CHEM 437 Chp 2

What makes water so special for life?

• Most biochemical reactions occur in aqueous medium.

• It is an excellent solvent for ionic and polar substances.

  • Because of its high dielectric constant.

• It is a poor solvent for nonpolar substances.

  • Because hydrophobic interactions, lipids coalesce, membranes form, and the cellular nature of life is established.

• High heat capacity allows effective temperature regulation in living things.

What are the critical roles of water in life?

• Membrane formation

• Folding of biomacromolecules

• Enzyme mechanism

What are some of the unusual physical properties of water?

• Two H atoms covalently bonded to a central O atom.

• Non-tetrahedral bond angles, with a bent shape.

• Forms a dipole due to the bent shape.

• High boiling and melting points, heat of vaporization, and surface tension.

Explain hydrogen bonding in water.

• Can serve as both a hydrogen bond donor an an acceptor.

• Forms four hydrogen bonds (two donors and two acceptors) per molecule.

• Hydrogen bonds are cooperative.

  • Promotes formation of other H bonds.

Explain hydrogen-bonding network in liquid water.

• Constant reorganization hydrogen bonding networks.

• The network is disordered.

• In liquid water, a water molecule on average, forms 2-3 hydrogen bonds.

Explain hydrogen-bonding in ice.

• The lifetime of a hydrogen bond in ice is longer than H2O.

• In ice, a water molecule forms 4 hydrogen bonds.

• The atoms lie in a direct line.

• The hydrogen bonds keep the water molecules apart, forming an open lattice.

  • This is why the density of ice is less than that of liquid water.

Explain why water is a great solvent for polar solutes.

• High dielectric constant

• Highly polar solvent

• Excellent solvent for salts and polar compounds

Dielectric Constant

Measures a material’s ability to store electrical energy. It is the reduction in energy of mutual attraction between two charged species in a vacuum relative to what it is in the solvent.

How does water interact with polar substances?

• Readily forms hydrogen bonds with polar solutes.

• Polar molecules dissolve easily.

• Ions are always hydrated in water, and carry around in a “hydration shell.”

  • Ions allow water molecules to become more ordered as the water molecules form hydration shells around the ions.

Why is water a poor solvent for non-polar solutes?

• Water cannot form hydrogen bonds with them.

• They instead for a cage like structure.

• The hydrogen bond network is reorganized to accommodate the nonpolar solute.

• The nonpolar solutes allow water molecules to become more ordered, and restrict the orientations of the water molecules that surround them.

Amphiphilic/Amphipathic

Molecules that interact favorable with both polar and nonpolar environments because they contain both polar and nonpolar groups.

What are the ionization properties of water?

• Water ionizes because of the highly electronegative oxygen atom

• The free protons are immediately hydrated to form hydronium ions, which form a hydration shell

Equilibrium Constant for the Ionization of Water

Keq= [H+][OH-]/[H2O]

pH Range

• 0-6, acid

• 7 neutral

• 8-14, base

Dissociation of Strong Electrolytes

Acids: HA → H+ + A-

Ka= [H+][A-]/[HA]

Bases: BOH → B+ + OH-

Ka= [B+][OH-]/[BOH]

pH Equation from Concentration of H+ Ion

pH= -log [H+]

[H+]= 10^-pH

pOH Equation from Concentration of OH- Ion

pOH= -log [OH-]

[OH-]= 10^-pOH

Ion Product Constant of Water Equation

Kw= [H+][OH-]

Kw= 1.0 × 10^-14

pKw= pH + pOH = 14

Dissociation of Weak Electrolytes

Acids: HA ←> H+ + A-

Ka= X²/[HA]

X= √(Ka [HA]) = [H+]

pH= -log10 [H+]

Bases: BOH <→ B+ + OH-

Kb= X²/[BOH]

X= √(Kb [BOH]) = [OH-]

pOH= -log10 [OH-]

Buffers

• Solutions that resist pH changes when you add acids or bases.

• Composed of an equimolar mixture of a weak acid and its conjugate base.

• pKa close to the desired pH.

Why are buffers important for living systems?

• Maintaining pH is vital to cells

• The structure and function of proteins, DNA, RNA, etc. depend on non-covalent bonds that pH affects

• There are several buffer systems in cells that help to maintain pH

  • Intracellular, phosphate and histidine system

  • Extracellular, bicarbonate-carbonic acid system

Henderson-Hasselbalch Equation

pH= pKa + log10 [A-]/[HA]

• pH < pKa, then [HA] > [A-]

• pH > pKa, then [HA] < [A-]