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In the Arrhenius theory an acid produces H + in aqueous solution and a base produces OH – in aqueous solution.In the Arrhenius theory an acid produces.

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Presentation on theme: "In the Arrhenius theory an acid produces H + in aqueous solution and a base produces OH – in aqueous solution.In the Arrhenius theory an acid produces."— Presentation transcript:

1 In the Arrhenius theory an acid produces H + in aqueous solution and a base produces OH – in aqueous solution.In the Arrhenius theory an acid produces H + in aqueous solution and a base produces OH – in aqueous solution. The more general Brønsted-Lowry theory defines an acid as a H + donor and a base as a H + accepter.The more general Brønsted-Lowry theory defines an acid as a H + donor and a base as a H + accepter. CH 104: ACID-BASE PROPERTIES OF AQUEOUS SOLUTIONS Svante Arrhenius Johannes Brønsted Thomas Lowry

2 The symbol H + (aq) is convenient to use; however, it is not accurate. Hydrogen ion (H + ) is a proton without an electron. It is hydrated in water and exists as hydronium ion (H 3 O + (aq) ).The symbol H + (aq) is convenient to use; however, it is not accurate. Hydrogen ion (H + ) is a proton without an electron. It is hydrated in water and exists as hydronium ion (H 3 O + (aq) ). The self-ionization of water:The self-ionization of water: Preferred: 2H 2 O (l) = H 3 O + (aq) + OH – (aq) Accepted: H 2 O (l) = H + (aq) + OH – (aq) H 3 O + (or H + ) is acid.H 3 O + (or H + ) is acid. Hydroxide ion (OH – ) is base.Hydroxide ion (OH – ) is base. HYDROGEN, HYDRONIUM, AND HYDROXIDE IONS

3 The pH scale measures acidity. It typically ranges from 0 to 14. The acidity is neutral at pH 7. Values less than pH 7 are increasingly acidic. Values greater than pH 7 are increasingly basic.The pH scale measures acidity. It typically ranges from 0 to 14. The acidity is neutral at pH 7. Values less than pH 7 are increasingly acidic. Values greater than pH 7 are increasingly basic. pH AND HYDRONIUMION CONCENTRATION pH AND HYDRONIUM ION CONCENTRATION

4 If pH = 0.0, then [H 3 O + ] = 1 M = 1x10 –0 MIf pH = 0.0, then [H 3 O + ] = 1 M = 1x10 –0 M If pH = 1.0, then [H 3 O + ] = 0.1 M = 1x10 –1 MIf pH = 1.0, then [H 3 O + ] = 0.1 M = 1x10 –1 M If pH = 2.0, then [H 3 O + ] = 0.01 M = 1x10 –2 MIf pH = 2.0, then [H 3 O + ] = 0.01 M = 1x10 –2 M If pH = 3.0, then [H 3 O + ] = M = 1x10 –3 MIf pH = 3.0, then [H 3 O + ] = M = 1x10 –3 M If pH = 4.0, then [H 3 O + ] = M = 1x10 –4 MIf pH = 4.0, then [H 3 O + ] = M = 1x10 –4 M If pH = 5.0, then [H 3 O + ] = M = 1x10 –5 MIf pH = 5.0, then [H 3 O + ] = M = 1x10 –5 M If pH = 6.0, then [H 3 O + ] = M = 1x10 –6 MIf pH = 6.0, then [H 3 O + ] = M = 1x10 –6 M If pH = 7.0, then [H 3 O + ] = M = 1x10 –7 MIf pH = 7.0, then [H 3 O + ] = M = 1x10 –7 M If pH = 8.0, then [H 3 O + ] = M = 1x10 –8 MIf pH = 8.0, then [H 3 O + ] = M = 1x10 –8 M If pH = 9.0, then [H 3 O + ] = M = 1x10 –9 MIf pH = 9.0, then [H 3 O + ] = M = 1x10 –9 M If pH = 10.0, then [H 3 O + ] = M = 1x10 –10 MIf pH = 10.0, then [H 3 O + ] = M = 1x10 –10 M If pH = 11.0, then [H 3 O + ] = M = 1x10 –11 MIf pH = 11.0, then [H 3 O + ] = M = 1x10 –11 M If pH = 12.0, then [H 3 O + ] = M = 1x10 –12 MIf pH = 12.0, then [H 3 O + ] = M = 1x10 –12 M If pH = 13.0, then [H 3 O + ] = M = 1x10 –13 MIf pH = 13.0, then [H 3 O + ] = M = 1x10 –13 M If pH = 14.0, then [H 3 O + ] = M = 1x10 –14 MIf pH = 14.0, then [H 3 O + ] = M = 1x10 –14 M pH 2.0, 0.01 M, and 1x10 –2 M each have 1 significant figure. For pH and other logarithms, the numbers to the right of the decimal are significant. The numbers to the left of the decimal are NOT significant. The 0 in pH 2.0 is significant. The 2 in pH 2.0 is NOT significant, it defines the 2 in 1x10 –2 M.pH 2.0, 0.01 M, and 1x10 –2 M each have 1 significant figure. For pH and other logarithms, the numbers to the right of the decimal are significant. The numbers to the left of the decimal are NOT significant. The 0 in pH 2.0 is significant. The 2 in pH 2.0 is NOT significant, it defines the 2 in 1x10 –2 M. pH AND HYDRONIUMION CONCENTRATION pH AND HYDRONIUM ION CONCENTRATION

