Chapter 2 students - The Water Molecule bent geometry 0.96...

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104.5 o 0.96 Å bent geometry O-H bond is highly polarized (H: partially positively charged, O partially negatively charged) The Water Molecule 1 MCDB310 – Chapter 2: Water electrostatic attraction between O and H of neighboring water molecules highly directional intermolecular association: Hydrogen bond
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The Hydrogen Bond longer and much weaker than covalent bond (20 kJ/mol vs. e.g. C-C, 348 kJ/mol) 2 MCDB310 – Chapter 2: Water δ + δ - not restricted to water; can also form between other electronegative proton donors and acceptors (e.g. C=O···H-N)
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3 MCDB310 – Chapter 2: Water Biologically Important Hydrogen Bonds
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Quiz: What are the Hydrogen Acceptors and Donors in Aspartame? 4 MCDB310 – Chapter 2: Water
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5 MCDB310 – Chapter 2: Water Geometry of Hydrogen Bonds Hydrogen bonds are strongest when all three participants are arranged in a straight line
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Hydrogen Bonds in Ice each water molecule is tetrahedrally arranged and makes H-bonds with four other water molecules upon melting, ~15% of H-bonds are broken “open” ice structure causes water to expand upon freezing 6 MCDB310 – Chapter 2: Water
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Water as a Solvent 7 MCDB310 – Chapter 2: Water Hydrophilic (“water loving”) substances polar, charged substances (e.g. salts such as NaCl) polar, uncharged substances (e.g. sugars such as glucose) Hydrophobic (“water fearing”) non-polar substances (e.g. gases, oil) Very good for: Poor for:
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Charged Substances 8 MCDB310 – Chapter 2: Water held together by electrostatic interactions strength of electrostatic interaction depends on - number of charges - distance between charges - dielectric constant of solvent ε (measure of its ability NaCl Na + + Cl -
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Electrostatic Interactions 9 MCDB310 – Chapter 2: Water Coulomb’s law: F: force between two electrical charges q 1 and q 2 ε 0 : proportionality constant r: distance between charges : dielectric constant of solvent (water: 78.5 , benzene: 4.6) water weakens electrostatic interactions 2 0 2 1 r q q F r =
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Most Salts Readily Dissolve in Water 10 MCDB310 – Chapter 2: Water Explanation: Entropy of salt increases upon dissolution
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Dissolution of Polar, Uncharged Substances 11 MCDB310 – Chapter 2: Water Glucose water soluble because solute/solute H-bonds are replaced by water/solute H-bonds that are energetically more favorable (entropy!)
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Hydrophobic Compounds 12 MCDB310 – Chapter 2: Water non-polar gases (e.g. O 2 , CO 2 ) organic compounds with long aliphatic chains (fatty acids), phenyl groups etc. (oils, lipids, wax) Soluble only in non-polar solvents (e.g. benzene, hexane)
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13 MCDB310 – Chapter 2: Water
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Chapter 2 students - The Water Molecule bent geometry 0.96...

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