Acids and Bases

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Acids and Bases
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Introduction

• Proton transfer reactions
• Acids
• H H2O ? H3O
• H20 HCl ? H30 Cl-
• Bases
• NaOH ? Na OH-
• NH3 H2O ? NH4 OH-

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Acid-Base Strength

• Strength of acids and bases is due to degree of
  ionization.
• Strong acids and bases are completely dissociated
• Weak acids and bases are not dissociated
  completely.

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Strong Acids

• HCl Hydrochloric acid
• HBr Hydrobromic acid
• HI Hydroiodic acid
• HNO3 Nitric acid
• H2SO4 Sulfuric acid
• HClO4 Perchloric acid

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Strong Bases

• LiOH Lithium hydroxide
• NaOH Sodium hydroxide
• KOH Potassium hydroxide
• Ba(OH)2 Barium hydroxide
• Ca(OH)2 Calcium hydroxide
• Mg(OH)2 Magnesium hydroxide

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• Strong acids and bases completely dissociate in
  water.
• A concentrated weak acid will never be a strong
  acid.

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Bronsted-Lowry Acids and Bases

• Acids are proton (H) donors.
• Must have an H attached, but not all Hs are
  ionizable.
• CH3COOH only the last H is acidic
• Bases are proton (H) acceptors.
• OH- is a strong base H OH- ? H2O
• NH3 is an important weak base
• NH3 H ? NH4

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Number of Protons

• Monoprotic acids HCl, HNO3, HBr, HI
• Diprotic acids H2SO4, H2CO3
• Triprotic acids H3PO4

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Amphiprotic

• When an substance can act as either an acid or a
  base it is called amphiprotic
• HCO3- ? CO32- H
• proton donated, acid
• HCO3- H ? H2CO3
• proton accepted, base

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Water Is Amphiprotic

• H2O ? H OH-
• H2O H ? H3O

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Conjugate Acid-Base Pairs

• When an acid donates a proton, the conjugate base
  is left.
• HCO3- ? CO32- H
• CO32- is the conjugate base of HCO3-

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• When an base accepts a proton, the conjugate acid
  is formed.
• SO42- H ? HSO4-
• HSO4- is the conjugate acid of SO42-

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• The stronger the acid, the weaker its conjugate
  base.
• The stronger the base, the weaker its conjugate
  acid.

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Equilibrium

• The stronger acid will prevail.
• HCl PO43- ? HPO42- Cl-
• Compare acid strengths
• HCl ? H Cl-
• HPO42- ? H PO43-
• HCl is a stronger acid than HPO42-
• The reaction is shifted to the right.

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• HCN SO42- ? HSO4- CN-
• Compare acid strengths
• HCN ? H CN-
• HSO4- ? H SO42-
• HSO4- is a stronger acid than HCN
• The reaction is shifted to the left.

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• Ka acid dissociation constant
• H3OA-
• Ka ________
• HA

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Properties of Acids and Bases

• Neutralization When an acid reacts with a base
  a salt and water are formed.
• Metals When an acids reacts with an active
  metal, a salt and hydrogen gas are formed.
• Carbonates and Bicarbonates When an acid reacts
  with CO32- or HCO3-, carbon dioxide and water are
  formed.

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Self-Ionization of Water

• Kw ion product constant for water
• Kw H3OOH-
• Kw 1.0 x 10-14

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• For pure water
• H3O OH- 1.0 x 10-7

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Examples

• If the H3O of a solution is 1 x 10-6, the
  OH- is
• 1 x 10-14 / 1 x 10-6 1 x 10-8

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• What is the OH- of a 0.001 M HCl solution?
• 1 x 10-14 / 1 x 10-3 1 x 10-11 M

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Examples

• Given H3O find OH-
• (a) 10-11 M
• (b) 10-4 M
• (c) 10-7 M
• (d) 10 M

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• Given OH- find H3O
• (a) 10-10 M
• (b) 10-2 M
• (c) 10-7 M
• (d) 10 M

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pH of solutions

• pH - logH3O
• H3O 10-pH

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Examples

• H3O 10-8 M
• pH 8
• H3O 10-10 M
• pH 10
• H3O 10-2 M
• pH 2

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pOH of solutions

• pOH - logOH-
• OH- 10-pOH

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Examples

• OH- 10-8 M
• pOH 8
• OH- 10-10 M
• pOH 10
• OH- 10-2 M
• pOH 2

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• pH pOH 14.00
• pH 8, then pOH 6
• pH 2, then pOH 12

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Examples

• Find pH, given H3O
• 10-8 M
• 10-10 M
• 10-2 M
• 10 M
• 10-7M

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• Find pH and pOH, given OH-
• 10-3 M
• 10-1 M
• 10-5 M
• 10-7M

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pH of Aqueous Salts

• Strong acid and strong base
• Hydrochloric acid and sodium hydroxide react to
  form sodium chloride and water
• HCl NaOH ? NaCl H2O
• HCl ? H Cl- strong
• NaOH ? Na OH- strong
• H OH- ? H2O
• HCl and the NaOH are completely dissociated. The
  H3O and OH- neutralize each other and the
  solution is neutral.

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• Salt of a strong acid and a weak base
• Hydrochloric acid and aluminum hydroxide react to
  form aluminum chloride and water
• 3HCl Al(OH)3 ? AlCl3 3H2O
• HCl ? H Cl- strong
• Al(OH)3 ? Al3 3OH- weak
• HCl is completely dissociated. The Al(OH)3 is a
  weak base and is not dissociated. Therefore
  there is an excess of H3O in solution and the
  solution is acidic.

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• Salt of a weak acid and a strong base
• Acetic acid and sodium hydroxide react to form
  sodium acetate and water
• CH3COOH NaOH ? NaCH3COO- H2O
• CH3COOH ? H CH3COO- weak
• NaOH ? Na OH- strong
• NaOH is completely dissociated. The CH3COOH is a
  weak acid and is not dissociated. Therefore
  there is an excess of OH- in solution and the
  solution is basic.

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• Determine which are acidic, basic and neutral
• BaCl2 (b) NaNO3
• (c) HCOONH4 (d) CaSO4
• (e) Na2HPO4 (f) Li2CO3

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Titration

• Titration is a laboratory procedure used to
  measure the number of moles of acid in an unknown
  acid solution by using a known, standardized base
  solution.

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• The equivalence point is the point where the
  number of moles of acid equal the number of moles
  of base.
• An indicator is used to tell when the equivalence
  point is reached. The indicator changes color
  when a certain pH is attained.
• The endpoint is the point where the indicator
  changes color.

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Examples

• A 50.0 mL solution of an unknown base was
  titrated with 0.150 M HCl. 22.0 mL of acid was
  needed to reach the endpoint. What is the
  molarity of the base?

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• A 3.00 M solution of HCl was used to titrate
  10.00 mL of an unknown base. 15.00 mL of acid
  was required. What is the molarity of the base?

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Blood pH

• Blood pH is maintained at 7.4 by buffers
• If the pH is lower than 7.35, then acidosis
• If the pH is higher than 7.45, then alkalosis

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Buffers

• Solution of a weak acid and its conjugate base
  (salt of the weak acid)
• Small amounts of acid or base do not change the
  pH of the solution significantly.
• pH is determined by the Ka of the weak acid
• pH pKa log(salt/acid)

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• What is the pH of a buffer solution made by
  dissolving 0.10 mol of formic acid, HCOOH, and
  0.10 mol of sodium formate, HCOONa, in 1.0 L of
  water?
• pKa of formic acid is 3.75