Transcript Chapter 2 - Evangel University

Mary K. Campbell
Shawn O. Farrell
http://academic.cengage.com/chemistry/campbell
Chapter Two
Water: The Solvent for Biochemical Reactions
Paul D. Adams University of Arkansas
What makes water polar?
What is a polar bond:
• Electrons are unequally shared, more negative charge found closer to one atom.
• Due to difference in _________________________of atoms involved in bond.
Electronegativity
• __________________: a measure of the force of an
atom’s attraction for electrons it shares in a chemical
bond with another atom
• Oxygen and Nitrogen, ____________ electronegative
than carbon and hydrogen
• _______________ is most electronegative (4)
Polar Bonds & Molecules
• Molecules such as CO2 have polar __________ but,
given their geometry, are nonpolar _______; that is,
they have a __________ dipole moments
Solvent Properties of H2O
• _________ compounds (e.g.,KCl) and low-molecularweight ________ covalent compounds (e.g., C2H5OH
and CH3COCH3) tend to dissolve in ______________
• The underlying principle is electrostatic ________ of
_____________ charges; the positive dipole of water
for the negative dipole of another molecule, etc.
• ____________ interaction: e.g., KCl dissolved in H2O
• ____________ interactions: e.g., ethanol or acetone
dissolved in H2O
• ____________ interactions: weak and generally do not
lead to solubility in water
Hydration Shells Surrounding Ions in Water
Ion-dipole and Dipole-dipole Interactions
• ____________ and ____________ interactions
help ionic and polar compounds dissolve in water
Solvent Properties of H2O
• ____________ : water-loving
• tend to dissolve in water
• ____________ : water-fearing
• tend not to dissolve in water
• ____________ : characteristics of both properties
• molecules that contain one or more ____________
and one or more ____________ regions, e.g., sodium
palmitate
Amphipathic molecules
• both ____________ and ____________ character
• Interaction between ____________ molecules is very
weak, called van der Waals interactions
Micelle formation by amphipathic molecules
• Micelle: a ____________ arrangement of organic
molecules in ___________ solution clustered so that
• their ____________ parts are buried inside the sphere
• their ____________ parts are on the surface of the
sphere and in contact with the water environment
• formation depends on the attraction between
____________
____________
____________
Examples of Hydrophobic and Hydrophilic
Substances
Hydrogen Bonds
• Hydrogen bond: the attractive interaction between
dipoles when:
• positive end of one dipole is a hydrogen atom bonded
to an atom of high electronegativity, most commonly O
or N, and
• the negative end of the other dipole is an atom with a
lone pair of electrons, most commonly O or N
• Hydrogen bond is ______________________
Interesting and Unique Properties of Water
• Each water molecule can be involved in ___ hydrogen
bonds: ___ as donor, and ___ as acceptor
• Due to the ____________ arrangement of the water
molecule (Refer to Figure 2.1).
Hydrogen Bonding
• Even though hydrogen bonds are ___________ than
covalent bonds, they have a significant effect on the
physical properties of hydrogen-bonded compounds
Other Biologically Important Hydrogen bonds
• Hydrogen bonding is important in _______________
of 3-D structures of biological molecules such as:
DNA, RNA, proteins.
Acids, Bases and pH
• Acid: a molecule that behaves as a ____________
____________
• Strong base: a molecule that behaves as a
____________ ____________
Acid Strength
• One can derive a numerical value for the strength of an
acid (amount of hydrogen ion released when a given
amount of acid is dissolved in water).
• Describe by Ka:
• Written correctly,
Ionization of H2O and pH
• Lets quantitatively examine the dissociation of water:
• Molar concentration of water (55M)
• Kw is called the ion product constant for water.
• Must define a quantity to express hydrogen ion concentrations…pH
Henderson-Hasselbalch
• Equation to connect Ka to pH of solution containing
both acid and base.
• We can calculate the ratio of
weak acid, HA, to its conjugate
base, A-, in the following way
Henderson-Hasselbalch (Cont’d)
• Henderson-Hasselbalch equation
pH =
pK a + l og [Conjugate base ]
[We ak aci d]
• From this equation, we see that
• when the concentrations of weak acid and its
conjugate base are equal, the pH of the solution
equals the pKa of the weak acid
• when pH < pKa, the weak acid predominates
• when pH > pKa, the conjugate base predominates
Titration Curves
•
•
________________ : an experiment in which measured amounts of acid (or base)
are added to measured amounts of base (or acid)
_______________________ : the point in an acid-base titration at which enough
acid has been added to exactly neutralize the base (or vice versa)
• a monoprotic acid releases one H+ per mole
• a diprotic acid releases two H+ per mole
• a triprotic acid releases three H+ per mole
Buffers
• buffer: a solution whose ________ resists change
upon addition of either more acid or more base
• consists of a weak acid and its conjugate base
• Examples of acid-base buffers are solutions
containing
• CH3COOH and CH3COONa
• H2CO3 and NaHCO3
• NaH2PO4 and Na2HPO4
Buffer Range
• A buffer is effective in a range of about +/- 1 pH unit
of the pKa of the weak acid
Buffer Capacity
• Buffer capacity is related to the ____________ of
the weak acid and its conjugate base
• the greater the concentration of the weak acid and its
conjugate base, the greater the buffer capacity
Naturally Occurring Buffers
• ____________ is the principal buffer in cells
• ____________ is an important (but not the only)
buffer in blood
• ____________ can result in increased blood pH
• ____________ can result in decreased blood pH
(Biochemical Connections p. 60)
Selecting a Buffer
• The following are typical criteria
• suitable pKa
• no interference with the reaction or detection of the
assay
• suitable ionic strength
• suitable solubility
• its non-biological nature
Laboratory Buffers