AcidBase Equilibria

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Acid-Base Equilibria

• BLB 10th Chapter 16

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Examples of acids bases
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16.1 Acids Bases A Brief Review

• Arrhenius Definitions
• Acid a substance that produces hydrogen ions
  (H) in water
• HA ? H A-
• Base a substance that produces hydroxide ions
  (OH-) in water
• BOH ? B OH-

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16.2 Brønsted-Lowry Acids Bases

• H (proton) in water
• H H2O ? H3O
• hydronium ion
• Hydronium ion can hydrogen bond with more water
  molecules to form large clusters of hydrated
  hydronium ions.
• H and H3O are used interchangeably.

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16.2 Brønsted-Lowry Acids Bases

• Brønsted-Lowry definitions
• acid proton donor
• Neutral (HNO3), anionic (HCO3-), cationic (NH4)
• Must have a removable (acidic) proton
• base proton acceptor
• Neutral (NH3), anionic (CO32-)
• Must have a lone pair of electrons

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Acid-Base Reactions
HCl(aq) H2O(l) ? H3O (aq) Cl-(aq) NH3(aq)
H2O(l) ? NH4(aq) OH-(aq) HCl(aq) NH3(aq) ?
NH4(aq) Cl-(aq)
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Acid-base reaction in non-aqueous mediaHCl(g)
NH3(g) ? NH4Cl(s)
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• amphiprotic capable of behaving as a Brønsted
  acid and Brønsted base
• amphoteric capable of behaving as a Lewis acid
  and Brønsted base (17.5)
• neutralization acid base ? salt water
• Conjugate acid/base pairs differ by a single
  proton
• HA(aq) H2O(l) ? H3O(aq) A-(aq)
• acid base conj. acid conj.
  base

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Relative Acid/Base Strength

• Strength is a measure of the ability of an acid
  (or base) to donate (or accepts) a H.
• Stronger acids donate H more readily.
• Completely dissociate in water
• Conjugate bases have negligible tendency to
  accept protons.
• Weaker acids donate H less readily.
• Partially dissociate and establish equilibrium
• Conjugate bases have some tendency to accept
  protons.
• The stronger an acid, the weaker its conjugate
  base and vice versa.

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p. 672
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• Acid/base reactions proceed from the stronger
  acid-base pair to the weaker acid-base pair.
• Common strong acids (p. 679)
• HClO4, HClO3, H2SO4, HI, HBr, HCl, HNO3
• Monoprotic acid capable of donating only one H
• Polyprotic acid capable of donating more than
  one H
• Common strong bases (p. 680)
• M(OH)n, where M group I (n1) II (n2)
  metals, except Be

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Acid/Base Reactions
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16.3 The Autoionization of Water

• H2O(l) H2O(l) ? H3O(aq) OH-(aq)
• Kw H3OOH- HOH- 1.0 x 10-14 (_at_
  25C)
• Kw ion-product constant (or dissociation
  constant)
• Pure water is neutral. Thus,
• H3O OH- 1.0 x 10-7 M _at_ 25C
• For an aqueous solution
• H3O gt OH- acidic
• H3O OH- neutral
• H3O lt OH- basic

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Working with Kw
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16.4 The pH Scale

• pH represents a solutions acidity (_at_ 25C. 0
  ? 7 ? 14
• acid neutral base
• See Table 16.1, p. 678 for summary.
• See Figure 16.5, p. 679 for examples.
• pH -logH3O -logH
• H3O 10-pH
• pOH -logOH- pH pOH 14
• OH- 10-pOH

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p. 676
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More common chemicals
CaCO3 CO3- H2O ? HCO3- OH- CO2 H2O ?
H2CO3
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pH calculations
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More about pH

• pH does not necessarily indicate strength.
• Measuring pH
• pH meters
• Acid-base indicators

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p. 679
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16.5 Strong Acids and Bases

• Strong acids bases completely ionize.
• HA0 H3O ? pH
• MOH0 OH- ? pOH ? pH
• 2M(OH)20 OH- ? pOH ? pH
• H3O is the strongest acid that can exist in
  water. (produced by all acids in water)
• OH- is the strongest base that can exist in
  water. (produced by all bases in water)

