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Chapter FifteenPrentice-Hall ©2002Slide 1 of 31 930805 http:\\asadipour.kmu.ac.ir.......40 slides 1
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Acids And Bases (Mark=1)
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The Arrhenius Theory acid – a substance that produces H + ion when dissolved in water base - a substance that produces OH - ion when dissolved in water Limitation of Arrhenius Theory 930805 http:\\asadipour.kmu.ac.ir.......40 slides 3
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The Brønsted-Lowry Theory Lowry was an English scientist,. Bronsted was a Danish scientist Acid – proton donor Base – proton acceptor Conjugate acid and base, HA/A -, differ by one proton. The conjugate acid of a base is the base plus the attached proton and the conjugate base of an acid is the acid minus the proton. (HCl/Cl - ) A substance that can act either as an acid or a base is amphiprotic. (H 2 PO4 - ) 930805 http:\\asadipour.kmu.ac.ir.......40 slides 4
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Chapter FifteenPrentice-Hall ©2002Slide 5 of 31 The BrØnsted-Lowry Theory Conjugate acid-base pairs are species that differ by a H +. – Some examples: 930805 http:\\asadipour.kmu.ac.ir.......40 slides 5
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Amphoterism Look at this reaction in more structural detail. reaction of zinc hydroxide with nitric acid. 930805 http:\\asadipour.kmu.ac.ir.......40 slides 6
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Amphoterism Zn(OH) 2 behaves as an acid in presence of strong bases. In more structural detail. 930805 http:\\asadipour.kmu.ac.ir.......40 slides 7
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Chapter FifteenPrentice-Hall ©2002Slide 8 of 31 Strong Acid Dissociation (makes 100 % ions) 930805 http:\\asadipour.kmu.ac.ir.......40 slides 8
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Chapter FifteenPrentice-Hall ©2002Slide 9 of 31 Weak Acid Dissociation (only partially ionizes) 930805 http:\\asadipour.kmu.ac.ir.......40 slides 9
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Ionization of Ammonia 930805 http:\\asadipour.kmu.ac.ir.......40 slides 10
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Chapter FifteenPrentice-Hall ©2002Slide 11 of 31 930805 http:\\asadipour.kmu.ac.ir.......40 slides 11
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Strengths Of Conjugate Acid-Base Pairs The stronger an acid, the weaker is its conjugate base. The stronger a base, the weaker is its conjugate acid. An acid-base reaction is favored in the direction from the stronger member to the weaker member of each conjugate acid-base pair. HCl+H 2 O→H 3 O + +Cl - CH 3 COOH + H 2 O H 3 O + + CH 3 COO - ????????? K a and K b values are used to compare the strengths of weak acids and bases. 930805 http:\\asadipour.kmu.ac.ir.......40 slides 12
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Leveling and differentiating effect Water has a leveling effect; when the strong acids are dissolved in water, they all completely ionize to the hydronium ion. HCl+H 2 O→H 3 O + +Cl - HClO 4 +H2O→H 3 O + +ClO 4 - Which one is stroger? Water has a differentiating effect; when the weak acids are dissolved in water, they all partially ionize to the hydronium ion. CH 3 COOH + H 2 O H 3 O + + CH 3 COO - HCOOH + H 2 O H 3 O + + HCOO - Which one is stroger? K a 930805 http:\\asadipour.kmu.ac.ir.......40 slides 13
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Differenti ating effect → → → → → ← H2O Weak conj. bases Leveling effect Weak acids Weak conj. acid Weak bases Strongest acid Strongest base Strong acid Strong base Differenti ating effect 930805 http:\\asadipour.kmu.ac.ir.......40 slides 14
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Differentiating effect CH3COOH Differentiating effect NH3 → → Leveling effect ← ← CH3COOH Leveling effect Strongest acid CH3COOH 2 + Strongest base NH 2 - 930805 http:\\asadipour.kmu.ac.ir.......40 slides 15
