Ch1b2010_week7_ab

Ch1b2010_week7_ab - Wk 5 3 4 JH 5 Spontaneous Processes...

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1 Finish acid/base equilibrium Very brief electrochemistry, Δ G and equilibrium Expanded Chemistry in Context No lecture on Monday 19 18 17 16 JH Kinetics OGC Chptr 18 15 Pres Day holiday Wk 7 12 PS5 due 11 JH equilibrium 10 9 JH Chemical equilib OGC: 14.1-14.7 8 no lecture No lecture Midterm due 8 PM Wk 6 5 4 JH Spontaneous Processes & Thermo Equilib OGC: 13.5-13.7 midterm out 10PM 3 Wk 5 Prof Reisman takes over
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2 Example (Part I) Prepare solutions with pH=3 from nitric acid and then from acetic acid For nitric acid (HNO 3 ) pK a = -1.44 [] [ ][ ] 3 3 3 3 3 10 10 10 5 . 27 + + = = = = M HNO NO H AB B A K a Solve for M; M=1.003x10 -3 M This is the sign of a strong acid; the concentration of [H + ] [HA] added Possible error in your handwritten notes from Tuesday’s chalk talk
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3 Buffers! Tell them about buffers! Good idea, Gilbert! Gilbert Lewis 1875-1946
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4 Acids and Bases: Buffers A buffer contains both a weak acid and its conjugate base. Example: for an acetic acid/acetate buffer you would add acetic acid and sodium acetate to water. Buffers resist change in pH . The weak acid and its conjugate base react with either acid or base HA(aq) + OH - (aq) A (aq) - + H 2 O(l) A - (aq) + H + (aq) HA(aq) Both of these reactions have large equilibrium constants.
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5 Example, Part II Make a buffer solution that remains at approximately pH=5 from acetic acid & sodium acetate, and show that it remains close to pH=5 upon addition of 20 mM HNO 3 Start with [1M] CH 3 CO 2 H and, with Na + CH 3 CO 2 - , make the buffer [] [ ][ ] 1 10 10 10 75 . 1 5 5 2 3 2 3 5 + + + = = × = = x H CO H CO H H AB B A K a Acetic acid is only weakly ionized (at pH=5) and so we assume its concentration stays at [1M] Solving for x yields x=1.75M CH 3 CO 2 - Na + Possible error in your handwritten notes from T
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6 Example, Part II Make a buffer solution that remains at approximately pH=5 from acetic acid & sodium acetate, and show that it remains close to pH=5 upon addition of 20 mM HNO 3 Start with [1M] CH 3 CO 2 H and, with Na + CH 3 CO 2 - , make the buffer [] [ ][ ] 1 10 10 10 75 . 1 5 5 2 3 2 3 5 + + + = = × = = x H CO H CO H H AB B A K a Now we add 20 mM HNO 3 = [0.02M] Assume that all HNO 3 ionizes (it is a strong acid) [0.02M] H + is taken up by H 3 CO 2 - , thereby reducing the concentration of acetate from 1.75 M to 1.73 M
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7 Example, Part II Make a buffer solution that remains at approximately pH=5 from acetic acid & sodium acetate, and show that it remains close to pH=5 upon addition of 20 mM HNO 3 [] [ ] [ ] x x x AB B A K a + = × = = + 02 . 1 73 . 1 10 75 . 1 5 Assume that all HNO 3 ionizes (it is a strong acid) [0.02M] H + is taken up by H 3 CO 2 - , thereby reducing the concentration of acetate from 1.75 M to 1.73 M Then set up the K a equation, allowing for the concentration of [H + ] to vary Solve for x; [x] = 1.032x10 -5 ; pH = 4.98 The pH has hardly changed, even though we added a strong acid! This is the point of a buffer!!
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8 Electrochemistry Light (& equilibrium) 75% less facts than your regular electrochemistry
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9 Cu 2+ (aq) + Zn Cu(s) + Zn 2+ The salt bridge conducts ions between both chambers to maintain electrical neutrality.
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Ch1b2010_week7_ab - Wk 5 3 4 JH 5 Spontaneous Processes...

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