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GeneralChemRev - BIOC 460 General Chemistry Review Chemical...

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Bioc460 - General Chemistry Review p. 1 BIOC 460 General Chemistry Review: Chemical Equilibrium, Ionization of H 2 O, pH, pK a General Equilibrium What are the UNITS of K eq ? Example reactions: A --> B units of K eq ? _________ A --> B + C units of K eq ? _________ A + B --> C units of K eq ? _________ A + B --> C + D units of K eq ? _________ 2A --> B units of K eq ? _________ 2A + B --> C + D units of K eq ? _________ Units of K eq depend on the reaction!
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Bioc460 - General Chemistry Review p. 2 Dissociation of H 2 O
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Bioc460 - General Chemistry Review p. 3 NOTE: •If pH > 7, [H + ] < 10 –7 M (solution is "basic") •If pH < 7, [H + ] > 10 –7 M (solution is "acidic") •If pH = 7, [H + ] = 10 –7 M (solution is "neutral") pH scale is logarithmic: •If change in pH ( Δ pH) = 1, Δ [H + ] = 10 1 (10-fold). •If change in pH ( Δ pH) = 2, Δ [H + ] = 10 2 (100-fold). Table 2-6, from Nelson & Cox: Lehninger Principles of Biochemistry , 4th edition (2004)
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Bioc460 - General Chemistry Review p. 4 Ionization of weak acids and bases Br ο nsted definitions: acid = a proton donor base = a proton acceptor Definition of pK a
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Bioc460 - General Chemistry Review p. 5 Below is a table of some weak acids, with their pK a values (25° C). Acid pK a Acid pK a acetic acid CH 3 -COOH 4.76 ammonium ion NH 4 + 9.25 chloroacetic acid ClCH 2 -COOH 2.85 Methylamine CH 3 -NH 3 + 10.6 dichloroacetic acid (Cl) 2 CH-COOH 1.48 Ethylamine CH 3 -CH 2 -NH 3 + 10.8 trichloroacetic acid (Cl) 3 C-COOH 0.70 Ethanolamine HO-CH 2 -CH 2 -NH 3 + 9.5 formic acid H-COOH 3.75 acetic acid CH 3 -COOH 4.76 propionic acid CH 3 -CH 2 -COOH 4.87 Fig. 2-16: Proton dissociation reactions of some mono-, di-, and triprotic acids, with their pK a s arranged on a pH scale (from Nelson & Cox: Lehninger Principles of Biochemistry , 4th edition (2004)
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Bioc460 - General Chemistry Review p. 6
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Bioc460 - General Chemistry Review p. 7 Quantitative relationship between pK a and pH Biological reactions occur in aqueous environments, or in environments defined by the active centers of enzymes which themselves exist in aqueous environments. The active centers of enzymes have functional groups that participate in the processes of catalysis, and the chemical nature of those functional groups is often a function of the pH. The importance of pH in the functional properties of proteins and enzymes is well known. Dissociation of weak acids Consider the simple acid HA (or alternatively, HA + ): HA H + + A Example: R-COOH H + + RCOO OR HA + H + + A Example: R-NH 3 + H + + R-NH 2 The form with the associated proton is the conjugate acid form. The form after dissociation of the proton (the form without the proton) is the conjugate base . The equilibrium DISSOCIATION constant is called K a . (The subscript "a" stands for " acid " dissociation, not an association equilibrium. Biochemists often simply use the term "pK".) The higher the K a (the greater the tendency of the acid to DISSOCIATE/DONATE its proton), the stronger the acid . Remember that pH = – log[H + ] and pK a = – logK a . Thus the lower the pK a , the stronger the acid .
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