Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 438 - Introductory Biochemistry - Spring 2013

Lecture Notes:

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Water, cont.

How does water interact with non-polar molecules?

artists image of micelle, liposome and lipid bilayer

public domain image via Wikipedia Creative Commons

Finally, recall that water is a good nucleophile and so will participate in many chemical reactions-readily hydrolyzes esters, amides, anhydrides etc.

Ionization of Water, pH & Buffers

Dissociation of water molecules: One aspect of water we have yet to talk about is its dissociation or ionization. In normal aqueous solution there is a certain probability that a hydrogen nucleus (a proton) can exchange between two hydrogen bonded molecules:

(Of course the hydronium ion, H3O+, will be associated with additional water molecules as well through H-bonding. For simplicity we will just write H+, with the understanding that it refers in fact to hydrated hydronium ions in aqueous solution. ) Note the reaction is not highly favored, in neutral solution (no excess H+or OH-) there will only be 10-7 molar hydronium ions, in other words only about 2 of every billion water molecules will be protonated!

For aqueous solution [H+][OH-]= 10-14;

Brönsted acid: we will be using the Brönsted definition for acids and bases:

pH & Buffers

The strengths (ability to donate protons) of acids vary considerably.

The equilibrium equation for a mixture of a weak acid and its conjugate base can be rewritten by taking logs of both sides and rearranging to give the Henderson-Hasselbalch equation: pH = pKa + log [A-]/[HA]

We frequently represent the reaction of an acid with a base as a titration curve (Fig. 2.16).

titration curve

titratiuon curve with labels

You should understand these curves and be able to label them for axis, percent dissociation at beginning, middle and end, buffer region, end-point, and how to find pKa. An exercise to help you to review titrations curves is available.

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© R. A. Paselk 2010;

Last modified 4 February 2013