GeneralChemRev-BCH110A

GeneralChemRev-BCH110A - BCH 110A Dr. Ziegler General...

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BCH 110A General Chemistry Review (Dr. Ziegler) p. 1 BCH 110A – Dr. Ziegler General Chemistry Review: Chemical Equilibrium, Ionization of H 2 O, pH, pK a General Equilibrium: What are the UNITS of K eq ? Use dimensional analysis. Example reactions: A --> B units of K eq ? _______ [B] / [A] has units of [M] / [M], unitless . A --> B + C units of K eq ? _______ A + B --> C units of K eq ? _______ [C] / [A][B] has units of [M] / [M][M], so units of that K eq would be (M) –1 (“inverse molar”) 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! Dissociation of H 2 O:
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BCH 110A General Chemistry Review (Dr. Ziegler) p. 2 Table 2-6, from Nelson & Cox: Lehninger Principles of Biochemistry , 4th edition (2004) 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).
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BCH 110A General Chemistry Review (Dr. Ziegler) p. 3 Ionization of weak acids and bases: Br ο nsted definitions: acid = a proton donor base = a proton acceptor Definition of pK a : Below is a table of some weak acids, with their pK a values (25° C). Explain the differences in pK a within the 3 groups below. Why does substitution of each successive H in the methyl group of acetic acid with a Cl decrease the pK a ? Why does substitution of each additional methyl or methylene group for the H in formic acid or in methylamine increase the pK a ? Why does addition of the OH group to ethylamine to make ethanolamine decrease the pK a ?
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BCH 110A General Chemistry Review (Dr. Ziegler) p. 4 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. 2-15: Proton dissociation reactions of some mono-, di-, and triprotic acids, with their pK a s arranged on a pH scale. Nelson & Cox: Lehninger Principles of Biochemistry , 5th edition (2008)
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BCH 110A General Chemistry Review (Dr. Ziegler) p. 5 Quantitative relationship between pK a and pH: Look again at the equilibrium dissociation reaction and remember LeChatelier's Principle: HA H + + A The higher the [ H + ] (i.e., the lower the pH), the more the equilibrium shifts to the left, so the more conjugate acid will be present. Conversely, the lower the [ H + ] (i.e., the higher the pH), the more the equilibrium shifts to the right, so the more conjugate base will be present. The exact ratio of base to acid depends on the pH and the pK a (Henderson-Hasselbalch Equation). Note that
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This document was uploaded on 10/26/2011 for the course BCH 110a at UC Riverside.

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GeneralChemRev-BCH110A - BCH 110A Dr. Ziegler General...

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