the unknown. 1 UNKNOWN [H 2 SO 4 ] = ?M The conversion steps are as follows: L NaOH → mol NaOH → mol H 2 SO 4 → M H 2 SO 4. Sample Problem 19.8 Convert volume to liters because molarity is in moles per liter.

29. 19.4 Neutralization Reactions > 29 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculate Solve for the unknown. 2 Use the molarity to convert the volume of base to moles of base. Sample Problem 19.8 0.018 L NaOH × 1.0 mol NaOH 1 L NaOH = 0.018 mol NaOH

30. 19.4 Neutralization Reactions > 30 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculate Solve for the unknown. 2 Use the mole ratio to find the moles of acid. Sample Problem 19.8 0.018 mol NaOH × 1.0 mol H 2 SO 4 2 mol NaOH = 0.0090 mol H 2 SO 4

31. 19.4 Neutralization Reactions > 31 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Calculate Solve for the unknown. 2 Calculate the molarity by dividing moles of acid by liters of solution. molarity = mol of solute L of solution 0.0090 mol 0.025 L = = 0.36M H 2 SO 4 Sample Problem 19.8

32. 19.4 Neutralization Reactions > 32 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Evaluate Does the result make sense? 3 If the acid had the same molarity as the base (1.0M), 50 mL of base would neutralize 25 mL of acid. Because the volume of the base is much less than 50 mL, the molarity of the acid must be much less than 1.0M. Sample Problem 19.8

33. 19.4 Neutralization Reactions > 33 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. In strong acids, are all ionizable hydrogens completely ionized? In weak acids?

34. 19.2 Hydrogen Ions and Acidity > 34 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Measuring pH What are two methods that are used to measure pH? Either acid-base indicators or pH meters can be used

35. IndicatorIndicator a compound that can reversibly change color depending on the concentration of H 3 O + ions. a compound that can reversibly change color depending on the concentration of H 3 O + ions. Used to measure pH Used to measure pH Indicator

36. 36

37. 19.2 Hydrogen Ions and Acidity > 37 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. An indicator (HIn) is a weak acid or base that dissociates in a known pH range. Indicators work because their acid form and base form have different colors in solution. Acid-Base Indicators The acid form of the indicator (HIn) is dominant at low pH and high [H + ]. The base form (In − ) is dominant at high pH and high [OH − ].

38. 19.2 Hydrogen Ions and Acidity > 38 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Acid-Base Indicators Many indicators are needed to span the entire pH spectrum.

39. 19.2 Hydrogen Ions and Acidity > 39 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. A pH meter can be easier to use than liquid indicators or indicator strips. pH Meters The pH reading is visible in a display window on the meter. The color and cloudiness of the solution do not affect the accuracy of the pH value obtained.

40. 40

41. 41 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.

42. 42 Acetic acid titrated with NaOH Figure 18.5 Weak acid titrated with a strong base

43. 19.5 Salts in Solution > 43 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)

44. 19.5 Salts in Solution > 44 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.

45. 45 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.

46. 46 Acid-Base Titration Section 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 - + H 2 O 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 - + H 2 O C 6 H 5 CO 2 H = HBz Benzoate ion = Bz - K b = 1.6 x 10 -10

47. 47 QUESTION: You titrate 100. mL of a 0.025 M solution of benzoic acid with 0.100 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?

48. 48 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 0.025 M solution of benzoic acid with 0.100 M NaOH to the equivalence point. What is the pH of the final solution?

49. 49 STOICHIOMETRY PORTION M * V = mol 1.Calc. moles of NaOH req’d (0.100 L)(0.025 M) = 0.0025 mol HBz This requires 0.0025 mol NaOH 2.Calc. volume of NaOH req’d 0.0025 mol (1 L / 0.100 mol) = 0.025 L 25 mL of NaOH req’d M * V = mol 1.Calc. moles of NaOH req’d (0.100 L)(0.025 M) = 0.0025 mol HBz This requires 0.0025 mol NaOH 2.Calc. volume of NaOH req’d 0.0025 mol (1 L / 0.100 mol) = 0.025 L 25 mL of NaOH req’d

50. 50 STOICHIOMETRY PORTION 3. Moles 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 3. Moles 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 = 0.020 M

51. 51 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 10 -10 [Bz - ] [HBz] [OH - ] I 0.020 0 0 C - x +x+x E 0.020 - 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 10 -10 [Bz - ] [HBz] [OH - ] I 0.020 0 0 C - x +x+x E 0.020 - x x x

52. 52 Acid-Base Reactions x = [OH - ] = 1.8 x 10 -6 pOH = 5.75 -----> pH = 8.25

53. 53 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? pH at half-way point? Equivalence point pH = 8.25 Equivalence point pH = 8.25

54. 54 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? At the half-way point, [HBz] = [Bz - ] Therefore, [H 3 O + ] = K a = 6.3 x 10 -5 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!

55. 55 Figure 18.7 Weak base (NH 3 ) titrated with a strong acid (HCl)

56. 19.4 Neutralization Reactions > 56 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.

57. 19.4 Neutralization Reactions > 57 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. neutralization reaction: a reaction in which an acid and a base react in an aqueous solution to produce a salt and water titration: process used to determine the concentration of a solution (often an acid or base) in which a solution of known concentration (the standard) is added to a measured amount of the solution of unknown concentration until an indicator signals the end point Glossary Terms

58. 19.4 Neutralization Reactions > 58 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. standard solution: a solution of known concentration used in carrying out a titration equivalence point: the point in a titration where the number of moles of hydrogen ions equals the number of moles of hydroxide ions end point: the point in a titration at which the indicator changes color Glossary Terms

59. 19.4 Neutralization Reactions > 59 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