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C. Y. Yeung (CHW, 2009)

Acid-Base Eqm (1): New Acid-Base Theory. Acid-Base Eqm (1): New Acid-Base Theory. p.01. Acid = substance releases H + as the only cation in water. (e.g. HCl  H + + Cl - ). Base = substance reacts with H + to give salt and water only. (e.g. NaOH + HCl  NaCl + H 2 O).

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C. Y. Yeung (CHW, 2009)

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  1. Acid-Base Eqm (1): New Acid-Base Theory Acid-Base Eqm (1): New Acid-Base Theory p.01 Acid = substance releases H+ as the only cation in water. (e.g. HCl  H+ + Cl-) Base = substance reacts with H+ to give salt and water only. (e.g. NaOH + HCl  NaCl + H2O) Arrhenius Theory C. Y. Yeung (CHW, 2009)

  2. p.02 Acid=proton (H+)donor Base =proton (H+)acceptor(can be anything with lone pair e-) conjugate base conjugate acid base acid HA + H2O A-+ H3O+ H2O + NH3OH- + NH4+ H2SO4 + H2O HSO4- + H3O+ HSO4- + H2O SO42- + H3O+ (H+ acceptor) (H+ donor) (H+ acceptor) (H+ donor) Bronsted - Lowry Theory Arrhenius Theory

  3. p.03 e.g. The conjugate base of HCl is Cl-. The conjugate acid of NH3 is NH4+. Familiar Conjugate Pairs p. 134 Check Point 17-1

  4. The ion responsible for Acidity: H3O+ (Hydroxonium ion) p.04 dative covalent bond O H H H3O+ (hydroxonium ion) no e-, a proton only!  very high charge density and e- deficient . (all orbitals are empty!!) H+  O atom has 2 lone pair e- !  very e- rich. ** H+ ion does not exist in aq. solution, but H3O+ instead.

  5. p.05 H2O + H2O i.e. H2O can act as both acid and base. Thus, H2O is an amphoteric species. Actually, H3O+ (aq) are formed from the “self-ionization” of H2O molecules DH > 0 (endothermic) OH-+ H3O+ Other examples of amphoteric species: HCO3-, HSO4-, HPO42-

  6. Ionic product of H2O (Kw) p.06 H2O(l) + H2O(l) OH-(aq)+ H3O+(aq) [H3O+(aq)][OH-(aq)] Kc = [H2O(l)][H2O(l)] [H3O+(aq)] [OH-(aq)] Kc [H2O(l)]2= Kw= [H3O+(aq)] [OH-(aq)] ** For neutral solution (e.g. pure water), [H3O+(aq)] = [OH-(aq)] = 1.0010-7 M (at 250C) temp. dependent. At 250C, KW = 1.0010-14 M-2

  7. p.07 If [H3O+] = [OH-] : NEUTRAL SOLUTION If [H3O+] > [OH-] : ACIDIC SOLUTION If [H3O+] < [OH-] : BASIC SOLUTION For all aq. solutions, [H3O+] [OH-] = 1.0010-14 mol2 dm-6 pH = – log [H3O+] pOH = – log [OH-] e.g. If [H3O+] of solution A is 2.5110-4 M, then at 298K, pH = – log (2.5110-4) = 3.60, pOH = 14 – 3.60 = 10.4

  8. p.08 If temp. increases to 313K, KW becomes . It is neutral !! (at 313K) However, as KW is temperature dependent, pH also changes with temp.!! Therefore, “pH of water = 7” is only true at 298K!! 2.92 10-14 M-2  at 313K, [H3O+] = [OH-] = (2.92 10-14)1/2 = 1.7110-7 M  pH = -log (1.71 10-7) = 6.77

  9. p.09 T,  DH>0 eqm. shifts FW, Kw  For your reference …. Kw & Temperature p. 140 Check Point 17-3

  10. Since the pH falls as temperature increases, does it mean that water becomes more acidic at higher temperature? p.10 pH of an impure water sample at 1000C = 7.0 i.e. the sample is acidic / neutral / alkaline. pH of an impure water sample at 00C = 7.0 i.e. the sample is acidic / neutral / alkaline. NO!! [H3O+] = [OH-], neutral! pH drops due to the increase of Kw at high temperature. Neutral pH of water at 1000C is 6.14! Neutral pH of water at 00C is 7.48! p. 140 Check Point 17-3

  11. p.11 The dissociation of H2O is an endothermic process. Increase in temperature, the equilibrium shifts to the right (forward) and hence [H3O+] increases. In pure water, [H3O+] at 323K is higher than that at 298K, therefore pH of pure water at 323K < 7. H2O(l) + H2O(l) OH-(aq)+ H3O+(aq) 1994 HKALE Paper 2 Q.3(b) At 323K, the pH of pure water is less than 7.0. Explain.

  12. p.12 Next …. Calculation of Ka and Kb. Strength of Acid and Base. Assignment Study all examples in p.138 - 139 p.152 Q.1 –2 [due date: 25/3(Wed)]

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