Acids and Bases

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Acids and Bases

• Chapter 17

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Why are lemons sour?
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What about other acids?
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What about bases?
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Definitions

• Brønsted-Lowry
• An acid is anything that can donate a proton
• HCl(aq) H2O(l) ? Cl-(aq) H3O(aq)
• A base is anything that can accept a proton
• Water (above) acts as a base
• H3O hydronium ion
• HCl acid, Cl- conjugate base
• Conjugate acid-base pair
• H2O base, H3O conjugate acid
• Conjugate acid-base pair

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A problem

• Write the conjugate bases for the acids and the
  conjugate acids for the bases
• Acids Bases
• HIO3 S2-
• HCO3- CN-
• CH3COOH NH3

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Some more traits

• Species donates 1 proton monoprotic acid
• 2 protons diprotic acid
• Accepts 1 proton monoprotic base
• Accepts 2 protons diprotic base
• Give a triprotic acid

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Lewis acids and bases

• The same dude who gave us Lewis structures
• Lewis acid lone pair acceptor
• Lewis base lone pair donor
• Form coordinate covalent bonds or adducts
• NH3 BH3

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Coordination complexes

• adducts
• Water, ammonia, ions, molecules, etc. (ligands)
  give lone pairs to (usually transition) metal
  cations in soln
• Ex Cu2(aq) 4NH3(aq) ? Cu(NH3)42(aq)
• Very colorful solns
• Chem 133

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Waters uniqueness

• Can act as both a base and an acid
• Amphiprotic
• H2O(l) H2O(l) ? H3O(aq) OH-(aq)
• Water autoionizes

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pH

• power of hydrogen
• Measurement of protons/hydronium ions in soln
• Based on -logarithm (log10)
• ? pH -logH3O
• Likewise
• pOH -logOH-
• (10-pH H3O 10-pOH OH-)

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pH scale

• So pH pOH 14.00 pKw
• Where Kw H3OOH- 1.0 x 10-14
• 7.00 neutrality
• lt7.00 ? acidity (increase going to 0)
• gt7.00 ? basicity (increases going to 14)
• At neutrality, H3O OH- 1.0 x 10-7
• Is OH- more concentrated under acidic or basic
  conditions?

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Before you work on next slide!

• Sig figs in logs
• Result has the same number of decimal places in
  the input has sig figs
• Log (1.00 ? 105) -5.000
• 3 sig figs 3 dec place
• Sig figs in antilogs
• Result has the same number of sig figs as the
  number of decimal places in the input
• 106.00 1.0 ? 106
• 2 dec place 2 sig figs

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pH calculation problems
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Equilibria for acids HA(aq) H2O(l) ? H3O(aq)
A-(aq)

• As Ka increases what happens to the acid
  dissociation? Does it increase or decrease?
• It increases
• A stronger acid
• If Ka lt 1.0, weak acid

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Strong acids

• HCl
• HBr
• HI
• H2SO4
• HNO3
• HClO4
• HClO3

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Equilibria for basesB(aq) H2O(l) ? BH(aq)
OH-(aq)

• As Kb increases so does base strength
• If Kb lt 1.0
• Weak base

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Strong bases

• Metal (I) hydroxides
• Ba(OH)2

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Comparing acid/base strengths

• Can use a logarithmic scale
• pKa -log Ka
• As Ka increases acid strength increases
• But what about pKa? Does it increase or decrease
  as Ka increases?
• What can we say about acid strength and pKa?

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Conversely

• pKb -log Kb
• Likewise, as pKb decreases base strength
  increases

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The relation between Ka and Kb

• HCN(aq) H2O(l) ? H3O(aq) CN-(aq)
• Ka 4.0 x 10-10
• CN-(aq) H2O(l) ? HCN(aq) OH-(aq)
• Kb 2.5 x 10-5
• Net rxn 2H2O(l) ? H3O(aq) OH-(aq)
• Knet 1.0 x 10-14 Kw

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Hence

• And pKw pKa pKb 14.00

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Problem

• What is the Ka and pKa of a 0.10 M soln of
  chloroacetic acid with a pH of 1.95?

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Solution
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Problem

• What is the pH of a 0.237 M solution of benzoic
  acid? The pKa is 4.19.

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Solution
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Problem

• What is the pH of a 1.00 M solution of Sodium
  Acetate? (pKb 5.6 x 10-10) Why pKb and not pKa?

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Solution
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Another problem

• What is the pH of a solution that is made from
  250.0 mL of 0.250 M KOH and 150.0 mL of 0.0125 M
  HBr?

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Solution
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Which side will be favored?

• Rxns proceed from stronger acid/base to weaker
  acid/base
• Predict the products and the direction of arrow
  for the following reaction
• NH3(aq) HCO3- ?
• Use table 17.3 on pg. 808

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Solution
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More problems

• For the following write the complete reaction,
  and determine the correct K value both in symbols
  (e.g., 1/Ka) and in number.
• a) Potassium bicarbonate and hydrochloric acid
• b) The acid dissociation of HCN
• c) Acetic acid and potassium hydroxide

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So what really makes it strong vs. weak?

• 1) Electronegativity
• Acid strength increases as electronegativity of A
  increases
• 2) Bond strength
• Lessens as descend GVII ? stronger acid
• 3) Larger atomic radius
• Increases as descend G
• ? easier to lose H, stronger acid

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Continued

• 3) Oxyacids
• The more oxygens the stronger
• Ex in order of increasing acid strength
• HClO lt HClO2 lt HClO3 lt HClO4
• O-H bond polarity increases as oxygens are added
  ? easier to remove hydrogen (as proton)
• Inductive effect ability of atoms in a molecule
  to attract electrons from another part of the
  molecule

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More

• Also, more oxygens allow for more stable
  (delocalized) structures
• Compare Lewis structures of deprotonated
  (conjugate bases) HNO3 to HNO2
• Allows for greater acidity

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Carboxylic acids

• Brønsted-Lowry acids
• RCO2-H H2O ? RCO2- H3O
• Give me the resonance structures
• If Electron Withdrawing Groups are substituted
  for hydrogens what would happen?
• Order of increasing Ka
• CH3CO2H gt ClCH2CO2H gt Cl2CHCO2H gt Cl3CCO2H

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Anions as Brønsted bases

• Basicity increased as negative charge of anion
  increases
• PO43-
• HPO42-
• H2PO4-
• Which is most basic?
• Which has the highest pKb?

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Salts

• Will a salt soln be acidic, basic, or neutral?
• Lets do these
• HCl NaOH ? NaCl H2O
• HCl NH4OH ? NH4Cl H2O
• H3PO4 3CsOH ? Cs3PO4 3H2O
• H3PO4 3NH4OH ? (NH4)3PO4 3H2O