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lecnotes22 - MIT OpenCourseWare http/ocw.mit.edu 5.111...

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MIT OpenCourseWare http://ocw.mit.edu 5.111 Principles of Chemical Science Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms .
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22.1 5.111 Lecture Summary #22 Acid/Base Equilibrium Continued (Chapters 10 and 11) Topics: Equilibrium involving weak bases, pH of salt solutions, and buffers From Wednesday’s handout 2. Base in water NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) [NH 4 + ] [OH - ] Base ionization constant K b = [NH 3 ] K b is 1.8 x 10 -5 at 25°C. This small value tells us that only a small amount of NH 3 is present as NH 4 + . A strong base reacts essentially completely to give OH - (aq) when put in water. NH 3 is not a strong base. It is a moderately weak base. B (aq) + H 2 O (l) BH + (aq) + OH - (aq) BASE (B) IN WATER A - (aq) + H 2 O (l) HA (aq) + OH - (aq) BASE (A - ) IN WATER pK b = -log K b larger K b , stronger base larger pK b , weaker base 3. Conjugate acids and bases The stronger the acid, the weaker its conjugate base. The stronger the base, the weaker its conjugate acid. Consider conjugate acid-base pair NH 3 and NH 4 + : NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) NH 4 + (aq) + H 2 O (l) H 3 O + (aq) + NH 3 (aq) Multiply K's together and get: + [NH 3 ][H 3 O + ] x [NH 4 ][OH - ] [H 3 O + ][OH - ] K a x K b = = [NH 4 + ] [NH 3 ]
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22.2 K a x K b = K w log K a + log K b = log K w or pK a + pK b = pK w = 14.00 Strong acid HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) Strong base B (aq) + H 2 O (l) BH + (aq) + OH - (aq) 4. Relative strengths of acids Is HNO 3 or NH 4 + a stronger acid? Will the reaction lie far to the right or left? - HNO 3 (aq) + NH 3 (aq) NO 3 (aq) + NH 4 + (aq) [NO 3 - ][NH 4 + ] K = [HNO 3 ][NH 3 ] consider each acid separately: - 1. HNO 3 (aq) + H 2 O (l) H 3 O + (aq) + NO 3 (aq) - [H 3 O + ][NO 3 ] K a (HNO 3 ) = = 20. [HNO 3 ] 2. NH 4 + (aq) + H 2 O (l) H 3 O + (aq) + NH 3 (aq) [H 3 O + ][NH 3 ] K a (NH 4 + ) = = 5.6 x 10 -10 [NH 4 + ] Subtract equation 2 from 1 and divide the corresponding equilibrium constants. - [H 3 O + ][NO 3 ] K (HNO 3 ) [HNO 3 ] [NO 3 - ][NH 4 + ] K = a = = = 20. = 3.6 x 10 10 K a (NH 4 + ) [H 3 O + ][NH 3 ] [HNO 3 ][NH 3 ] 5.6 x 10 -10 [NH 4 + ] Reaction lies far to the . HNO 3 is a than NH 4 + .
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22.3 Types of acid-base problems 1. weak acid in water 2. weak base in water salt in water 3. strong acid in water 4. strong base in water 5. buffer Equilibrium involving weak acids
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