ACIDS

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ACIDS BASES
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Arrhenius Theory

• 1. in aqueous solution
• 2. Acid produces H
• 3. Base produces OH-

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Acid
H2O
HA ----gt H3O A-
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HCl(g) H2O ---gt H3O(aq)
Cl-(aq)

• CH3COOH(l) H2O ltgt H3O(aq)
  CH3COO-(aq)

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careless, but often seen

• HCl ----gt H Cl-
• CH3COOH ltgtH CH3COO-

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Base
H2O

• NaOH(s) ---gt Na(aq) OH-(aq)

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Arrhenius acid/base reaction

• acidbase---gtH2O a salt
• HA MOH ---gt H2O MA

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Monoprotic acid HCl

• HCl(aq) NaOH(aq) ---gt
• H2O(l) NaCl(aq)
• H Cl- Na OH- ---gt
• H2O Na Cl-
• H OH- ---gt H2O

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diprotic acid H2SO4

• H2SO4(aq) 2NaOH(aq) ---gt
• 2H2O(l) Na2SO4(aq)
• H OH- ---gt H2O

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polyprotic acid H3PO4

• H3PO4(aq) 3NaOH(aq) ---gt
• 3H2O(l) Na3PO4(aq)
• H3PO4 3 OH- ---gt 3 H2O
  PO43-

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Bronsted-Lowry Theory

• 1. aqueous non-aqueous solutions
• 2. Acid species donating a proton
• HCl ----gt H Cl-
• H2SO4 ----gt H HSO4-
• CH3COOH ----gt H CH3COO-

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Bronsted-Lowry Theory

• 3. Base species accepting a proton
• OH- H ----gt HOH
• H2O H ----gt H3O
• NH3 H ----gt NH4

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Conjugate acid-base pairs
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Conjugate acid-base pairs
acid1 base1 acid2 base2
conjugate pairs
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Conjugate acid-base pairs
acid1 base1 acid2 base2
conjugate pairs
HF HOH
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Conjugate acid-base pairs
acid1 base1 acid2 base2
conjugate pairs
HF HOH H3O F-
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ALL Arrhenius reactions are Bronsted-Lowry
reactions HCl NaOH ---gt H2O NaCl
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NOT all Bronsted reactions are Arrhenius
reactions CH3COOH NH3 ----gt
NH4 CH3COO-
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Amphiprotic AmphotericCan act as either an
acid or a base

• HCl HOH ---gt H3O Cl-
• NH3 HOH ltgt NH4 OH-
• NH3 OH- ltgt NH2- HOH
• HOH HOH ltgt H3O OH-

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ACID STRENGTH

• Relative ability of a compound to donate a proton
• Base strength is considered a result, not a cause

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REVIEW

• Strong acid
• 100 dissociation
• Weak acid
• lt100 dissociation

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Electronegativity is the most significant factor
influencing the strength of acids bases
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HF gt HCl gt HBr gt HI as acids in non-aqueous
solvents, or as pure gases
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Look at difference in electronegativities 2.1 H
- F 4.0 2.1 H - Cl 3.0 2.1 H - Br 2.8 2.1
H - I 2.5
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Most ionic is the most acidic
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However, HF lt HCl HBr HI as acids in
aqueous solution
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2.1 H - F 4.0 2.1 H - O 3.5 competition! 2.1
H - Cl 3.0 2.1 H - Br 2.8 2.1 H - I 2.5
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The strength of oxy-acids are also dependent on
electronegativity.
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Oxy-acids and bases have the same fundamental
structure
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NaOH Na - O - H 0.9 3.5
2.1 HClO Cl - O - H 3.0
3.5 2.1
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In water, the more ionic bond dissociates,
forming the acid or base
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NaOH Na - O - H 0.9 3.5
2.1 HClO Cl - O - H 3.0
3.5 2.1
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Acids in homologous series are of different
strength
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Acid Strength H2SO4 gt H2SO3 HNO3 gt HNO2 HClO4
gt HClO3 gt HClO2 gt HClO
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Acid Strength CH3COOHgt CH3CH2OH CF3COOH gt
CH3COOH
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pH pK Ka , Kb , Kw
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2H2O H3O OH-
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2H2O H3O OH-
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Keq H2O2 H3O OH-
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Keq H2O2 H3O OH- Kw H3O
OH- where Kw (25oC ) 1 x 10-14
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in a neutral solution H3O OH-
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in a neutral solution H3O OH- 1 x 10-14
H3O 2 OH-2
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in a neutral solution H3O OH- 1 x 10-14
H3O 2 OH-2 H3O OH- 1 x 10-7
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pX -log X
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pX -log X pK -log K pH -log H3O
pOH -log OH-
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leveling effect of H2O limits H3O
OH- to that controlled by H2O
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upper limit H3O 1
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upper limit H3O 1 lower limit H3O
1 x 10-14
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pH scale
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pH scale
acid neutral base
7
14
0
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pH scale
acid neutral base
7
14
0
highest H3O on left lowest H3O on
right
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H3O and OH- must be considered together
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Kw H3O OH-
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Kw H3O OH- -log Kw -log H3O OH-
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Kw H3O OH- -log Kw -log H3O
OH- -log Kw -log H3O
-logOH-
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pKw pH pOH
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pKw pH pOH but Kw 1 x 10-14
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pKw pH pOH but Kw 1 x 10-14 14 pH
pOH
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Relationship between conjugate acids bases
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Relationship between conjugate acids
bases HA H2O H3O A- A- H2O
HA OH-
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H3O OH- Kw
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H3O OH- Kw Ka. Kb Kw
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SUMMARY pH -log H3O pOH -log OH- H3O
OH- 1 x 10-14 pH pOH 14 Ka. Kb Kw
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Applications of Acid-Base Concepts
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1. What is the pH of a solution that is 0.025 M
KOH?
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2. What is the pH of a 0.20 M acetic acid
solution?
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3. 100 mL of 0.10 M CH3COOH are mixed with 20.0
mL of 0.10 M NaOH. What is the pH of the
solution?
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4. Calculate the percent ionization of 0.10 M
methylamine (CH3NH2).
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Lewis Acid-Base Theory

• Acid
• substance capable of accepting an e- pair

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Lewis acid

• must have an empty valence level orbital
• i.e. H has an empty 1s orbital which can accept
  an e- pair

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Thus, H is an acid under all three theories

• Arrhenius
• Bronsted-Lowry
• Lewis

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Lewis Acid-Base TheoryAcid substance capable of
accepting an e- pair

• Base
• substance capable of donating an e- pair

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Examples of Lewis bases

• OH- , NH3 , F-
• all have unbonded pairs of e- available for
  donation

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Elements of Group 13 (3A) form compounds that
make excellent Lewis acids
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B 1s2 2s2 2p1
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B 1s2 2s2 2p1
forms sp2 hybrid for bonding purposes
sp2 2p
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unhybridized p-orbital
sp2 hybrid orbital
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another typical Lewis acid-base reaction
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..
BF3 NH3
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H3BO3 H2O ?
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?
H3BO3 H2O H2BO3- H3O
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NO!
?
H3BO3 H2O H2BO3- H3O
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H3BO3 2H2O B(OH)4- H3O
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BF3 3H2O ?
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BF3 3H2O B(OH)3 3HF
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BF3 3H2O B(OH)3 3HF
B3 3 OH- B(OH)3 Lewis acid-base rxn
only F- H HF Lewis B/L
acid-base rxn
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Lewis Bronstead Arrhenius