5 THE pH OF COMMON SUBSTANCES

6 This is the distribution of precipitation pH for North America. The combustion of sulfur-containing coal from Midwestern power plants is a major cause of acid precipitation.The combustion of sulfur-containing coal from Midwestern power plants is a major cause of acid precipitation. 2S + 3O 2 + 2H 2 O → 2H 2 SO 4 The prevailing winds carry this acid from the Midwest to the East.The prevailing winds carry this acid from the Midwest to the East. THE pH OF ACID PRECIPITATION

7 All concentrations are in moles per liter. (1) The Ion Product of Water = K w = 1.0x10 –14 = [H 3 O + ][OH – ] Rearranging Equation 1. (2) [H 3 O + ] = (1.0x10 –14 ) / [OH – ] (3) [OH – ] = (1.0x10 –14 ) / [H 3 O + ] “p” is the negative base 10 logarithm. (4) pH = –log 10 [H 3 O + ] = log 10 (1 / [H 3 O + ]) (5) pOH = –log 10 [OH – ] = log 10 (1 / [OH – ]) MATHEMATICS, ACIDS, AND BASES

8 Taking the “p” of Equation 1 and rearranging. K w = 1.0x10 –14 = [H 3 O + ][OH – ] pK w = =pH + pOH pK w = = pH + pOH (6) pH = – pOH (7) pOH = – pH Taking the antilogarithm of Equation 4. pH = –log 10 [H 3 O + ] (8) [H 3 O + ] = 10 (–pH) Taking the antilogarithm of Equation 5 and inserting Equation 7. pOH = –log 10 [OH – ] [OH – ] = 10 (–pOH) (9) [OH – ] = 10 (pH – 14.00) MATHEMATICS, ACIDS, AND BASES

9 In summary,In summary, (1) The Ion Product of Water = K w = 1.0x10 –14 = [H 3 O + ][OH – ] (2 and 8) [H 3 O + ] = (1.0x10 –14 ) / [OH – ] = 10 (–pH) = 10 (pOH – 14.00) (3 and 9) [OH – ] = (1.0x10 –14 ) / [H 3 O + ] = 10 (–pOH) = 10 (pH – 14.00) (4 and 6) pH = –log 10 [H 3 O + ] = log 10 (1 / [H 3 O + ]) = – pOH (5 and 7) pOH = –log 10 [OH – ] = log 10 (1 / [OH – ]) = – pH Complete this table.Complete this table. MATHEMATICS, ACIDS, AND BASES [H 3 O + ] [OH – ] pHpOH 3.5x10 –4 M 5.0x10 –10 M x10 –11 M x10 –5 M x10 –6 M 2.4x10 –9 M x10 –7 M 2.6x10 –8 M 6.42

