Chapter 18 - Brady(1)

Chapter 18 - Brady(1) - Introduction to General and Organic...

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1 Chapter 18 Equilibria in Solutions of Weak Acids and Bases Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry II
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2 Weak Acids and Bases All weak acids behave the same way in aqueous solution: they partially ionize In terms of the “general” weak acid HA, this can be written as: Following the procedures in Chapter 16 - + + + A O H O H HA 3 2 ] [ ] ][ [ ] [ ] ][ [ 3 HA A H HA A O H K a - + - + =
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3 K a is called the acid ionization constant • These are often reported as the p K a • Table 18.1 and Appendix C list the K a and p K a for a number of acids A “large” p K a, means a “small” value of K a and only a “small” fraction of the acid molecules a a K K log p - = Ionization Constants , K a
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4 Weak bases behave in a similar manner in water For the “general” base B: ] [ ] ][ [ is constant ionization base the 2 B OH HB K OH HB O H B b - + - + = + + Weak Bases
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5 • Values of K b and p K b for a number of weak bases are listed in Table 18.2 and in Appendix C Similar to weak acids: There is an interesting relationship between the acid and base ionizations constants for a conjugate acid-base pair; K a K b = K w Using the general weak acid HA: b b K K log p - = Ionization Constants , K b
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6 w b a b a K OH O H A OH HA HA A O H K K A OH HA K OH HA O H A HA A O H K A O H O H HA = = × = × = + + = + + - + - - - + - - - - - + - + ] ][ [ ] [ ] ][ [ ] [ ] ][ [ is product the ] [ ] ][ [ : base conjugate for the ] [ ] ][ [ : acid weak For the 3 3 2 3 3 2 Rationale
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7 Thus, for any conjugate acid-base pair: Most tables of ionization constants only give values for the acid or base of the conjugate acid- base pair The ionization constant of the ion member of the conjugate acid-base pair is then calculated as needed C) 25 (at 00 . 14 p p p and o = = + = × w b a w b a K K K K K K Conjugate Acid-Base Pairs
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8 The stronger the acid is, the weaker the conjugate base. Very strong acids ionize 100% and their conjugate bases do not react to any measurable extent. Relative Strengths of Conjugate Acid-base Pairs
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9 The primary goal is usually to determine the equilibrium concentration for all species in the mass action expression The percentage ionization of the acid or base is defined as: This, and the pH, are often used or requested in equilibrium calculations % 100 liter per available moles liter per ionized moles ionization percentage × = Calculating Ionization Constants
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10 Example : Morphine is very effective at relieving intense pain and is a weak base. Determine the value of K b , p K b , and percentage ionization of morphine if a 0.010 M solution has a pH of 10.10 ? ANALYSIS
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Chapter 18 - Brady(1) - Introduction to General and Organic...

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