L1_2-Water_Acids_Bases_and_Buffers

L1_2-Water_Acids_Bases_and_Buffers - Part I: Aqueous...

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1 Part I: Aqueous Solutions Water Lectures 1 and 2: August 26 and 29, 2011 Introduction Water is the most abundant substance in living systems Makes up >70% or more of the weight of most organisms The first living things evolved in water and shaped our dependence on it Water is central to biochemistry because: Nearly all biological molecule assume their shapes in response to the physical and chemical properties It is the major medium for the majority of biochemical reactions In frequently participates in many chemical reactions that support life The oxidation of water to produce molecular oxygen is fundamental to photosynthesis and converts the energy of the sun to a usable form The combination of solvent properties responsible for intramolecular and intermolecular association of the substances is peculiar to water: it’s hard to get the same or similar results from other solvents Wherever water is found on earth, there is life. Physical Properties of Water Composed of one oxygen and two hydrogens Bent geometry Not linear like other tri-atomic molecules such as carbon dioxide O-H bond distance of 0.958 Å Bond angle of 104.5° Not 109.5° like a perfect tetrahedron This is due to the crowding of the area by the electrons of oxygen The oxygen is more electronegative and causes unequal sharing of the bonded electrons Large electronegativity difference between H and O results in a 33% ionic character on the O-H bond The oxygen has a - charge of –0.66e (where e is the charge of an electron) because of the unshared electrons and the unequal sharing of the bonding electrons The hydrogens have a + charge of +0.33e Highly polar As a Solvent Solubility depends on the ability of a solvent to interact with a solute more strongly than solute particles interact with each other Polar characteristic of water makes it excellent solvent for polar and ionic materials such materials are hydrophilic
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2 Nonpolar substances are virtually insoluble in water such materials are hydrophobic nonpolar substances dissolve in nonpolar solvents Salts are held together by ionic forces and interact according to Coulomb’s law which implies that as the dielectric constant of a medium increases, the force between its embedded charges decreases. F = Q 1 Q 2 ε r 2 F = Force of the interaction ε = dielectric constant r = distance Q 1 , and Q 2 are the magnitudes of charges on the attracted species The dielectric constant of water is among the highest of any pure liquid whereas those of non-polar substances such as hydrocarbons are relatively small. The dielectric constant is a measure of the solvent’s ability to keep opposite charges apart.
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L1_2-Water_Acids_Bases_and_Buffers - Part I: Aqueous...

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