Upload
vedpal-yadav
View
69
Download
0
Tags:
Embed Size (px)
DESCRIPTION
Citation preview
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
CHAPTER 2Water, pH, and Ionic Equilibria
to accompany
Biochemistry, 2/e
by
Reginald Garrett and Charles Grisham
All rights reserved. Requests for permission to make copies of any part of the work should be mailed to: Permissions Department, Harcourt Brace & Company, 6277 Sea Harbor Drive, Orlando, Florida 32887-6777
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Outline
• 2.1 Properties of Water
• 2.2 pH
• 2.3 Buffers
• 2.4 Water's Unique Role in the Fitness of the Environment
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Properties of Water
• High b.p., m.p., heat of vaporization, surface tension
• Bent structure makes it polar
• Non-tetrahedral bond angles
• H-bond donor and acceptor
• Potential to form four H-bonds per water
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Comparison of Ice and WaterIssues: H-bonds and Motion
• Ice: 4 H-bonds per water molecule
• Water: 2.3 H-bonds per water molecule
• Ice: H-bond lifetime - about 10 microsec
• Water: H-bond lifetime - about 10 psec
• (10 psec = 0.00000000001 sec)
• Thats "one times ten to the minus eleven second"!
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Solvent Properties of Water
• Ions are always hydrated in water and carry around a "hydration shell"
• Water forms H-bonds with polar solutes
• Hydrophobic interactions - a "secret of life"
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Hydrophobic Interactions
• A nonpolar solute "organizes" water
• The H-bond network of water reorganizes to accommodate the nonpolar solute
• This is an increase in "order" of water
• This is a decrease in ENTROPY
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Amphiphilic Molecules
Also called "amphipathic"
• Refers to molecules that contain both polar and nonpolar groups
• Equivalently - to molecules that are attracted to both polar and nonpolar environments
• Good examples - fatty acids
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Acid-base EquilibriaThe pH Scale
• A convenient means of writing small concentrations:
• pH = -log10 [H+]
• Sørensen (Denmark)
• If [H+] = 1 x 10 -7 M
• Then pH = 7
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Dissociation of Weak Electrolytes
Consider a weak acid, HA• The acid dissociation constant is given
by:• HA H+ + A-
• Ka = [ H + ] [ A - ] ____________________
[HA]
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
The Henderson-Hasselbalch Equation
Know this! You'll use it constantly.
• For any acid HA, the relationship between the pKa, the concentrations existing at equilibrium and the solution pH is given by:
• pH = pKa + log10 [A¯ ]
¯¯¯¯¯¯¯¯¯¯
[HA]
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Consider the Dissociation of Acetic Acid
Assume 0.1 eq base has been added to a fully protonated solution of acetic acid
• The Henderson-Hasselbalch equation can be used to calculate the pH of the solution:With 0.1 eq OH¯ added:
• pH = pKa + log10 [0.1 ]
¯¯¯¯¯¯¯¯¯¯
[0.9]
• pH = 4.76 + (-0.95)
• pH = 3.81
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Consider the Dissociation of Acetic AcidAnother case....
• What happens if exactly 0.5 eq of base is added to a solution of the fully protonated acetic acid?
• With 0.5 eq OH¯ added:• pH = pKa + log10
[0.5 ] ¯¯¯¯¯¯¯¯¯¯
[0.5]
• pH = 4.76 + 0
• pH = 4.76 = pKa
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Consider the Dissociation of Acetic Acid
A final case to consider....• What is the pH if 0.9 eq of base is added
to a solution of the fully protonated acid?• With 0.9 eq OH¯ added:
• pH = pKa + log10 [0.9 ]
¯¯¯¯¯¯¯¯¯¯
[0.1]
• pH = 4.76 + 0.95
• pH = 5.71
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Buffers
• Buffers are solutions that resist changes in pH as acid and base are added
• Most buffers consist of a weak acid and its conjugate base
• Note in Figure 2.15 how the plot of pH versus base added is flat near the pKa
• Buffers can only be used reliably within a pH unit of their pKa
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company
Biochemistry 2/e - Garrett & Grisham
Copyright © 1999 by Harcourt Brace & Company