Acid and Base Behavior and Structure and Lewis Acids

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Acid and Base Behavior and Structure and Lewis
Acids

• Wednesday, July 18th
• CHM 102

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What makes some acids stronger?

• There are three central properties that can
  affect the strength of an acid
• How polar is the H-X bond?
• How strong is the H-X bond?
• How stable is the X- (conjugate base) of HX?

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Polarity of the H-X bond

• Proton transfer will only occur in an H-X type
  acid if the polarity is as shown
• Compare these types of polar bonds such as H-Cl
  to the nonpolar bond in CH4, the C-H bond.

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Strength of the H-X bond

• Polarity alone doesnt define how strong an acid
  is!
• Which is the stronger bond? HF or HCl?
• Based on polarity alone, which would be the
  stronger acid?
• Bond strength plays a strong role in the
  determination of the strength of an acid, just as
  polarity does!

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Last but not least

• Along with the polarity and strength of the H-X
  bond, the stability of the X- conjugate base
  plays a role in the strength of the acid.
• When H-X dissociates into H and X-, if the X-
  conjugate base isnt very stable, then the
  dissociation of HX probably isnt very product
  stable.

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Binary Acids (H 1 other element)

• The major contributor (of the 3) to acid strength
  for binary acids is bond strength.

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Oxyacids

• Oxyacids have a central atom with oxygens coming
  off of it, and the acidic proton is connected to
  one of the oxygens.
• So youre mainly looking at polyatomic-inclusive
  acids (i.e. HClO, H2SO4, etc)
• There are two general rules for the prediction of
  the strength of an oxyacid.

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Oxyacids Rule 1

• When two oxyacids have the same number of OH
  groups and the same number of oxygen atoms, the
  strength of the acid is based on the
  electronegativity of the central atom.
• For example

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Oxyacids Rule 2

• When two oxyacids have the same central atom, the
  strength of the acid depends mainly on the number
  of oxygen atoms attached to the central atom.
• For example

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Carboxylic Acids

• Carboxylic acids are of the general form
• 3 main things affect the acidity of carboxylic
  acids
• The double-bonded oxygen (highlighted in yellow)
  is pulls further electron density out of the O-H
  bond, weakening the hold on the acidic proton.
• The conjugate base of carboxylic acids are
  resonance stabilized.
• 3. If electronegative atoms are substituted as
  shown below, the strength of the carboxylic acid
  increases.

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Practice Acid Behavior and Structure

• Of the following choices, which is the most
  acidic? Least acidic?
• H2S H2O HCl
  HI

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More Practice Acid Behavior and Structure

• Of the following choices, which is the most
  acidic? Least acidic?
• Set 1 HBrO2 or HBrO
• Set 2 HBrO2, HClO2, or HIO2

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Yet More Practice Acid Behavior and Structure

• If you had to choose between the following two
  carboxylic acids, which is more acidic?

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Lewis Acids and Bases

• So far weve had the Arrhenius AB definition
  (based on acids raising H, bases raising
  OH-.
• Weve also covered the Bronsted-Lowry AB
  definition (based on acids being proton donors
  and bases being proton acceptors).
• Now weve got Lewis acids and bases, which are
  based on acids being electron pair acceptors, and
  bases being electron pair donors.

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Lewis Acids and Bases

• Lets look back at our classic reaction of
  hydrobromic acid with water
• HBr(aq) H2O(l) ? Br-(aq) H30(aq)
• There, the HBr is donating a proton
  (Bronsted-Lowry acid), and the water is accepting
  the proton (Bronsted-Lowry base).
• In Lewis acid base theory, the HBr is still the
  acid, and the water is still the base, but we
  have to look at things differently.

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Lewis Acids and Bases

• Lets a different look back at our classic
  reaction of hydrobromic acid with water
• Here, you can see the one of the lone pairs on
  the water donates its electrons to an incoming H
  from the HBr. So the Lewis acid in this case is
  what? What is the Lewis base?

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Lewis Acids and Bases

• Now lets look at the reaction that takes place
  between ammonia and boron trifluoride
• What exactly is happening here? Is there an H
  transfer? So the Lewis acid in this case is
  what? What is the Lewis base?

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Lewis Acids and Bases

• Lets look at carbon dioxide reacing with water
  to form carbonic acid
• What exactly is happening here? Is there an H
  transfer? So the Lewis acid in this case is
  what? What is the Lewis base?

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Acids and Bases Problems Up to This Point

• Wednesday, July 18th
• CHM 102

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Review Problems 1

• Youve got a solution that is 0.1 M HClO4, what
  is the pH, pOH, and H in solution?
• Which of the following are strong acids?H2SO4
  HCl HF HNO3 HI H3PO4
• Which of the following are strong bases?
• KOH NH3 CsOH Ca(OH)2 Mg(OH)2
• What is the OH if the H 110-5 M?
• What is the conjugate acid and conjugate base of
  HCO3-?
• What is special about the bicarbonate anion?

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Review Problem 2

• Given the table of Ka values shown above, which
  of the acids listed above has the weakest
  conjugate base? Which one of the weak acids
  above is going to have the highest pH in solution?

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Review Problem 3

• What is the pH of a solution of 0.2 M H2CO3 at
  25oC, given that the Ka of carbonic acid at 25oC
  is 4.310-7?

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Review Problem 4

• What is the OH- of a 0.8 M solution of acetic
  acid at 28oC, given that the Kb for acetic acid
  at this temperature is 5.4110-8?

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Review Problem 4

• What is the pOH of a solution of 0.9 M solution
  of ammonia, if at equilibrium, the concentration
  of ammonium is 2.210-4?