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Chapter FifteenPrentice-Hall ©2002Slide 16 of 31 Ionization constant of an acid For a monoprotic weak acid (HA) dissolved in water, HA(aq) + H 2 O(l) H 3 O + (aq) + A - (aq) acid conjugate acid base conjugate base 930805 http:\\asadipour.kmu.ac.ir.......40 slides 16
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Chapter FifteenPrentice-Hall ©2002Slide 17 of 31 Ionization constant of an acid Ionization constant of the acid 930805 http:\\asadipour.kmu.ac.ir.......40 slides 17
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Acid And Base Ionization Constants weak acid:CH 3 COOH + H 2 O H 3 O + + CH 3 COO - [H 3 O + ][CH 3 COO - ] Acid ionization constant: K a = [CH 3 COOH] weak base:NH 3 + H 2 O NH 4 + + OH - [NH 4 + ][OH - ] Base ionization constant: K b = [NH 3 ] K a and K b are the measure of the strengths of weak acids and bases. 930805 http:\\asadipour.kmu.ac.ir.......40 slides 18
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Acids H –X Hydrid acids H – O – E Oxo acids 930805 http:\\asadipour.kmu.ac.ir.......40 slides 19
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Representative Trends In Strengths of hydrid Acids S≈E & R 930805 http:\\asadipour.kmu.ac.ir.......40 slides 20
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Relative Strengths Of hydrid Acids H –X in a period: (E↑↑↑↑ & R↓) The larger the electronegativity difference between H and X, the more easily the proton is removed and the stronger is the acid. EN 0.4<0.9<1.4< 1.9 Acid strengthCH 4 NH 3 H 2 O HF The strengths of binary acids increase from left to right across a period of the periodic table. 930805 http:\\asadipour.kmu.ac.ir.......40 slides 21
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Relative Strengths Of Binary Acids H –X in a periodic group: (E↓ & R↑↑↑↑) Anion radius: the larger the anion’s radius, the stronger the acid. Anion radius 136< 181 < 195< 216 HF HCl HBr HI Acid strength K a 6.6x10 -4 < ~10 6 < ~10 8 < ~10 9 The strength of binary acids increase from top to bottom in a group of the periodic table. 930805 http:\\asadipour.kmu.ac.ir.......40 slides 22
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Strengths Of Oxoacids H – O - X E≈ Two factors: - electronegativity of the central atom (E) - number of terminal oxygen atoms (n) As the electronegativity of the central atom increases. As the number of terminal oxygen atoms increases, the acid strength also increases. E↑ S↑ n↑ S↑ ↑ ↑ HOClO n S≈E & R 930805 http:\\asadipour.kmu.ac.ir.......40 slides 23
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Strengths Of Oxoacids As the electronegativity of the central atom (E) increases the acid strength increases. Electronegativity2.5<2.8<3.0 HOIHOBrHOCl Acid strength K a 2.3x10 -11 <2.5x10 -9 <2.9x10 -8 As the number of terminal oxygen atoms increases, the acid strength also increases. O O װ װ H-O-Cl<H-O-Cl=O<H-O-Cl=O<H-O-Cl=O װ O Acid strength2.9x10 -8 < 1.1x10 -2 < ~1000 < ~10 8 930805 http:\\asadipour.kmu.ac.ir.......40 slides 24
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Strengths Of Oxoacids Conventional charge or Oxidation number O O װ װ H-O-Cl < H-O-Cl=O < H-O-Cl=O < H-O-Cl=O װ O Oxidation number Cl +1 +3 +5 +7 Conventional charge Cl 0 +1 +2 +3 -------------------------------------------------------- O O O װ װ װ H-O-P-H= H-O-P-O-H= H-O-P-O-H ו ו ו H H OH Oxidation number P +1 +3 +5 Conventional charge P +1 +1 +1 930805 http:\\asadipour.kmu.ac.ir.......40 slides 25
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Strengths Of Carboxylic Acids O R – C – O – H R – CH2 – O - H 1)Carboxylic acids all have the -COOH group 2)Differences in acid strength must come from differences in the R group attached to the carboxyl group. 3)In general, the more that electronegative atoms are attached in the R group, the stronger the acid. 930805 http:\\asadipour.kmu.ac.ir.......40 slides 26
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Strengths Of Carboxylic Acids O װ R – CH2 – O – H < R – C – O – H In general, the more that electronegative atoms are attached in the R group, the stronger the acid. I-CH 2 CH 2 COOH Cl-CH 2 CH 2 COOH CH 3 -CHClCOOH CH 3 CCl 2 COOH K a 8.3x10 -5 <1.0x10 -4 < 1.4x10 -3 <8.7x10 -3 930805 http:\\asadipour.kmu.ac.ir.......40 slides 27
