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

• The more general The more general Brønsted-Lowry theoryBrønsted-Lowry theory defines an acid defines an acid as a Has a H++ donor and a base as a H donor and a base as a H++ accepter. accepter.

CH 104: ACID-BASE PROPERTIES OF AQUEOUS SOLUTIONSCH 104: ACID-BASE PROPERTIES OF AQUEOUS SOLUTIONS

Svante ArrheniusSvante Arrhenius Johannes Brønsted Johannes Brønsted Thomas Lowry Thomas Lowry

• The symbol HThe symbol H++(aq)(aq) is convenient to use; however, it is not is convenient to use; however, it is not

accurate. accurate. Hydrogen ionHydrogen ion (H (H++) is a proton without an ) is a proton without an electron. It is hydrated in water and exists as electron. It is hydrated in water and exists as hydronium hydronium ionion (H (H33OO++

(aq)(aq)).).

• The self-ionization of water:The self-ionization of water:

Preferred: 2HPreferred: 2H22OO(l)(l) = H = H33OO++(aq)(aq) + OH + OH––

(aq)(aq)

Accepted: HAccepted: H22OO(l)(l) = H = H++(aq)(aq) + OH + OH––

(aq)(aq)

• HH33OO++ (or H (or H++) is acid.) is acid.

• Hydroxide ionHydroxide ion (OH (OH––) is base.) is base.

HYDROGEN, HYDRONIUM, AND HYDROXIDE IONSHYDROGEN, HYDRONIUM, AND HYDROXIDE IONS

• The pH scale measures acidity. It typically ranges from 0 The pH scale measures acidity. It typically ranges from 0 to 14. The acidity is neutral at pH 7. Values less than pH 7 to 14. The acidity is neutral at pH 7. Values less than pH 7 are increasingly acidic. Values greater than pH 7 are are increasingly acidic. Values greater than pH 7 are increasingly basic.increasingly basic.

pH AND HYDRONIUMpH AND HYDRONIUM ION CONCENTRATIONION CONCENTRATION

• If pH = 0.0, then [HIf pH = 0.0, then [H33OO++] = 1 M = 1x10] = 1 M = 1x10–0–0 M M• If pH = 1.0, then [HIf pH = 1.0, then [H33OO++] = 0.1 M = 1x10] = 0.1 M = 1x10–1–1 M M• If pH = 2.0, then [HIf pH = 2.0, then [H33OO++] = 0.01 M = 1x10] = 0.01 M = 1x10–2–2 M M• If pH = 3.0, then [HIf pH = 3.0, then [H33OO++] = 0.001 M = 1x10] = 0.001 M = 1x10–3–3 M M• If pH = 4.0, then [HIf pH = 4.0, then [H33OO++] = 0.0001 M = 1x10] = 0.0001 M = 1x10–4–4 M M• If pH = 5.0, then [HIf pH = 5.0, then [H33OO++] = 0.00001 M = 1x10] = 0.00001 M = 1x10–5–5 M M• If pH = 6.0, then [HIf pH = 6.0, then [H33OO++] = 0.000001 M = 1x10] = 0.000001 M = 1x10–6–6 M M• If pH = 7.0, then [HIf pH = 7.0, then [H33OO++] = 0.0000001 M = 1x10] = 0.0000001 M = 1x10–7–7 M M• If pH = 8.0, then [HIf pH = 8.0, then [H33OO++] = 0.00000001 M = 1x10] = 0.00000001 M = 1x10–8–8 M M• If pH = 9.0, then [HIf pH = 9.0, then [H33OO++] = 0.000000001 M = 1x10] = 0.000000001 M = 1x10–9–9 M M• If pH = 10.0, then [HIf pH = 10.0, then [H33OO++] = 0.0000000001 M = 1x10] = 0.0000000001 M = 1x10–10–10 M M• If pH = 11.0, then [HIf pH = 11.0, then [H33OO++] = 0.00000000001 M = 1x10] = 0.00000000001 M = 1x10–11–11 M M• If pH = 12.0, then [HIf pH = 12.0, then [H33OO++] = 0.000000000001 M = 1x10] = 0.000000000001 M = 1x10–12–12 M M• If pH = 13.0, then [HIf pH = 13.0, then [H33OO++] = 0.0000000000001 M = 1x10] = 0.0000000000001 M = 1x10–13–13 M M• If pH = 14.0, then [HIf pH = 14.0, then [H33OO++] = 0.00000000000001 M = 1x10] = 0.00000000000001 M = 1x10–14–14 M M• pH 2.0, 0.01 M, and 1x10pH 2.0, 0.01 M, and 1x10–2–2 M each have 1 significant figure. For pH and M each have 1 significant figure. For pH and

other logarithms, the numbers to the right of the decimal are other logarithms, the numbers to the right of the decimal are significant. The numbers to the left of the decimal are NOT significant. 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 The 0 in pH 2.0 is significant. The 2 in pH 2.0 is NOT significant, it defines the 2 in 1x10defines the 2 in 1x10–2–2 M. M.

