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CHEM100_11

Lecture #28. Chapter 16. Acid-base equilibrium. CHEM100_11. This week: Labs, Pre-labs. Assignment#5(Chemical Equilibrium) due to TODAY!!!. X-mas exam: 11 December, 9 am. Dr. Orlova PS-3026 Ph: 867-5237 gorlova@stfx.ca. Student Chemical Society Tutorials:

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CHEM100_11

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  1. Lecture #28 Chapter 16. Acid-base equilibrium CHEM100_11 This week: Labs, Pre-labs Assignment#5(Chemical Equilibrium) due to TODAY!!! X-mas exam: 11 December, 9 am Dr. Orlova PS-3026 Ph: 867-5237 gorlova@stfx.ca Student Chemical Society Tutorials: Every Monday and Tuesday , NH246 from 7pm-8:30pm Helping hrs at PS-3026: Tu & Th 1-2 pm; Wd 4-5 pm

  2. Cations of heavy metals: FeCl3 (aq)  3Cl- (aq) + Fe3+ (aq) Fe(H2O)63+ (aq) + H2O (l)  Fe (H2O)5 (OH)+2 (aq) + H3O+ (aq) Weak acids and weak bases Weak acids Weak acids: HNO2 (aq) + H2O (l)  H3O+(aq) + NO2- (aq) Polyatomic cations: NH4Cl (aq)  NH4+ (aq) + Cl- (aq) NH4+(aq) + H2O (l)  NH3 (aq) + H3O+ (aq) Weak Bases Ammonia and amines: NH3 (aq) + H2O (l)  NH4+ (aq) + OH- (aq) Anions of weak acids: KCN (aq)  K+ (aq) + CN- (aq) CN- (aq) + H2O (l)  HCN (aq) + OH- (aq)

  3. Relation between Ka and Kb HClO (aq) + H2O (l)  ClO- (aq) + H3O+ (aq) ClO- (aq) + H2O (l)  HClO (aq) + OH- (aq) Ka HClO (aq) + H2O (l)  ClO- (aq) + H3O+ (aq) ClO- (aq) + H2O (l)  HClO (aq) + OH- (aq) Kb 2H2O (l)  OH- (aq) + H3O+ (aq) Kw

  4. When we add 2 rxn, Keq should be multiplied Ka x Kb = Kw Ka x Kb = 10-14 pKa + pKb = pKw = 14

  5. Combined effects of Cation and Anion in Solution acid salts : Na HCO3 , NaH2PO4 (salts of polyprotic acids) The pH of an acid salt is affected by the hydrolysis of the anion and by its acid dissociation. Sample exercise 16.18 in the text Exercise: Predict whether the dipotasium salt of citric acid (K2HC6H5O7) will form an acidic or basic solution in water. Note that citric acid is 3-protic acid From the reference table (16.3) for H3C6H6O7: Ka1 = 7.4 x 10-4, Ka2 = 1.7x10-5; Ka3 = 4.0x10-7

  6. There are two possibilities: HC6H5O72- H+ + C6H5O73- Ka3 = 4.0x10-7 HC6H5O72- + H2O H2C6H5O7- + OH- Strong electrolyte ionization: K2HC6H5O7 (aq)  2K+ (aq) + HC6H5O72- (aq) Kb2 for HC6H5O7 2- ? Must calculate : Kw/Ka2 Kb2 = 1.0x10-14 / 1.7x10-5 ; Kb2 = 5.8x10-10 Kb2 < Ka3 ACIDIC

  7. CH3COOH (aq) + H2O (l) CH3COO- (aq) + H3O+ (aq) The Common Ion Effect Chapter 17 Solutions of acids and their salts • Consider the equilibrium established when acetic acid, CH3COOH, is added to water. Let us add to the solution of CH3COOH a soluble salt of this acid, such as CH3COONa • [C2H3O2-] increases and the system is no longer at equilibrium. CH3COO- is a weak base, thus pH will increase.

  8. CH3COOH (aq) H+ (aq) + CH3COO- The common-ion effect NH3 (aq) + H2O (l) NH4+ (aq) + OH- (aq) Add NH4Cl: equil. shifts to the left, reducing OH- Le Chatelier principle- increase in [CH3COO- ] shifts this equilibrium to the left, reducing [H+] Ionization of a weak electrolyte is decreased by adding to the solution a strong electrolyte that has an ion in common with the weak electrolyte. The ionization of bases is also decreased by adding of a common ion

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