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Strengths Of Amines As Bases BrNH 2 C 6 H 5 NH 2 K b 2.5x10 -8 1.8x10 -5 7.4x10 -10 Electron-withdrawing groups on the ring further diminish the basicity of aromatic amines relative to aniline Me 2 NH>MeNH 2 >Me 3 N>NH 3 Why? 930805 http:\\asadipour.kmu.ac.ir.......40 slides 28
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Amine bases: N H H H N R H H N R H R N R R R N H H aromatic amine base R= CH 3, CH 2 CH 3 aliphatic amine bases ammonia 930805 http:\\asadipour.kmu.ac.ir.......40 slides 29
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Self-Ionization Of Water Even the purest of water conducts electricity. This is due to the fact that water self-ionizes, that is, it creates a small amount of H 3 O + and OH -. 2 H 2 O (l) = H 3 O + (aq) + OH¯ (aq) [H 3 O + ] [OH¯] K eq = ———————— [H 2 O](1000/18 = 55.6) K w = K eq [H 2 O] = 1e –14 only at 25 o C, it’s T dependent. 930805 http:\\asadipour.kmu.ac.ir.......40 slides 30
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O H H O H H O Self ionization reaction of water: + O H H H H + + - T ↑ K w ↑ 930805 http:\\asadipour.kmu.ac.ir.......40 slides 31
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Chapter FifteenPrentice-Hall ©2002Slide 32 of 31 930805 http:\\asadipour.kmu.ac.ir.......40 slides 32
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The pH Scale 930805 http:\\asadipour.kmu.ac.ir.......40 slides 33
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An Example The pH of milk of magnesia, a suspension of solid magnesium hydroxide in its saturated aqueous solution, is measured to be 10.52. What is the molarity of Mg(OH) 2 in its saturated aqueous solution? 930805 http:\\asadipour.kmu.ac.ir.......40 slides 34
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The Lewis Theory Developed in 1923 by G.N. Lewis. Acids are defined as electron pair acceptors. Bases are defined as electron pair donors. Neutralization reactions are accompanied by coordinate covalent bond formation. In organic chemistry, Lewis acids are often called electrophiles Lewis bases are often called nucleophiles 930805 http:\\asadipour.kmu.ac.ir.......40 slides 35
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The Lewis Theory Look at this reaction in more detail paying attention to the electrons. Nucleophilic replacement 930805 http:\\asadipour.kmu.ac.ir.......40 slides 36
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The Lewis Theory The reaction of sodium fluoride and boron trifluoride provides an example of a reaction that is - only a Lewis acid-base reaction. – It does not involve H + at all, thus it cannot be an Arrhenius nor a Brønsted-Lowry acid-base reaction. Na―F + BF 3 Na + + BF 4 - which is the acid and which is the base ؟؟؟؟ Electrophilic replacement 930805 http:\\asadipour.kmu.ac.ir.......40 slides 37
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Chapter FifteenPrentice-Hall ©2002Slide 38 of 31 Evolution of the acid-base concept yearthinkerAcid Base acid-base reaction ==================================================== 1884Arrhenius ionizeionize H + + OH¯ HOH H + OH¯ 1923 Bronsted-Proton proton HA + B HB + A Lowry Donor acceptorconjugation 1923 Lewiselectrophil nucleophilE + Nu E:Nu 930805 http:\\asadipour.kmu.ac.ir.......40 slides 38
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Chapter FifteenPrentice-Hall ©2002Slide 39 of 31 Acid-Base Theories Arrhenius, Brønsted-Lowry, and Lewis Acid- Base Theories expand on one another. Lewis Brønsted-Lowry Arrhenius 930805 http:\\asadipour.kmu.ac.ir.......40 slides 39
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Acid-Base Theories Arrhenius, Brønsted-Lowry, and Lewis Acid- Base Theories expand on one another. Lewis Brønsted-Lowry Arrhenius 930805 http:\\asadipour.kmu.ac.ir.......40 slides 40
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