University:
Boston University
Course:
CAS CH 102 | General Chemistry 2
Academic year:

2021
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458

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18
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alliance3731g4

WATER Properties of Water 1) Water has a high boiling point and relatively low melting point; its range in the liquid state is large 2) The heat of vaporization is high (more heat to change to vapor state than to raise temperature of the liquid state) 3) Liquid is more dense than the solid (e.g., most metals and other substances the solid is more dense) 4) High viscosity relative to its molecular weight (MW) 5) High surface tension Properties of Water Properties of Water WATER The 4 S’s for Water: Shape Size Solubility Stability Most of the properties of water are explained or due to its shape, in other words, its structure Shape of Water • sp3 hybridized orbitals: 2 are bonding 2 are filled non-bonding • Distorted tetrahedral (equal angles of 109.5°): due to repulsion of electrons in filled orbitals .5° 4 10 Shape of Water Permanent DIPOLE Very compact structure: 2.0 Å in diameter though van der Waals diameter of oxygen atom is 2.8 Å The O–H bond is relatively short (0.96 Å) though van der Waals diameter of hydrogen atom is 2.4 Å What are these discrepancies due to? ELECTRONEGATIVITY OF OXYGEN 2.0 Å Size of Water Solubility of Water: Hydrogen Bonding These dipoles interact with each other in a special kind of non-covalent bond: THE HYDROGEN BOND. Bit of a misnomer but named as such because it appears though two oxygen atoms of two water molecules share a hydrogen Its not at all equal: the water with the covalent bond to hydrogen is called the DONOR. The opposite water molecule is called the ACCEPTOR DONOR ACCEPTOR Solubility of Water: Hydrogen Bonding d+ THREE important aspects that make H-bonds unique: d– d– 1) Electrostatic: dipoledipole interaction d+ 2) Partial electron sharing: partial covalency 3) Directional: geometry 0.96 Å 1.84 Å … . 2.8 Å This is a stronger bond than this (due to optimizing the covalency) Rings of Water “Flickering Clusters” Molecules of Water Molecules On average, liquid water at 25 °C has about 3.5 hydrogen bonds per molecule Structure of Ice Properties of Water 1) Water has a high boiling point and relatively low melting point; its range in the liquid state is large 2) The heat of vaporization is high • Ave number of H-bonds in liquid water is ~3.5, but in steam ~0 3) Liquid is more dense than the solid • Ave number of H-bonds in liquid water is ~3.5, but in ice its 4, but the geometry takes over 4) High viscosity relative to its molecular weight (MW) • liquid water with ~3.5 H-bonds/molecule is sticky 5) High surface tension 6) Water is also a great SOLVENT Solvation of Ions Cations Anions Hydrogen Bonding to Polar Molecules Water to by Water ketones/aldehydes Water to alcohols Water to carboxylic acids Water to amines Typical Bond Energies (kcal · mol-1) } ~95 4.7 Molecular Forces Important for Life The 4 NON-covalent bonds: H-bonds Salt bridges van der Waal’s interactions Hydrophobic effect } Electrostatic Electrostatic Interactions (AKA Salt Bridges) Charge – Charge interactions Na+ The strength of all these is governed by Coulomb’s Law: Cl– Force between two charged particles = k q1q2 D r2 r r Where q is the charge on each particle 1 and 2, D is the dielectric constant (unit-less; =1.0 in vacuum, ~80 in water), r is the distance between the two charges, and k is Coulomb’s constant (9x109 J·m/C2, where C=1.6x10-19 electron charges) 3 r5 Induced dipole – Induced dipole interactions (AKA Van der Waals interactions) Recalling the relationship between Force and Energy (E = Force x distance (r)): E = k q1q2 Dr r6 For partial charges, this distance dependence is exponential Molecular Forces Important for Life The 4 NON-covalent bonds: ✓ H-bonds ✓ Salt bridges van der Waal’s interactions Hydrophobic effect } Electrostatic Electrostatic Interactions (AKA Salt Bridges) Charge – Charge interactions Na+ The strength of all these is governed by Coulomb’s Law: Cl– Force between two charged particles = k q1q2 D r2 r r Where q is the charge on each particle 1 and 2, D is the dielectric constant (unit-less; =1.0 in vacuum, ~80 in water), r is the distance between the two charges, and k is Coulomb’s constant (9x109 J·m/C2, where C=1.6x10-19 electron charges) 3 r5 Induced dipole – Induced dipole interactions (AKA Van der Waals interactions) Recalling the relationship between Force and Energy (E = Force x distance (r)): E = k q1q2 Dr r6 E = k q1q2 D r6 For partial charges, this distance dependence is exponential van der Waals Interactions Induced dipole – Induced dipole interactions (AKA Van der Waals interactions) E = k q1q2 D r6 But, as the energy is so dependent on distance, even when at r0, its VERY weak. 10 Å / 1 nm van der Waals Interactions van der Waals Interactions This is one example highlighting the issue of COMPLEMENTARITY, and why this is so important in biology Typical Bond Energies (kcal · mol-1) } @ ~3 Å 95 20 4.7 Van der Waals interactions 0.07 Hydrophobic interactions Molecular Forces Important for Life The 4 NON-covalent bonds: ✓ H-bonds ✓ Salt bridges ✓ van der Waal’s interactions Hydrophobic effect } Electrostatic Hydrophobic Interactions Hydrophobic interactions: atoms “attracted” together due to water structure. Nonpolar Solutes Aggregate in Water Hydrophobic Interactions Transfer of Hydrocarbons to Nonpolar Solvents is Entropically Driven Hydrophobic Interactions Structure: Nonpolar Solutes in Water Like ICE Hydrophobic Interactions Figure 2.12 Hydrophilic and Hydrophobic Hydrophobic Interactions Figure 2.12 Hydrophilic and Hydrophobic x5 Hydrophobic Interactions 3 “degrees” of freedom (DoF) Statistical mechanics has determined that each DoF is equal to 5-6 kcal/mol at 25 °C 6 “degrees” of freedom Hydrophobic Interactions For EXAMPLE 3 [CH4·10H2O] [(CH4)3 · 12H2O] + 18H2O DG DG?is definitely NEGATIVE: How? Enthalpy Enthalpy 30 H2O x 4 H-bonds each x 5 kcal/mol = 600 kcal/mol heat release to make H-bonds 18 H2O x 3.5 H-bonds each x 5 kcal/mol = 315 kcal/mol, plus 12x4x5 = 240 kcal/mol; Total = 555 kcal/mol heat released to make bonds Entropy DH = –555 – (–600) = +45 Entropy 3 degrees of freedom (DoF) x 6 kcal/mol at 25 °C per DoF = 18 kcal/mol TDS=114-18 TDS = +96 kcal/mol in entropic energy gained 19 degrees of =96 freedom (DoF) x 6 kcal/mol at 25 °C per DoF = 114 kcal/mol DG = DH – TDS = +45 – 96 = –51 kcal/mol ENTROPY DRIVEN DH = +45 kcal/mol added in excess to break Hbonds Bond Energies of Important Noncovalent Interactions (kcal · mol-1) } 95 20 @ ~3 Å 4.7 Van der Waals interactions 0.07 Hydrophobic interactions (per –CH2–) 26 6 20 years ago TOMORROW (9/11) at 8:46 AM EDT, the first tower was hit at 9:03 AM, the second tower was hit By Anthony Quintano - https://www.flickr.com/photos/quintanomedia/15071865580, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=38538291

Properties of Water Lecture

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