Aqueous AcidBase Equilibria

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• Aqueous Acid-Base Equilibria

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Keywords Concepts

• Electrolytes

• Autoionization of Water
• ion product of water, Kw

• pH and pOH

• Ionization Constants for Weak Acids and Bases,
• Ka (pKa) Kb (pKb)

• Polyprotic Acids

• Solvolysis

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Strong Electrolytes

• Strong electrolytes dissociate completely in
  aqueous solutions
• Weak electrolytes dissociate partially
• Nonelectrolytes do not dissociate
• Three classes of strong electrolytes
• Strong Acids

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What is H (aq)?

H3O
H
H5O2
H9O4
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Representing Protons

• All representations of the proton are equivalent.
• H5O2 (aq), H7O3 (aq), H9O4 (aq) have been
  observed.
• We will use H(aq) most of the time!

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Acid-base Properties
Bronsted Lowry Theory ACID A proton
donor BASE A proton acceptor
-COOH -OH -NH2 HCl
NH3 OH- -COO-
NH3 H2O ? NH3 OH-
Bronsted Base Acid

Conjugate Acid
Base
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Ephedrine hydrochloride
Ephedrine
Base and Conjugate Acid
Base Ephedrine
Conjugate acid The product resulting from the
addition or acquisition of a proton.
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Strong Electrolytes

• Strong Bases

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Strong Electrolytes

• Soluble Salts

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Strong Electrolytes

• Calculation of concentrations of ions in solution
  of strong electrolytes is easy
• Calculate the concentrations of each ion in 0.050
  M nitric acid, HNO3.

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Strong Electrolytes

• Calculate the concentrations of each ion in 0.020
  M calcium hydroxide, Ca(OH)2, solution.

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The Auto-Ionization of Water

• Pure water dissociates very slightly
• less than one-millionth molar

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The Auto-Ionization of Water

• Because the activity of pure water is 1, the
  equilibrium constant for this reaction is

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The Auto-Ionization of Water

• Experimental measurements have determined that
  the concentration of each ion is 1.0 x 10-7 M at
  250C

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The Auto-Ionization of Water

• This particular equilibrium constant is called
  the ion-product for water, Kw.

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The Auto-Ionization of Water

• Calculate the concentrations of H3O and OH- in
  0.050 M HCl.

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The Auto-Ionization of Water
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The Auto-Ionization of Water

• Increase in H3O from HCl shifts the
  equilibrium and decreases the OH-.
• A concentration of 2.0?10-13 M is difficult to
  visualize.

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The pH and pOH scales

• A convenient way to express acidity and basicity
• pH is defined as

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The pH and pOH scales

• In general, a lower case p before a symbol is
  read as negative logarithm of the symbol

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The pH and pOH scales

• If we know either H3O or OH-, then we can
  calculate pH and pOH.
• Calculate the pH of a solution in which the
  H3O 0.030M.

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The pH and pOH scales

• The pH of a solution is 4.597. What is the
  concentration of H3O?

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The pH and pOH scales

• A convenient relationship between pH and pOH may
  be derived for all dilute aqueous solutions at
  25C.

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The pH and pOH scales

• Taking the logarithm of both sides of this
  equation gives

• Multiplying both sides of this equation by -1
  gives

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The pH and pOH scales

• Or, in a slightly different form, we have

Remember these two equations!!
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The pH and pOH scales

• The typical (practical) range for the pH scale is

• and for pOH the scale is

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The pH and pOH scales

• Calculate H3O, pH, OH-, and pOH for
• 0.020 M HNO3 solution.

You do it!
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The pH and pOH scales

• Calculate H3O, pH, OH-, and pOH for 0.020 M
  HNO3 solution.

You do it!
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Ionization Constants for Weak Monoprotic Acids
and Bases

• Consider the dissolution of acetic acid, a weak
  acid, in water as an example.
• The equation for the ionization of acetic acid is

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Ionization Constants for Weak Monoprotic Acids
and Bases

• The equilibrium constant for this ionization is
  expressed as

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Ionization Constants for Weak Monoprotic Acids
and Bases

• There is a specific equilibrium constant symbol
  for the ionization of an acid
• Ka acid ionization constant

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Ionization Constants for Weak Monoprotic Acids
and Bases

• Values for several ionization constants

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Ionization Constants for Weak Monoprotic Acids
and Bases

• From the above table we see that the order of
  increasing acid strength for these weak acids is

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Ionization Constants for Weak Monoprotic Acids
and Bases

• The order of increasing base strength of the
  anions (conjugate bases) of these acids is

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Ionization Constants for Weak Monoprotic Acids
and Bases

• Write the equation for the ionization of the weak
  acid HCN and the expression for its ionization
  constant.

You do it!