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Unformatted text preview: Chapter 2 - Water ♦ Hydrogen bonds between water molecules provide the cohesive forces that make water a liquid at room temperature and favor the extreme ordering of molecules that is typical of crystalline water. ♦ Polar biomolecules dissolve in water because they can replace water-water interactions with more energetically favorable water-solute interactions. ♦ Non-polar biomolecules are not soluble in water and they tend to cluster together. ♦ Each hydrogen atom of a water molecule shares an electron pair with the central oxygen atom. The water molecule has a rough tetrahedron shape with a hydrogen atom at each of two corners and unshared electron pairs at the other two corners. The H-O-H bond angle is 104.5 and not 109.5 like a perfect tetrahedron because of crowding by the nonbonding orbitals of the oxygen atom. ♦ Oxygen is more electronegative than hydrogen, attracting electrons more strongly. ♦ Since there is unequal electron sharing, there is two electric dipoles in the water molecule, one along each of the H-O bonds, hydrogen having a partial positive charge and oxygen having a partial negative charge equal in magnitude to the sum of the two partial positives. • As a result there is a electrostatic attraction between the oxygen atom of one water molecule and the hydrogen of another called a hydrogen bond! ♦ Hydrogen bonds are relatively weak. ♦ Bond dissociation energy is the energy required to break a bond. ♦ The bond dissociation energy for a hydrogen bond in water is only 23kJ/mol compared with 470kJ/mol for the covalent O-H bond in water. ♦ ♦ ♦ ♦ ♦ ♦ • Hydrogen bonding in ice . In ice, each water molecule forms four hydrogen bonds, the maximum possible for a water molecule, creating a regular crystal lattice. By contrast, in liquid water at room temperature and atmospheric pressure, each water molecule hydrogen bonds with an average of 3.4 other water molecules. This crystal lattice structure makes ice less dense than liquid water, and thus ice floats on liquid water. ♦ Thermal energy is the kinetic energy of motion of the individual atoms and molecules. ♦ Breaking a sufficient proportion of hydrogen bonds to destabilize the crystal lattice of ice requires much thermal energy, which accounts for the relatively high melting point of water. ♦ ∆G is free energy change. • When ∆G is negative the reaction is favorable (i.e., no added energy needed). ♦ ∆G = ∆H - T ∆S ♦ ∆H = enthalpy (measure of bonds/interactions formed and broken in the system, negative when bonds/interactions are broken). ♦ T = temperature of system (Kelvin). ♦ ∆S = entropy (measure of the change in the randomness of the system, positive when randomness increases)....
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This note was uploaded on 05/31/2010 for the course BIOLOGICAL BIOL-103 taught by Professor Pokorski during the Spring '09 term at University of Michigan-Dearborn.
- Spring '09