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Transcript
pKa is an Energy Term • A proton abstracted by a base is generalized in the following reaction AH + B(-) A(-) + BH o o We will worry about the descriptive details of how this happens later. • The half reaction in water is: AH + H2O A(-) + H3O(+) o o has an equilibrium constant associated with it Keq = [A][H3O]/[AH][H2O] o The terms in this equation are concentration. The molarity of water, [H2O] is 55 M or 55 moles/Liter o The acidity constant is • Ka = Keq[H2O] = [A][H3O]/[AH] Consider this equation: Ka = Keq[H2O] Taking the negative log of both sides and multiplying by RT gives: o −RTlogKa =−RTlog(Keq[H2O])= −RTlogKeq −RTlog[H2O] R is the gas constant and T is the absolute temperature −2.30RTlogKa = −RTLnKeq − RTLn[H2O] but −logKa = pKa and −RTLnKeq = ∆G 2.30RTpKa + RTLn[H2O]= ∆G Result: PKa increases linearly with energy! Acids conj. bases species with lone pairs Deprotonation Reaction Coordinate The species with lone pairs (conjugate bases) respond more to structural changes than the acids.
