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8.3 Bases

8.3 Bases. Similar to weak acids, weak bases react with water to a solution of ions at equilibrium. The general equation is: B ( aq ) + H 2 O (l)  HB + ( aq ) + OH - ( aq ) where, B represents any base The equilibrium expression for this reaction is: K c = [HB + ][OH - ]

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8.3 Bases

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  1. 8.3 Bases • Similar to weak acids, weak bases react with water to a solution of ions at equilibrium. • The general equation is: B(aq) + H2O(l)  HB+(aq) + OH-(aq) where, B represents any base The equilibrium expression for this reaction is: Kc = [HB+][OH-] [B][H2O]

  2. Base Dissociation Constant, Kb • Just as with weak acids, the concentration of water is almost constant in a solution of weak base. • Therefore, we can group the constant terms in the equation. [H2O] Kc = [HB+][OH-] = Kb [B] • Table 8.3 (Pg. 404 lists some common base dissociation constants, Kb)

  3. Calculating pH and pOH • An aqueous solution of ammonia has a concentration of 0.105 mol/L. Calculate the pH of the solution. 1. Write the balanced chemical equation. NH3(aq) + H2O(l)  NH4+(aq) + OH-(aq) Given: [NH3] = 0.105 mol/L 2. Set up an ICE table

  4. P.P. #29, Continued • Write the equation for the base dissociation constant, Kb Kb =[NH4+][OH-] [NH3] From the table, Kb(NH3) = 1.8 x 10-5

  5. P.P. #29, Continued • Substitute Kb and the equilibrium values from the ICE table into the equation. Solve for x. 1.8 x 10-5 = (x) (x) (0.105 – x) • Look at [NH3]/Kb = 0.105/1.8x10-5 = 5833 > 100, so the amount of NH3 that dissociates is negligible compared to the initial [NH3] Therefore, (0.105 – x), becomes (0.105)  solve for x

  6. P.P. #29, Continued • Solve for x 1.8 x 10-5 = (x) (x) (0.105) (0.105)1.8 x 10-5 = x2 x = √1.89 x 10-6 = 1.374 x 10-3 6. Solve for pOH pOH = -log[OH-] = -log[1.374 x 10-3] = 2.832

  7. P.P. #29, Continued • Solve for pH pH = 14 – pOH = 14 – 2.832 = 11.168 Therefore, the pH of the ammonia solution is 11.14

  8. The relationship b/n Ka and Kb • For any acid and its conjugate base Ka(Kb) = [H3O+][OH-] = Kw • What does this mean? • The stronger an acid is, the weaker its conjugate base is. The conjugate of a strong acid is always a weak base. The conjugate of a strong base is always a weak acid.

  9. Buffer Solutions • Contain a mixture of: • A weak acid and its conjugate base OR • A weak base and its conjugate acid • Buffer solutions resist a change in pH when a moderate amount of acid or base is added. • Buffer solutions are made 2 ways: • Use a weak acid and one of its salts • Eg. Mix acetic acid and sodium acetate • Use a weak base and one of its salts • Eg. Mix ammonia and ammonium chloride

  10. Making a Buffer • http://www.chembio.uoguelph.ca/educmat/chm19104/chemtoons/chemtoons5.htm

  11. How do buffers work? • Consider a buffer made with acetic acid and sodium acetate. • Acetic acid is weak, so [CH3COOH] is high • Sodium acetate is very soluble, so [CH3COO-] is high • Adding and acid or base has only a slight effect on pH because the H3O+ or OH- ions are removed by one of the components of the buffer solution. • http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/buffer12.swf

  12. Importance of Buffers • Very important in biological systems • For example: • pH of arterial blood is ~7.4 • It must stay b/n 7.0 and 7.5 or the organism may die. • Blood is buffered by the equilibrium between CO3 ions and HCO3 ions. (formed by the reaction of dissolved CO2 and H2O)

  13. 8.4 Acid-Base Titration Curves • Acid – Base Titration Curve = Graph of the pH of an acid (or base) against the volume of and added base (or acid) • Titration reactions are used to find the equivalence point • Equivalence point – point where the acid and base completely react with one another. • If, • V1, V2 and C1 are known • C2 can be found (C1V1 = C2V2)

  14. How pH indicators show equivalence points • pH changes rapidly near the equivalence point • A single drop of titrant can change the pH by 2 pH units. • The colour change indicates equivalence even if the colour change happens at pH of 8 • As long as it is in the steep section of the curve • Strong acid titrated with strong base

  15. Selecting a pH Indicator Weak acid titrated with strong base.

  16. Section Review • Which indicator should be used for each curve? • Is this the titration curve for a weak acid titrated with a strong base? Explain.

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