pH AND HYDRONIUMpH AND HYDRONIUM ION CONCENTRATIONION CONCENTRATION

THE pH OF COMMON SUBSTANCESTHE pH OF COMMON SUBSTANCES

This is the distribution of precipitation pH for North America.This is the distribution of precipitation pH for North America.• The combustion of sulfur-containing coal from Midwestern power The combustion of sulfur-containing coal from Midwestern power

plants is a major cause of acid precipitation.plants is a major cause of acid precipitation.2S + 3O2S + 3O22 + 2H + 2H22O → 2HO → 2H22SOSO44

• 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 PRECIPITATIONTHE pH OF ACID PRECIPITATION

All concentrations are in moles per liter.All concentrations are in moles per liter.

(1) The Ion Product of Water = K(1) The Ion Product of Water = Kww = 1.0x10 = 1.0x10–14–14 = [H = [H33OO++][OH][OH––]]

Rearranging Equation 1.Rearranging Equation 1.

(2) [H(2) [H33OO++] = (1.0x10] = (1.0x10–14–14) / [OH) / [OH––]]

(3) [OH(3) [OH––] = (1.0x10] = (1.0x10–14–14) / [H) / [H33OO++]]

““p” is the negative base 10 logarithm.p” is the negative base 10 logarithm.

(4) pH = –log(4) pH = –log1010[H[H33OO++] = log] = log1010(1 / [H(1 / [H33OO++])])

(5) pOH = –log(5) pOH = –log1010[OH[OH––] = log] = log1010(1 / [OH(1 / [OH––])])

MATHEMATICS, ACIDS, AND BASESMATHEMATICS, ACIDS, AND BASES

Taking the “p” of Equation 1 and rearranging.Taking the “p” of Equation 1 and rearranging.KKww = 1.0x10 = 1.0x10–14–14 = [H = [H33OO++][OH][OH––]]

pKpKww = 14.00 = = 14.00 = pH + pOHpH + pOH

(6) pH = 14.00 – pOH(6) pH = 14.00 – pOH(7) pOH = 14.00 – pH(7) pOH = 14.00 – pH

Taking the antilogarithm of Equation 4.Taking the antilogarithm of Equation 4.pH = –logpH = –log1010[H[H33OO++]]

(8) [H(8) [H33OO++] = 10] = 10(–pH)(–pH)

Taking the antilogarithm of Equation 5 and inserting Taking the antilogarithm of Equation 5 and inserting Equation 7.Equation 7.

pOH = –logpOH = –log1010[OH[OH––]]

[OH[OH––] = 10] = 10(–pOH)(–pOH)

(9) [OH(9) [OH––] = 10] = 10(pH – 14.00)(pH – 14.00)

MATHEMATICS, ACIDS, AND BASESMATHEMATICS, ACIDS, AND BASES

• In summary,In summary,(1) The Ion Product of Water = K(1) The Ion Product of Water = Kww = 1.0x10 = 1.0x10–14–14 = [H = [H33OO++][OH][OH––]]

(2 and 8) [H(2 and 8) [H33OO++] = (1.0x10] = (1.0x10–14–14) / [OH) / [OH––] = 10] = 10(–pH)(–pH) = 10 = 10(pOH – 14.00)(pOH – 14.00)

(3 and 9) [OH(3 and 9) [OH––] = (1.0x10] = (1.0x10–14–14) / [H) / [H33OO++] = 10] = 10(–pOH)(–pOH) = 10 = 10(pH – 14.00)(pH – 14.00)

(4 and 6) pH = –log(4 and 6) pH = –log1010[H[H33OO++] = log] = log1010(1 / [H(1 / [H33OO++]) = 14.00 – pOH]) = 14.00 – pOH

(5 and 7) pOH = –log(5 and 7) pOH = –log1010[OH[OH––] = log] = log1010(1 / [OH(1 / [OH––]) = 14.00 – pH]) = 14.00 – pH

• Complete this table.Complete this table.

MATHEMATICS, ACIDS, AND BASESMATHEMATICS, ACIDS, AND BASES

[H[H33OO++]] [OH[OH––]] pHpH pOHpOH

3.5x103.5x10–4–4 M M

5.0x105.0x10–10–10 M M

5.385.38

7.587.58

2.9x102.9x10–11–11 M M 3.463.46 10.5410.54

2.0x102.0x10–5–5 M M 4.704.70 9.309.30

4.2x104.2x10–6–6 M M 2.4x102.4x10–9–9 M M 8.628.62

3.8x103.8x10–7–7 M M 2.6x102.6x10–8–8 M M 6.426.42

• A strong acid or a strong base in distilled water will almost completely A strong acid or a strong base in distilled water will almost completely ionize.ionize.

