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Lecture 3

CHEM 237 Lecture 3: 3-4_Water
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Department
Chemistry
Course
CHEM 237
Professor
Dara Gilbert
Semester
Spring

Description
Properties of water Polarity: Oxygens electronegativity creates a dipole (1.8D) Hydrogen bonds: an H bond network can form, allowing 23 bonds per molecule allowing solvency of solutes in water Ice: 4 H bonds per molecule due to reduced entropy, tetrahedral arrangement, is less dense than water Max density at 4*C in solid form which is important because otherwise oceans would freeze from the bottom up Physiology Water is a good solvent for AAs, small alcohols, and carbohydrates but a bad solvent for nonpolar gases, aliphatic chains, and aromatic moieties Excitable cells need a membrane potential = different [ion] inside and outside the cell membrane through proteins which only happens in a polar solvent that hydrates ions (the gradients can travel across axons and trigger nerve impulsessynapses) 5575 is water, participates in reactions and keeps ions in solution (ion gradients in nerve impulses and muscle contractions) Bodily fluids = 0.9 NaCl by mass Phosphate buffered saline: Isotonic to prevent RBC collapsingbursting = 10nM PO4, 137mM NaCl 2.7nM KCl (sodium potassium pumps for nerve cells and heart rate), 7.4 pH Cytoplasm = highly concentrated solution with high osmotic pressure Functions between 7.27.6 pH Acidosis: Blood pH drops and breathing rate increases to replace CO2 with oxygen Alkalosis: Blood pH increases and breathing rate slows to increase CO2 in the blood Hyperventilation: Due to alkalosisincreased breathing rate so the paper bag is used to rebreathe CO2 Fish are cold blooded since they cant take enough oxygen from the water to maintain a metabolism that has a higher body temperature than the environment (therefore bigger sea creatures are warm bloodedmammals since they breath oxygen above water which dissolves better in air than in water) The hydrophobic effect is responsible for protein folding, proteinprotein association, lipid micelles, cell membranes, steroid hormones binding to their receptors Binding sites in enzymesreceptors are hydrophobic to allow hydrophobic substratesligands to bind (water molecules cover the substrate andor enzyme) Properties of solutions Solute: Whats dissolved in solution Solvent: That in which the solute is dissolved Solution: The 2 together, where like dissolves like (similar interactionscharges) When water takes apart an ionic crystal, the hydration shell forms around it with O at positive ions and H at negative ions Water dissolves salts because of 3 interactions: 1: High dielectric constant: Which reduces attraction of opposite ions in salt no attraction at large distances (>40nM) 2: Electrostatic (iondipole) interactions: Where energy is reduced between solvated ions and water molecules increasing solubility 3: Covalent character: Partial covalent bond characteristics due to the H bond donors and electron pair acceptors (accept 2, donate 2) Coulombs law review This describes the force between 2 electric charges Water weakens the electrostatic strength between charges (At the same distance as in any other medium) due to its high dielectric constant due to its structure and the existence of dipoles Hydrophobia Hydrophobic effect: Folding of nonpolar molecules in aqueous solution due to repulsion (not due to attraction of nonpolar molecules) Nonpolar solutes cause a decrease in enthalpy water forms a hydration shell (as many H bonds as possible excluding the nonpolar solute), and a decrease in entropy solute forms an ordered cage To decrease the amount of water molecules forming a hydration cage around the nonpolar solute reduce its surface area (by clumping the nonpolar solute) to slow entropy reduction Bulk water = little order, high entropy but water near a hydrophobic molecule = high order, low entropy therefore low solubility Micelles: Tails point to inside, heads point to water to stabilize entropy (lower hydrocarbon concentration) Lipid bilayer: Larger aggregates with the tails pointing to each other in 2 layers (higher hydrocarbon concentration) Water interacts with the polar part of amphipathic molecules (e.g.: the polar head .vs. alkyl tail of fatty acids) Hydrophobic effect where waters entropy increases when the nonpolar solutes aggregate
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