QCA7e_ch10_szb_handouts

QCA7e_ch10_szb_handouts - 1 Quantitative Chemical Analysis...

Info iconThis preview shows pages 1–8. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 1 Quantitative Chemical Analysis Seventh Edition Quantitative Chemical Analysis Seventh Edition Chapter 10 Polyprotic Acid-Base Equilibria Copyright 2007 by W. H. Freeman and Company Daniel C. Harris Daniel C. Harris Proteins are Polyprotic Acids and Bases Myoglobin has 35 basic and 23 acidic side groups 2 Diprotic Acids and Bases The amino acid building blocks of proteins have the general structure: where R is a different group for each amino acid. Diprotic Acids and Bases The carboxyl group is a stronger acid than the ammonium group. Therefore, the nonionized form rearranges spontaneously to the zwitterion: At low pH, both the ammonium group and the carboxyl group are protonated. At high pH, neither is protonated. 3 Amino Acids Let us focus on the amino acid leucine, designated HL. The equilibrium constants refer to the following reactions: H 2 L + ' HL + H + K a1 = K 1 HL ' L + H + K a2 = K 2 L + H 2 O ' HL + OH K b1 HL + H 2 O ' H 2 L + + OH K b2 4 Selenocysteine Acidity The S Se substitution makes the amino acid much more acidic more reactive. There are 25 human selenoenzymes! 5 Back to Leucine The Acidic Form: H 2 L + Leucine hydrochloride contains the protonated species H 2 L + , which can dissociate twice. Because K 1 = 4.70 10 3 , H 2 L + is a weak acid. HL is an even weaker acid, because K 2 = 1.80 10 10 . It appears that the H2L+ will dissociate only partly, and the resulting HL will hardly dissociate at all. For this reason, we make the justified approximation that a solution of H 2 L + behaves as a monoprotic acid, with K a = K 1 . With this approximation, the calculation of the pH of 0.0500 M H 2 L + is a trivial matter. The Acidic Form: H 2 L + 6 The Acidic Form: H 2 L + What is the concentration of L in the solution? We have already assumed that it is very small, but it cannot be 0, of course. We can calculate [L ] from the K a2 eqn, with [HL] and [H + ] just computed. Conclusions The concentration of L is about eight orders of magnitude smaller than that of HL. The dissociation of HL is negligible relative to the dissociation of H 2 L + . For most diprotic acids, K 1 is sufficiently larger than K 2 for this approximation to be valid. If K 2 were just 10 times less than K 1 , [H + ] calculated by ignoring the second ionization would be in error by only 4%! (Error in pH: 0.01 pH unit.) In summary, a solution of a diprotic acid behaves like a solution of a monoprotic acid , with K a = K a1 . 7 The Basic Form, L The species L is found in a salt such as sodium leucinate. Dissolving sodium leucinate in water gives a solution of L , the fully basic species....
View Full Document

Page1 / 32

QCA7e_ch10_szb_handouts - 1 Quantitative Chemical Analysis...

This preview shows document pages 1 - 8. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online