chem 1.1 chapter 17a

chem 1.1 chapter 17a - General Chemistry II Professor...

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Unformatted text preview: General Chemistry II, Professor Maggie Ciszkowska General Chemistry II, Professor Maggie Ciszkowska Aqueous queous Equilibria Equilibria : : Acid Acid − − Base Base Chapter 17 Chapter 17 • Consider the equilibrium established when acetic acid, HC 2 H 3 O 2 , is added to water. • At equilibrium we have (in order of decreasing concentration): H 2 O( l ) HC 2 H 3 O 2 ( aq ) H 3 O + ( aq ) and C 2 H 3 O 2 − ( aq ) (equal concentrations) OH − ( aq ) HC 2 H 3 O 2 ( aq ) + H 2 O( l ) ⇄ H 3 O + ( aq ) + C 2 H 3 O 2 − ( aq ) The Common Ion Effect The Common Ion Effect ] O H HC [ ] O H C ][ O [H 2 3 2 2 3 2 3 − + = a K General Chemistry II, Professor Maggie Ciszkowska General Chemistry II, Professor Maggie Ciszkowska • Consider the addition of NaC 2 H 3 O 2 into the solution of HC 2 H 3 O 2 ( aq ). • NaC 2 H 3 O 2 is a salt, strong electrolyte, 100% dissociated , and, therefore, it is a source of acetate, C 2 H 3 O 2 − ( aq ). NaC 2 H 3 O 2 ( aq ) → Na + ( aq ) + C 2 H 3 O 2 − ( aq ) • [C 2 H 3 O 2 − ] increases and the system is no longer at equilibrium; Q a > K a The Common Ion Effect The Common Ion Effect General Chemistry II, Professor Maggie Ciszkowska General Chemistry II, Professor Maggie Ciszkowska • Addition of NaC 2 H 3 O 2 into the solution of HC 2 H 3 O 2 ( aq ). Q > K C 2 H 3 O 2 ( aq ) + H 2 O( l ) ⇄ H 3 O + ( aq ) + C 2 H 3 O 2 − ( aq ) • To reach equilibrium, Q must decrease , the equilibrium shifts to the left, and as a consequence, [H 3 O + ] must decrease. • Addition of a common ion influences equilibrium. The Common Ion Effect The Common Ion Effect ] O H HC [ ] O H C ][ O [H 2 3 2 2 3 2 3 − + = Q General Chemistry II, Professor Maggie Ciszkowska General Chemistry II, Professor Maggie Ciszkowska • A buffer solution consists of a mixture of a weak acid (HX) and its conjugate base (X − ) . HC 2 H 3 O 2 ( aq ) + H 2 O( l ) ⇄ H 3 O + ( aq ) + C 2 H 3 O 2 − ( aq ) acid (HX) conjugate base (X − ) or a weak base (NH 3 ) and its conjugate acid (NH 4 + ) : NH 3 ( aq ) + H 2 O( l ) ⇄ NH 4 + ( aq ) + OH − ( aq ) base conjugate acid Buffer Solutions: Composition Buffer Solutions: Composition General Chemistry II, Professor Maggie Ciszkowska General Chemistry II, Professor Maggie Ciszkowska • A buffer solution consists of a mixture of a weak acid (HX) and its conjugate base (X − ) . or a weak base (NH 3 ) and its conjugate acid (NH 4 + ) Buffer solutions resist significant changes in pH when a small amount of a strong acid or a strong base is added. Note : both an acid and a base must be present (mixed) in significant, comparable concentrations. Buffer Solutions: Composition Buffer Solutions: Composition General Chemistry II, Professor Maggie Ciszkowska General Chemistry II, Professor Maggie Ciszkowska HC 2 H 3 O 2 ( aq ) + H 2 O( l ) ⇄ H 3 O + ( aq ) + C 2 H 3 O 2 − ( aq ) The K a expression is: ] O H C [ ] O H HC [ ] O H [ ] O H HC [ ] O H C ][ O H [ 2 3 2 2 3 2 3 2 3 2 2 3 2 3 − +...
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chem 1.1 chapter 17a - General Chemistry II Professor...

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