Generic definitions of AB chem.

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Generic definitions of AB chem.

• Arrhenius theory an acid forms H in water and
  a base forms OH in water.
• But not all acidbase reactions involve water,
  and many bases (NH3, carbonates) do not contain
  any OH.
• BrønstedLowry theory defines acids and bases in
  terms of proton (H) transfer.
• A BrønstedLowry acid is a proton donor.
• A BrønstedLowry base is a proton acceptor.
• The conjugate base of an acid is the acid minus
  the proton it has donated.
• The conjugate acid of a base is the base plus the
  accepted proton.

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Examples of Conjugate AB pairs
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Couple of example problems

• Write a balanced chemical equation for the
  dissociation of each of the following (in H2O)
• H2SO4, H3O, HSO4-, NH4
• What is the conjugate base of each of the
  following?
• HCO3-, CO32-, OH-, H2PO4-

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Ionization of HClstrong/weak acids
H2O is a base in this reaction because it accepts
the H
Conjugate acid of H2O
HCl acts as an acid by donating H to H2O
Conjugate base of HCl
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Ionization of Ammonia
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Water Is Amphiproticgoes both ways
H2O acts as an acid when it donates H, forming
the conjugate base ___
H2O acts as a base when it accepts H, forming
the conjugate acid ___
Amphiprotic Can act as either an acid or as a
base
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Ka and KbYOUVE SEEN THIS!!

• The equilibrium constant for a Brønsted acid is
  represented by Ka, and that for a base is
  represented by Kb.
• Nothing more than a simple extension of what you
  already know!!

H3OCH3COO Ka
CH3COOH
Notice that H2O is not included in either
equilibrium expression.
NH4OH Kb
NH3
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Strength of Conjugate AcidBase Pairs

• A stronger acid can donate H more readily than a
  weaker acid.
• The stronger an acid, the weaker is its conjugate
  base.
• The stronger a base, the weaker is its conjugate
  acid.
• An acidbase reaction is favored in the direction
  from the stronger member to the weaker member of
  each conjugate acidbase pair.

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A graphic to relate Ka/weak/strong acids
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the weaker the conjugate base.
The stronger the acid
And the stronger the base
the weaker the conjugate acid.
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Acid/Base Strength and Direction of Equilibrium

• In previous table, HBr lies above CH3COOH in the
  acid column.
• Since HBr is a stronger acid than CH3COOH, the
  equilibrium for the reaction

Weaker base ? Stronger base
Weaker acid ? Stronger
acid
lies to the left.

• We reach the same conclusion by comparing the
  strengths of the bases (right column of Table
  15.1).
• CH3COO lies below Br CH3COO is the stronger
  base

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What about Water? Wheres it fit?

• Somewhere in the middle. Graphic below sort of
  displays the strength as a function of H3O and
  OH-

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Strong Acids

• The strong acidsHCl, HBr, HI, HNO3, H2SO4,
  HClO4are considered strong because they ionize
  completely in water.
• The strong acids all appear above H3O in Table
  15.1.
• The strong acids are leveled to the same
  strengthto that of H3Owhen they are placed in
  water.

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Periodic Trends in Acid Strength

• The greater the tendency for HX (general acid) to
  transfer a proton to H2O, the more the forward
  reaction is favored and the stronger the acid.
• A factor that makes it easier for the H to leave
  will increase the strength of the acid.
• Acid strength is inversely proportional to HX
  bond-dissociation energy. Weaker HX bond gt
  stronger acid.
• Acid strength is directly proportional to anion
  radius. Larger X radius gt stronger acid.

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Periodic Trends in Acid Strength
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Strength of Oxoacids

• Acid strength increases with the
  electronegativity of the central atom, and with
  the number of terminal oxygen atoms.

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Strength of Carboxylic Acids

• Carboxylic acids all have the COOH group in
  common.
• Differences in acid strength come from
  differences in the R group attached to the
  carboxyl group.
• In general, the more that electronegative atoms
  appear in the R group, the stronger is the acid.

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• Select the stronger acid in each pair Problems
  like this will obviously be on an exam
• (a) nitrous acid, HNO2, and nitric acid, HNO3
• (b) Cl3CCOOH and BrCH2COOH

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Strengths of Amines as Bases

• Aromatic amines are much weaker bases than
  aliphatic amines.
• This is due in part to the fact that the p
  electrons in the benzene ring of an aromatic
  molecule are delocalized and can involve the
  nitrogen atoms lone-pair electrons in the
  resonance hybrid.
• As a result, the lone-pair electrons are much
  less likely to accept a proton.
• Electron-withdrawing groups on the ring further
  diminish the basicity of aromatic amines relative
  to aniline.

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• Select the weaker base in each pair (Good Primer
  for your Organic Class this fall)

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Self-Ionization of Water

• Even pure water conducts some electricity. This
  is due to the fact that water self-ionizes

• The equilibrium constant for this process is
  called the ion product of water (Kw).
• At 25 C, Kw 1.0 x 1014 H3OOH
• This equilibrium constant is very important
  because it applies to all aqueous
  solutionsacids, bases, salts, and
  nonelectrolytesnot just to pure water.

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The pH Scale

• Concentration of H3O can vary over a wide range
  in aqueous solution, from about 10 M to about
  1014 M.
• A more convenient expression for H3O is pH.
• pH log H3O and so H3O 10pH
• The negative logarithm function of pH is so
  useful that it has been applied to other species
  and constants.
• pOH log OH and so OH
  10pOH
• pKw log Kw
• At 25 C, pKw 14.00
• pKw pH pOH 14.00

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The pH Scale
Since pH is a logarithmic scale, cola drinks (pH
about 2.5) are about ____ times as acidic as
tomatoes (pH about 4.5)
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• By the method suggested in Figure 15.5, a student
  determines the pH of milk of magnesia, a
  suspension of solid magnesium hydroxide in its
  saturated aqueous solution, and obtains a value
  of 10.52. What is the molarity of Mg(OH)2 in its
  saturated aqueous solution? The suspended,
  undissolved Mg(OH)2(s) does not affect the
  measurement.

Is the solution 1.0 x 108 M HCl acidic,
basic, or neutral?
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Equilibrium in Solutions of Weak Acids and Weak
Bases

• These calculations are similar to the
  equilibrium calculations performed in Chapter 14.
• An equation is written for the reversible
  reaction.
• Data are organized, often in an ICE format.
• Changes that occur in establishing equilibrium
  are assessed.
• Simplifying assumptions are examined (the 5
  rule).
• Equilibrium concentrations, equilibrium constant,
  etc. are calculated.

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• Ordinary vinegar is approximately 1 M CH3COOH and
  as shown in Figure 15.6, it has a pH of about
  2.4. Calculate the expected pH of 1.00 M
  CH3COOH(aq), and show that the calculated and
  measured pH values are in good agreement.

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• Example 15.7
• What is the pH of 0.00200 M ClCH2COOH(aq)?

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Several Textbook sample problems

• Example 15.8
• What is the pH of 0.500 M NH3(aq)?
• Example 15.9
• The pH of a 0.164 M aqueous solution of
  dimethylamine is 11.98. What are the values of Kb
  and pKb? The ionization equation is
• (CH3)2NH H2O (CH3)2NH2
  OH Kb ?
• Dimethylamine
  Dimethylammonium ion
• Example 15.10 A Conceptual Example
• Without doing detailed calculations, indicate
  which solution has the greater H3O, 0.030 M
  HCl or 0.050 M CH3COOH.