Lecture 1 Text Notes water

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University of Toronto St. George
Stavroula Andreopoulos

Lecture 1 Text Notes – Water Properties of Water - high boiling point, melting point, heat of vaporization, surface tension - intermolecular forces between water molecules are high - high internal cohesion - high dielectric constant - maximum density is in liquid state - negative volume of melting - two hydrogen atoms are covalently linked to oxygen, each sharing an electron pair in a bent configuration - bent structure influences its properties – creates a dipole that renders water polar - hydrogen bond formation is easier with bent shape - serves as both a donor and acceptor in hydrogen bonding and can potentially form four hydrogen bonds per water molecule - in ice, water molecules are hydrogen bonded to four other water molecules – hydrogen donates a hydrogen bond to the oxygen acceptor – tetrahedral symmetry - mutual reinforcement of hydrogen bonding – hydrogen bonded water molecule acceptor is a better donor than an unbonded water molecule - hydrogen bonds (23 kJ/mol) are weak compared to H-O covalent bonds (420 kJ/mol) – always clear which are chemically bonded and which are hydrogen bonded Structure of Ice - hydrogen bonds form space-filling, three dimensional network - bonds are directional and straight - hydrogen atom lies on a direct line between two oxygen atoms – linearity and directionality means that the hydrogen bonds on ice are strong - directionality leads to open lattice structure – density of ice only 57% of expected – hydrogen bonds of ice hold water molecules apart Molecular Interactions - hydrogen bonds in water last ~9.3 seconds - water molecules can switch hydrogen bond allegiances due to strained or broken hydrogen bonds – attribute of this is fluidity Solvent Properties - high polarity makes it an excellent solvent for ionic substances (salts), non-ionic but polar substances (sugars, simple alcohols, amines), and carbonyl-containing molecules (aldehydes, ketones) - strong electrostatic attractions between positive and negative ions in the crystal lattice od a salt – dissolve readily in water and form hydration shells surrounding the ions - hydration shells – dynamic and stable structures – each water molecule in the inner hydration shell around a Na ion is replaces about 2-4 nsec by another water molecule
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