10 A strong acid or a strong base in distilled water will almost completely ionize.A strong acid or a strong base in distilled water will almost completely ionize. Strong acid: HCl (g) + H 2 O (l) → H 3 O + (aq) + Cl – (aq) Strong base: NaOH (s) + H 2 O (l) → Na + (aq) + OH – (aq) Common strong acids and strong bases.Common strong acids and strong bases. a H 2 SO 4 ionizes in 2 steps. The first ionization goes to completion. The second ionization does not go to completion. STRONG ACIDS AND STRONG BASES AcidsBases HClHBrHI HClO 4 HNO 3 H 2 SO 4 a NaOHKOHRbOHCsOH Ca(OH) 2 Sr(OH) 2 Ba(OH) 2

11 Most acids and most bases are weak. That is, most acids and most bases in distilled water do not completely ionize.Most acids and most bases are weak. That is, most acids and most bases in distilled water do not completely ionize. Weak acid: HC 2 H 3 O 2(l) + H 2 O (l) = H 3 O + (aq) + C 2 H 3 O 2 – (aq)Weak acid: HC 2 H 3 O 2(l) + H 2 O (l) = H 3 O + (aq) + C 2 H 3 O 2 – (aq) A weak acid (HC 2 H 3 O 2 ) is in equilibrium with its conjugate base (C 2 H 3 O 2 – ).A weak acid (HC 2 H 3 O 2 ) is in equilibrium with its conjugate base (C 2 H 3 O 2 – ). Ionization Constant = K a = [H 3 O + ][C 2 H 3 O 2 – ] / [HC 2 H 3 O 2 ] = 1.74x10 –5 at 25° C.Ionization Constant = K a = [H 3 O + ][C 2 H 3 O 2 – ] / [HC 2 H 3 O 2 ] = 1.74x10 –5 at 25° C. Weak base: NH 3(aq) + H 2 O (l) = NH 4 + (aq) + OH – (aq)Weak base: NH 3(aq) + H 2 O (l) = NH 4 + (aq) + OH – (aq) A weak base (NH 3 ) is in equilibrium with its conjugate acid (NH 4 + ).A weak base (NH 3 ) is in equilibrium with its conjugate acid (NH 4 + ). Ionization Constant = K b = [NH 4 + ][OH – ] / [NH 3 ] = 1.74x10 –5 at 25° C.Ionization Constant = K b = [NH 4 + ][OH – ] / [NH 3 ] = 1.74x10 –5 at 25° C. WEAK ACIDS AND WEAK BASES

12 A buffer is a solution that resists drastic changes in pH when an acid or base is added.A buffer is a solution that resists drastic changes in pH when an acid or base is added. Furthermore, a buffer resists drastic changes in pH when it is diluted.Furthermore, a buffer resists drastic changes in pH when it is diluted. Buffers are used to control pH. For example, human blood is buffered at pH 7.4±0.1. The ability of blood to carry oxygen depends on the pH being within this range.Buffers are used to control pH. For example, human blood is buffered at pH 7.4±0.1. The ability of blood to carry oxygen depends on the pH being within this range. A buffer is a mixture of a weak acid and a salt of its conjugate base, or a weak base and a salt of its conjugate acid.A buffer is a mixture of a weak acid and a salt of its conjugate base, or a weak base and a salt of its conjugate acid. For example, a mixture of acetic acid (HC 2 H 3 O 2 ) and sodium acetate (NaC 2 H 3 O 2 ) is a common buffer.For example, a mixture of acetic acid (HC 2 H 3 O 2 ) and sodium acetate (NaC 2 H 3 O 2 ) is a common buffer. What is the conjugate base of acetic acid?What is the conjugate base of acetic acid? Acetate (C 2 H 3 O 2 – ).Acetate (C 2 H 3 O 2 – ). BUFFERS AND THE HENDERSON-HASSELBALCH EQUATION

13 (A similar Henderson-Hasselbalch equation would show how a buffer of a weak base and its salt resists drastic changes in pH.) This Henderson-Hasselbalch equation shows how a buffer of a weak acid and its salt resists drastic changes in pH.This Henderson-Hasselbalch equation shows how a buffer of a weak acid and its salt resists drastic changes in pH. BUFFERS AND THE HENDERSON-HASSELBALCH EQUATION

