Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431

Biochemistry

Fall 2008

Lecture Notes: 10 September

© R. Paselk 2008
 
PREVIOUS  

NEXT

Water, cont.

Water is an excellent solvent for polar substances since its dipolar structure enables it to insulate them from each other and it can make good dipole-dipole and dipole-charge bonds. Figure 2-6 shows the liganding of water to sodium and chloride ions to form hydration shells (For sodium ions, the waters in the inner hydration-shell exchange every 2-4 nsec.). Anything which can H-bond will also of course be quite soluble.

How does water interact with non-polar molecules?

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

The strengths (ability to donate protons) of acids vary considerably. [text Figure 2-15]


Pathway Diagrams

C431 Home

Lecture Notes

Last modified 10 September 2008