371 - Chapter 2 Outline.pdf

371 - Chapter 2 Outline.pdf - CHEM 371 Chapter 2 Water C...

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CHEM 371 Chapter 2: Water C. Mouzakitis Weak Interactions in Aqueous Systems Water has higher boiling point, melting point, heat of vaporization, and surface tension than other common solvents. Heat of vaporization of chloroform is 247 J/g, and water’s is 2260 J/g. Outer electron orbitals of the oxygen atom ( sp 3 ) give the H—O—H a 104.5 ° bond angle . Oxygen is more electronegative: the electrons are more often in the vicinity of the O atom. Two dipoles : each H bears a partial (+) charge and the O bears a partial (-) charge. Hydrogen bond: electrostatic attraction between O atom of one molecule and H of another. This provides the cohesion to make water a liquid at room temperature. Hydrogen bonds are longer and weaker than covalent or ionic bonds but stronger than van der Waals interactions . This is shown in the bond dissociation energies : 470 kJ/mol for O— H covalent, 23 kJ/mol for hydrogen bond, and 0.1-0.5 kJ/mol for van der Waals interactions. Hydrogen bonds between water molecules In ice , each water molecule can form 4 hydrogen bonds with neighboring water molecules. The lifetime of each bond is about 10 microseconds . In liquid , each water molecule can only form ~3.4 hydrogen bonds with neighboring water molecules. The lifetime of each bond is about 10 picoseconds . The crystal lattice structure of ice makes it less dense than liquid water, and thus ice floats on liquid water. H 2 O(s) ! H 2 O(liq) Δ H = +5.9 kJ/mol H 2 O (liq) ! H 2 O(g) Δ H = +44.0 kJ/mol Melting or evaporation greatly increases the entropy of the system. The shift toward randomness outweighs the hydrogen bonding, making Δ G negative, and the melting or evaporating spontaneous processes at room temperature. Hydrogen bonds between water and polar substances – page 50 of Lehninger’s Biochemistry 6 th ed. Hydrogen bonds can form between an electronegative atom (hydrogen acceptor like oxygen or nitrogen with a lone pair of electrons) and a hydrogen bound to another electronegative atom (hydrogen donor). Alcohols, aldehydes, ketones, sugars, and compounds containing N— H bonds can form H bonds with water molecules, and tend to be soluble in water. As shown, hydrogen bonds are directional : the attraction is strongest when the three atoms involved in the bond are linear . Solubility of Charged Crystalline Solutes Polar molecules are hydrophilic and can dissolve in water because they replace the water- water interactions with more energetically favorable water-solute interactions. Benzene and chloroform are nonpolar solvents, thus able to dissolve hydrophobic molecules like lipids and waxes. Water dissolves many crystalline salts (i.e. NaCl) by hydrating their component ions. Water surrounds the ions, partially neutralizing the charge and therefore weakening the electrostatic forces that once held the lattice together.
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