14 Therefore, the buffering of a weak acid and its salt depends on the relative concentrations of its conjugate base (A – ) and its unionized acid (HA).Therefore, the buffering of a weak acid and its salt depends on the relative concentrations of its conjugate base (A – ) and its unionized acid (HA). If a small amount of strong acid is added, it will combine with A – to make HA. If the change in [A – ]/[HA] is small, the change in pH will be small.If a small amount of strong acid is added, it will combine with A – to make HA. If the change in [A – ]/[HA] is small, the change in pH will be small. Conversely, if a small amount of strong base is added, it will react with HA to make A –. If the change in [A – ]/[HA] is small, the change in pH will be small.Conversely, if a small amount of strong base is added, it will react with HA to make A –. If the change in [A – ]/[HA] is small, the change in pH will be small. What has a larger buffering capacity (a larger resistance to changes in pH)? A solution with [A – ] = M and [HA] = M. Or a solution with [A – ] = 1 M and [HA] = 1 M.What has a larger buffering capacity (a larger resistance to changes in pH)? A solution with [A – ] = M and [HA] = M. Or a solution with [A – ] = 1 M and [HA] = 1 M. The solution with [A – ] = 1 M and [HA] = 1 M has a larger buffering capacity.The solution with [A – ] = 1 M and [HA] = 1 M has a larger buffering capacity. BUFFERS AND THE HENDERSON-HASSELBALCH EQUATION

15 The glass membrane of a pH electrode is made out of silicate groups with exchangeable hydrogen ions, Si-O-H +. These Si-O – groups are attached to the glass membrane. These H + ions are in equilibrium with the surface of the glass membrane and the sample.The glass membrane of a pH electrode is made out of silicate groups with exchangeable hydrogen ions, Si-O-H +. These Si-O – groups are attached to the glass membrane. These H + ions are in equilibrium with the surface of the glass membrane and the sample. Glass membrane-Si-O-H + (s) = Glass membrane-Si-O – (s) + H + (aq) If the number of H + ions in the sample is large, then the number of H + ions on the glass membrane is large and the electrode voltage is small. Conversely, if the number of H + ions in the sample is small, then the number of H + ions on the glass membrane is small and the electrode voltage is large. This voltage is converted to a pH value.If the number of H + ions in the sample is large, then the number of H + ions on the glass membrane is large and the electrode voltage is small. Conversely, if the number of H + ions in the sample is small, then the number of H + ions on the glass membrane is small and the electrode voltage is large. This voltage is converted to a pH value. MEASURING pH BY GLASS ELECTRODE

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17 A wide variety of dyes are used to make pH paper. These dyes change color with pH.A wide variety of dyes are used to make pH paper. These dyes change color with pH. MEASURING pH BY PAPER

18 Give at least 1 safety concern for the following procedure.Give at least 1 safety concern for the following procedure. Using acids and bases.Using acids and bases. These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in your eye, immediately flush with water.These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in your eye, immediately flush with water. Your laboratory manual has an extensive list of safety procedures. Read and understand this section.Your laboratory manual has an extensive list of safety procedures. Read and understand this section. Ask your instructor if you ever have any questions about safety.Ask your instructor if you ever have any questions about safety.SAFETY

19 Christian, G.D Analytical Chemistry, 3rd ed. New York, NY: John Wiley & Sons, Inc.Christian, G.D Analytical Chemistry, 3rd ed. New York, NY: John Wiley & Sons, Inc. Harris, D.C Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company.Harris, D.C Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company. Hill, J.W., D.K. Kolb Chemistry for Changing Times, 11th ed. Upper Saddle River, NJ: Pearson Prentice Hall.Hill, J.W., D.K. Kolb Chemistry for Changing Times, 11th ed. Upper Saddle River, NJ: Pearson Prentice Hall. McMurry, J., R.C. Fay Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall.McMurry, J., R.C. Fay Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall. Park, J.L ChemTeam: Photo Gallery Menu. Available: [accessed 9 October 2006].Park, J.L ChemTeam: Photo Gallery Menu. Available: [accessed 9 October 2006]. Petrucci, R.H General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.Petrucci, R.H General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.SOURCES


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