Strong acid: HClStrong acid: HCl(g)(g) + H + H22OO(l)(l) → H → H33OO++(aq)(aq) + Cl + Cl––

(aq)(aq)

Strong base: NaOHStrong base: NaOH(s)(s) + H + H22OO(l)(l) → Na → Na++(aq)(aq) + OH + OH––

(aq)(aq)

• Common strong acids and strong bases.Common strong acids and strong bases.

aa H H22SOSO44 ionizes in 2 steps. The first ionization goes to completion. The ionizes in 2 steps. The first ionization goes to completion. The

second ionization does not go to completion.second ionization does not go to completion.

STRONG ACIDS AND STRONG BASESSTRONG ACIDS AND STRONG BASES

AcidsAcids BasesBases

HClHClHBrHBrHIHIHClOHClO

44

HNOHNO33

HH22SOSO44

aa

NaOHNaOHKOHKOHRbOHRbOHCsOHCsOHCa(OH)Ca(OH)

22

Sr(OH)Sr(OH)22

Ba(OH)Ba(OH)22

• Most acids and most bases are weak. That is, most acids and most Most acids and most bases are weak. That is, most acids and most bases in distilled water do not completely ionize.bases in distilled water do not completely ionize.

• Weak acid: HCWeak acid: HC22HH33OO2(l)2(l) + H + H22OO(l)(l) = H = H33OO++(aq)(aq) + C + C22HH33OO22

––(aq)(aq)

• A A weak acidweak acid (HC (HC22HH33OO22) is in equilibrium with its ) is in equilibrium with its conjugate baseconjugate base

(C(C22HH33OO22––).).

• Ionization Constant = KIonization Constant = Kaa = [H = [H33OO++][C][C22HH33OO22––] / [HC] / [HC22HH33OO22] = 1.74x10] = 1.74x10–5–5 at 25° at 25°

C.C.

• Weak base: NHWeak base: NH3(aq)3(aq) + H + H22OO(l)(l) = NH = NH44++

(aq)(aq) + OH + OH––(aq)(aq)

• A A weak baseweak base (NH (NH33) is in equilibrium with its ) is in equilibrium with its conjugate acidconjugate acid (NH (NH44++).).

• Ionization Constant = KIonization Constant = Kbb = [NH = [NH44++][OH][OH––] / [NH] / [NH33] = 1.74x10] = 1.74x10–5–5 at 25° C. at 25° C.

WEAK ACIDS AND WEAK BASESWEAK ACIDS AND WEAK BASES

• A A bufferbuffer is a solution that is a solution that resists drastic changes in pHresists drastic changes in pH when an acid when an acid or base is added.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.Buffers are used to control pH. For example, human blood is buffered For example, human blood is buffered at pH 7.4±0.1. The ability of blood to carry oxygen depends on the pH at pH 7.4±0.1. The ability of blood to carry oxygen depends on the pH being within this range.being within this range.

• A buffer is a mixture of a weak acid and a salt of its conjugate base, or 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 weak base and a salt of its conjugate acid.

• For example, a mixture of acetic acid (HCFor example, a mixture of acetic acid (HC22HH33OO22) and sodium acetate ) and sodium acetate

(NaC(NaC22HH33OO22) is a common buffer.) is a common buffer.

• What is the conjugate base of acetic acid?What is the conjugate base of acetic acid?• Acetate (CAcetate (C22HH33OO22

––).).

BUFFERS AND THE HENDERSON-HASSELBALCH EQUATIONBUFFERS AND THE HENDERSON-HASSELBALCH EQUATION

(A similar Henderson-Hasselbalch equation would show how a buffer of (A similar Henderson-Hasselbalch equation would show how a buffer of a weak base and its salt resists drastic changes in pH.)a weak base and its salt resists drastic changes in pH.)

• This Henderson-Hasselbalch equation shows how a buffer of a weak This Henderson-Hasselbalch equation shows how a buffer of a weak acid and its salt resists drastic changes in pH.acid and its salt resists drastic changes in pH.

BUFFERS AND THE HENDERSON-HASSELBALCH EQUATIONBUFFERS AND THE HENDERSON-HASSELBALCH EQUATION

• Therefore, the buffering of a weak acid and its salt depends on the Therefore, the buffering of a weak acid and its salt depends on the relative concentrations of its conjugate base (Arelative concentrations of its conjugate base (A––) and its unionized acid ) and its unionized acid (HA).(HA).

