lecture_2 - Water, pH, Ionic Equilibria, and the...

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Water, pH, Ionic Equilibria, and the Hydrophobic Effect • Water is the medium of life. • Organisms are typically constituted of 70- 90% water. • Water and its ionization products, hydrogen ions (protons) and hydroxide ions, are critical determinants of the structure and function of proteins, nucleic acids, and membranes.
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Hydrogen Bonding in Water Water can serve as both an H donor and an H acceptor in H-bond formation It is the potential to form four H-bonds per water molecule that endows it with the anomalously high boiling point, melting point, heat of vaporization, and surface tension . Hydrogen bonding in water is cooperative. Hydrogen bonds between neighboring molecules are weak (23 kJ/mole) relative to the H–O covalent bonds (420 kJ/mol) O H H O H H O H H O H H O H H
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Structure of Water Water has two hydrogen atoms covalently linked to a central oxygen atom. Two lone pairs of electrons occupy other two sp 3 orbitals. Structure is a distorted tetrahedron. This geometry, and the electronegativity of the oxygen atom induces a net dipole moment. Normal tetrahedral bond angle (as in methane) is 109.5° Because of the dipole moment, water can serve as both a hydrogen bond donor and acceptor. What structure would you expect for CO 2 ? Would it have a dipole moment? Is it polar? Can it hydrogen bond?
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Solvent Properties of Water Because water is highly polar, it is an ideal solvent for ionic substances, or polar substances such as sugars, small alcohols, amines, and carbonyl-containing molecules such as aldehydes and ketones. Water dissolves salts because of strong electrostatic interactions. The attraction between water molecules interacting with ions is much greater than that of the ion with its oppositely charged partner. The diminished attraction is a measure of water’s dielectric constant.
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Hydrophobic Interactions Nonpolar substances do not readily hydrogen bond to water, and have a highly reduced solubility. Water forms a clathrate or hydration shell around nonpolar substances. Must reorganize. In this hydration shell, the water molecules are very ordered, which makes this an entropically unfavorable condition. Less costly to have one big hydration shell
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lecture_2 - Water, pH, Ionic Equilibria, and the...

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