water_and_aqueous_solutions

water_and_aqueous_solutions - Chemistry 237 Chapter 2 Water...

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Chemistry 237 Chapter 2 Water and Aqueous Solutions
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CHAPTER 2 Water and Aqueous Solutions – What kind of interactions occur between molecules – Why water is a good medium for life – Why nonpolar moieties aggregate in water – How dissolved molecules alter properties of water – How weak acids and bases behave in water – How buffers work and why we need them – How water participates in biochemical reactions Learning goals : to understand
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Water-Aqueous Medium Properties of pure water: polarity hydrogen bond formation solvent properties What is soluble and what is not and why intermolecular h bonds hydrophobic effect Colligative properties Acids, Bases, pH and titration curves Buffers
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Water, water, everywhere • 55-75% of the human body is water. • Participates in chemical reactions • Keeps ions in solution, nerve impulses and muscles contraction. • Drives the structure of biomolecules
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The electronegativity of the oxygen atom creates a dipole in the water molecule Compare electronegativity of the elements: oxygen is the second most electronegative element. (fluorine is the most electronegative at 4.0) The Water Molecule Figure 2-1 Structure of the water molecule. δ + δ + δ - The δ + and δ - symbols represent the (+) and (-) ends of the water molecule
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The Hydrogen Bond Compare the properties of water, NH 3 and CH 4 : why is water so different from NH 3 and CH 4 ? How can we explain the high m.p. and b.p. of water?
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A hydrogen bond is a long bond between the partially positive H of one water molecule and a non- bonding electron pair on the partially negative O in another water molecule O-H is the “donor” O is the “acceptor” Bond length ~ 1.8 Å Each H 2 O can make a total of 4 H-bonds H-bonds are strongest when they are colinear (180 ° ) Figure 2-2 Hydrogen bond between two water molecules.
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H-bonding network in liquid water y Figure 2-4 Theoretically predicted and spectroscopically confirmed structures of the water trimer, tetramer, and pentamer. The highly organized yet rapidly changing H-bonding network in water resembles the structure of ice 2-3 H bonds per water molecule in liquid
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3D Structure of Ice Structure of ice. Note the tetrahedral arrangement of water molecules in the ice crystal: each water molecule can have 4 H- bonds H-bonding is important in ice and liquid water 4 H bonds per water molecule Think entropy
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• Ice is less dense than water. • Water has a maximum density at 4 C. • Why is this important to us?
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Water as a Solvent • Water is a poor solvent for nonpolar substances – nonpolar gases – aromatic moieties – aliphatic chains • Water is a good solvent for charged and polar substances – amino acids and peptides – small alcohols – carbohydrates
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Water Dissolves Many Salts • High dielectric constant reduces attraction between oppositely charged ions in salt crystal, almost no attraction at large (> 40 nm) distance • Strong electrostatic interactions between the solvated ions and water molecules lowers the energy of the system • Entropy increases as ordered crystal lattice is dissolved
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water_and_aqueous_solutions - Chemistry 237 Chapter 2 Water...

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