• If a small amount of strong acid is added, it will combine with AIf a small amount of strong acid is added, it will combine with A–– to to make HA. If the change in [Amake HA. If the change in [A––]/[HA] is small, the change in pH will be ]/[HA] is small, the change in pH will be small.small.

• Conversely, if a small amount of strong base is added, it will react with Conversely, if a small amount of strong base is added, it will react with HA to make AHA to make A––. If the change in [A. If the change in [A––]/[HA] is small, the change in pH will ]/[HA] is small, the change in pH will be small.be small.

• What has a larger What has a larger buffering capacitybuffering capacity (a larger resistance to changes in (a larger resistance to changes in pH)? A solution with [ApH)? A solution with [A––] = 0.001 M and [HA] = 0.001 M. Or a solution ] = 0.001 M and [HA] = 0.001 M. Or a solution with [Awith [A––] = 1 M and [HA] = 1 M.] = 1 M and [HA] = 1 M.

• The solution with [AThe solution with [A––] = 1 M and [HA] = 1 M has a larger buffering ] = 1 M and [HA] = 1 M has a larger buffering capacity.capacity.

BUFFERS AND THE HENDERSON-HASSELBALCH EQUATIONBUFFERS AND THE HENDERSON-HASSELBALCH EQUATION

• The glass membrane of a pH The glass membrane of a pH electrode is made out of silicate electrode is made out of silicate groups with exchangeable groups with exchangeable hydrogen ions, Si-O-Hhydrogen ions, Si-O-H++. These . These

Si-OSi-O–– groups are attached to the groups are attached to the glass membrane. These Hglass membrane. These H++ ions ions are in equilibrium with the surface are in equilibrium with the surface of the glass membrane and the of the glass membrane and the sample.sample.

Glass membrane-Si-O-HGlass membrane-Si-O-H++(s)(s) = =

Glass membrane-Si-OGlass membrane-Si-O––(s)(s) + H + H++(aq)(aq)

• If the number of HIf the number of H++ ions in the ions in the sample is large, then the number sample is large, then the number of Hof H++ ions on the glass membrane ions on the glass membrane is large and the electrode voltage is large and the electrode voltage is small. Conversely, if the number is small. Conversely, if the number of Hof H++ ions in the sample is small, ions in the sample is small, then the number of Hthen the number of H++ ions on the ions on the glass membrane is small and the glass membrane is small and the electrode voltage is large. This electrode voltage is large. This voltage is converted to a pH value.voltage is converted to a pH value.

MEASURING pH BY GLASS ELECTRODEMEASURING pH BY GLASS ELECTRODE

MEASURING pH BY GLASS ELECTRODEMEASURING pH BY GLASS ELECTRODE

• A wide variety of dyes are used to make pH paper. These A wide variety of dyes are used to make pH paper. These dyes change color with pH.dyes change color with pH.

MEASURING pH BY PAPERMEASURING pH BY PAPER

• 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. These are irritants. Wear your goggles at all times. Immediately clean all spills. If you do get either of these in Immediately clean all spills. If you do get either of these in your eye, immediately flush with water.your eye, immediately flush with water.

• Your laboratory manual has an extensive list of safety Your laboratory manual has an extensive list of safety procedures. Read and understand this section.procedures. Read and understand this section.

• Ask your instructor if you ever have any questions about Ask your instructor if you ever have any questions about safety.safety.

SAFETYSAFETY

• Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New York, NY: John Christian, G.D. 1986. Analytical Chemistry, 3rd ed. New York, NY: John Wiley & Sons, Inc.Wiley & Sons, Inc.

• Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. New York, Harris, D.C. 1999. Quantitative Chemical Analysis, 5th ed. New York, NY: W.H. Freeman Company.NY: W.H. Freeman Company.

• Hill, J.W., D.K. Kolb. 2007. Chemistry for Changing Times, 11th ed. Hill, J.W., D.K. Kolb. 2007. Chemistry for Changing Times, 11th ed. Upper Saddle River, NJ: Pearson Prentice Hall.Upper Saddle River, NJ: Pearson Prentice Hall.

• McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: McMurry, J., R.C. Fay. 2004. Chemistry, 4th ed. Upper Saddle River, NJ: Prentice Hall.Prentice Hall.

• Park, J.L. 2004. ChemTeam: Photo Gallery Menu. Available: Park, J.L. 2004. ChemTeam: Photo Gallery Menu. Available: http://dbhs.wvusd.k12.ca.us/webdocs/Gallery/GalleryMenu.html [accessed [accessed 9 October 2006].9 October 2006].

• Petrucci, R.H. 1985. General Chemistry Principles and Modern Petrucci, R.H. 1985. General Chemistry Principles and Modern Applications, 4th ed. New York, NY: Macmillan Publishing Company.Applications, 4th ed. New York, NY: Macmillan Publishing Company.

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