pKa and the Relationship between Acids and Conjugate Bases, Study notes of Chemistry

Download pKa and the Relationship between Acids and Conjugate Bases and more Study notes Chemistry in PDF only on Docsity! pKa is an Energy Term • A proton abstracted by a base is generalized in the following reaction o AH + B(-) A(-) + BH o We will worry about the descriptive details of how this happens later. • The half reaction in water is: o AH + H2O A(-) + H3O(+) 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!