Chapter 14 Acids and Bases

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Chapter 14Acids and Bases

• Arrhenius Concept Acids produce H in
  solution, bases produce OH? ion.
• Brønsted-Lowry Acids are H donors, bases are
  proton acceptors.
• HCl H2O ? Cl? H3O
• acid base conjugate conjugate
• base acid
• Proton is transferred from the HCl molecule to
  the water molecule.

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Figure 14.1 The Reaction of HCI and H2O
Figure 14.2 The Reaction of an Acid with Water
Figure 14.3 The Reaction of NH3 with HCI to Form
NH4 and CI-
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Conjugate Acid/Base Pairs

• HA(aq) H2O(l) ? H3O(aq) A?(aq)
• conj conj conj
  conj
• acid 1 base 2 acid 2
  base 1
• Conjugate base everything that remains of the
  acid molecule after a proton is lost.
• Conjugate acid formed when the proton is
  transferred to the base.
• A conjugate acid-base pair consists of two
  substances related to each other by the donating
  and accepting of a single proton.

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Acid Dissociation Constant (Ka)

• HA(aq) H2O(l) ? H3O(aq) A?(aq)
• Where, Ka is the acid dissociation constant. In
  dilute solution we can assume that the
  concentration of liquid water remains essentially
  constant when an acid is dissolved.

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Acid Strength

• The strength of an acid is defined by the
  equilibrium position of its dissociation
• (ionization) reaction
• HA(aq) H2O(l) H3O(aq) A-(aq)
• Strong Acid
• Its equilibrium position lies far to the right.
  (HNO3)
• Yields a weak conjugate base. (NO3?)
• Common strong acids are H2SO4, HCl, HNO3, HClO4

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Figure 14.4 Graphic Representation of the
Behavior of Acids of Different Strengths in
Aqueous Solution
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Figure 14.5 Acid Strength Versus Conjugate Base
Strength
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Acid Strength(continued)

• Weak Acid
• Its equilibrium lies far to the left. (CH3COOH)
• Yields a much stronger (it is relatively strong)
  conjugate base than water. (CH3COO?)
• Common weak acids are H3PO4, HNO2, HOCl, organic
  acids (-COOH).

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Figure 14.6 A Strong Acid (a) and a Weak Acid (b)
in Water
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• Monoprotic acid One acidic proton (HCl)
• HCl(aq) H(aq) Cl-(aq)
• Diprotic acid Two acidic protons (H2SO4)
• H2SO4(aq) H(aq) HSO4-(aq)
• HSO4- (aq) H(aq) SO4 2-(aq)
• Oxyacids Acidic proton is attached to an oxygen
  atom (H2SO4)
• Organic acids Those with a carbon atom
  backbone, contain the carboxyl group (-COOH).
  CH3-COOH, C6H5-COOH

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Water as an Acid and a Base

• A substance is said to be amphoteric if it can
  behave either as an acid or as a base. Water is
  amphoteric (it can behave either as an acid or a
  base).
• H2O H2O ? H3O OH?
• 
  conj conj
• acid 1 base 2 acid 2 base 1
• Kw H3OOH- HOH- 1 ? 10?14 at 25C
• Where, Kw is the ion-product constant or
  dissociation constant for water.
• H OH- 1.0 x 10-7 M at 25oC in pure
  water.

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Figure 14.7 Two Water Molecules React to Form
H3O and OH-
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The pH Scale

• The pH scale provides a convenient way to
  represent solution acidity. The pH is a log scale
  based on 10.
• pH ? ?logH
• pH in water ranges from 0 to 14. The pH
  decreases as H increases.
• Kw 1.00 ? 10?14 H OH?
• pKw -log Kw 14.00 pH pOH
• As pH rises, pOH falls (sum 14.00).
• pOH -log OH-

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Figure 14.8 The pH Scale and pH Values of Some
Common Substances
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Calculating the pH of Strong Acid Solutions

• Calculate the pH of 1.0 M HCl.
• Since HCl is a strong acid, the major species in
  solution are H, Cl- and H2O
• To calculate the pH we will focus on major
  species that can furnish H. The acid is
  completely dissociates in water producing H and
  water also furnishes H by autoionization by the
  equilibrium
• H2O(l) H(aq) OH-(aq)
• In pure water at 25oC, H is 10-7M and in
  acidic solution even less than that. So the
  amount of H contributed by water is negligible
  compared with the 1.0M H from the dissociation
  of HCl.
• pH -log H -log (1.0) 0

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Solving Weak Acid Equilibrium Problems

• List major species in solution.
• Choose species that can produce H and write
  reactions.
• Based on K values, decide on dominant
  equilibrium.
• Write equilibrium expression for dominant
  equilibrium.
• List initial concentrations in dominant
  equilibrium.

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Solving Weak Acid Equilibrium Problems (continued)

• Define change at equilibrium (as x).
• Write equilibrium concentrations in terms of x.
• Substitute equilibrium concentrations into
  equilibrium expression.
• Solve for x the easy way.
• Verify assumptions using 5 rule.
• Calculate H and pH.

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Percent Dissociation (Ionization)

• It is useful to specify the amount of weak acid
  that has dissociated in achieving equilibrium in
  an aqueous solution. The percent dissociation is
  defined as follows
• For a given weak acid, the percent dissociation
  increases as the acid becomes more dilute.

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Figure 14.10 The Effect of Dilution on the
Percent Dissociation and (H) of a Weak Acid
Solution
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Bases

• Arrhenius concept A base is a substance that
  produces OH- ions in aqueous solution.
• Bronsted-Lowry concept A base is a proton
  acceptor.
• Strong and weak are used in the same sense
  for bases as for acids.
• strong complete dissociation (hydroxide ion
  supplied to solution)
• NaOH(s) ? Na(aq) OH?(aq)

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Bases(continued)

• weak very little dissociation (or reaction
  with water)
• H3CNH2(aq) H2O(l) ? H3CNH3(aq) OH?(aq)
• H3CNH2 molecule accepts a proton and thus
  functions as a base. Water is the acid in this
  reaction. Methyl amine contains no hydroxide ion,
  it still increases the concentration of hydroxide
  ion to yield a basic solution.

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Polyprotic Acids

• . . . can furnish more than one proton (H) to
  the solution. A polyprotic acid always
  dissociates in a stepwise manner, one proton at a
  time.
• For a typical weak polyprotic acid,
• Ka1 gt Ka2 gt Ka3

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Acid-Base Properties of Salts
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Structure and Acid-Base Properties

• When a substance is dissolved in water, it
  produces an acidic solution if it can donate
  protons and produces a basic solution if it can
  accept protons.
• Two factors for acidity in binary compounds
• Bond Polarity (high is good)
• Bond Strength (low is good)

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Figure 14.11 The Effect of the Number of Attached
Oxygens on the O-H Bond in a Series of of
Chlorine Oxyacids
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Oxides

• Acidic Oxides (Acid Anhydrides) When a covalent
  oxide dissolves in water an acidic solution
  forms.
• O?X bond is strong and covalent.
• SO2, NO2, CO2, CrO3
• Basic Oxides (Basic Anhydrides) When an ionic
  oxide dissolves in water a basic solution
  results.
• O?X bond is ionic.
• K2O, CaO

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Lewis Acids and Bases

• Lewis Acid electron pair acceptor
• Lewis Base electron pair donor
• Lewis acid has an empty atomic orbital that it
  can use to accept an electron pair from a
  molecule that has a lone pair of electrons.
• Lewis Lewis
• acid base

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Figure 14.13 The AI(H2O)63 Ion