lec03 - Biochemistry I Fall Term, 2004 September 3, 2004...

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1 Biochemistry I Fall Term, 2004 September 3, 2004 Assigned reading in Campbell: 2.3-2.6. Key Terms: Acid strength Acid dissociation constant Equilibrium constant Ion product constant for water pH = pK a + log[A - ]/[HA] Equivalence point Titration Buffer capacity Polyprotic acids Zwitterion Links : ( I ) Review Quiz on Lecture 3 concepts ( I ) ( S ) The pH titration animation shows a typical weak acid titration. ( S ) Graphing Quiz: pH Titration Determine pKa and the acid concentration. 2.3 Acids and Bases Acid: can donate protons Base: can accept protons Example: The compound M-O-H is an acid if it looses its proton, but it is a base if it releases an OH - group. Whether a compound is an acid or a base depends on the relative strength of the M-O bond versus the O-H bond. The strength of the acid increases with the electronegativity of M. The higher the electronegativity the more electrons are drawn from the OH group. This electron density stablilizes the M-O bond.
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2 For example, the alkali earth metals (Li, Na, K) all have electronegativities of one or less. Thus the M-O bond is more ionic in nature and LiOH, NaOH, KOH are very basic. In general, most hydroxides of metals are basic. When M is a non-metal ( e.g. C, S, N, Cl), the compound is usually acidic. The more electron- withdrawing M (and its ligands) are, the stronger the acid. Fluorine has the highest electronegativity, 4. Consider the following three compounds: Ethanol is a very weak acid, acetic acid somewhat stronger, and trichloracetic acid is very strong. A quantitative measure of the strength of an acid is given by the acid dissociation constant, K
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lec03 - Biochemistry I Fall Term, 2004 September 3, 2004...

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