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pH problems
End Test 1 material
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16.6 Weak Acids 16.7 Weak Bases

• Weak acids bases do not completely ionize.
• Weak acids establish an equilibrium in aqueous
  solution.
• HA(aq) H2O(l) ? H3O(aq) A-(aq)
• HA(aq) ? H(aq) A-(aq)
• They do not readily donate or accept Hs.
• HA0 ? H3O
• MOH0 ? OH-

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Weak Acids Acid-dissociation Constant
HA(aq) H2O(l) ? H3O(aq) A-(aq) HA(aq) ?
H(aq) A-(aq) Ka ? acid strength ? For
polyprotic acids Ka1 gtgt Ka2 gtgt Ka3 pKa
-logKa pKa ? acid strength?
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From p. 682 more in Appendix D, p. 1115-1116
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Weak Bases Base-dissociation Constant

• Weak bases establish an equilibrium in aqueous
  solution.
• B(aq) H2O(l) ? BH(aq) OH-(aq)

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From p. 691 more in Appendix D, p. 1115-1116
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Dissociation (or ionization)

• dissociation decreases as concentration
  increases (p. 686)

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Weak acid/base Problems1) Ka (or Kb) from
equilibrium pH2) pH from Ka (or Kb)

• Identify as weak acid or base.
• Write the chemical equilibrium.
• Write the equilibrium constant expression.
• Set up concentration table. (Ch. 15.5)
• Solve for x.
• Check with 5 rule. If greater than 5, use
  quadratic equation. (type 2 only)
• Complete problem.

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The pH of a 0.10 M solution of propanoic acid
(CH3CH2CO2H) is 2.94. Calculate the Ka for
propanoic acid.
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Calculate the pH of a 1.0 M HF solution.
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Calculate the pH of a 0.0010 M HF solution.
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Calculate the pH of a 0.20 M solution of
triethylamine N(CH2CH3)3.
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16.8 Relationship between Ka and Kb

• For a conjugate acid/base pair
• Ka x Kb Kw (derivation p. 693)
• pKa pKb pKw 14.00

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16.9 Acid-Base Properties of Salt Solutions

• Salt ionic compound
• Salts dissolve in water to produce ions.
• Ions can also affect the pH.
• Hydrolysis reaction between an ion and water to
  produce H3O or OH-
• F-(aq) H2O(l) ? HF(aq) OH-(aq)
• NH4(aq) H2O(l) ? H3O(aq) NH3(aq)

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Which ions will undergo hydrolysis, i.e. react
with water and affect the pH of the solution?

• Anion
• Conjugate base of a weak acid ? basic
• Conjugate base of a monoprotic strong acid ?
  neutral
• Cation
• Conjugate acid of a weak base ? acidic
• Group I II metal ions ? neutral (exceptions
  Be2 and Mg2 ? acidic)
• Other metal ions ? acidic

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Cation Anion ?Acidic, basic, or neutral?
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16.10 Acid-Base Behavior and Chemical Structure

• Binary Acids (HX)
• As bond strength increases, acid strength
  decreases.
• Group size of X ? acid strength ?
• Period electronegativity of X ? acid strength?

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• Oxyacids acidic H attached to an oxygen atom
• Same of OH groups and O atoms central atom
  electronegativity ? acid strength ?
• HClO gt HBrO gt HIO
• Same central atom, Y O atoms ? acid strength ?
• HClO4 gt HClO3 gt HClO2 gt HClO
• Carboxylic acids contain -COOH or CO2H
• electronegative atoms ? acid strength ?

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Oxyacids
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16.11 Lewis Acids and Bases

• Lewis acid electron-pair acceptor
• e--poor compounds
• Metal ions
• Lewis base electron-pair donor
• Amines, NR3
• Ligands (see chapter 24.1)
• Every Brønsted base is a Lewis base, but not vice
  versa.

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Lewis acid base examples