Water: Structure and Dissociation LEARNING OBJECTIVES Describe structure of water Justify that water is an universal solvent. Justify that water forms the medium of cytosol, ECM and blood. Discuss the dissociation of water and its contribution in pH. Discuss pH with reference to Henderson Hesselbalch Equation. PROPERTIES OF WATER Polar molecule Cohesion and adhesion High specific heat Density – greatest at 4oC Universal solvent of life WATER MOLECULE • • One oxygen atom Two hydrogen atoms Liquid water Tetrahedral Geometry High Surface Tension Specific gravity Boiling point Universal Solvent Solid water Polarity of Water Two Hydrogen atoms One Oxygen atom Oxygen is more electronegative, the region around oxygen has a partial negative charge The region near the two hydrogen atoms has a partial positive charge. A water molecule is a polar molecule with opposite ends of the molecule with opposite charges. Attractions between these polar molecules. The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond. Each water molecule can form hydrogen bonds with up to four neighbors. HYDROGEN BONDS Hold water molecules together Each water molecule can form a maximum of 4 hydrogen bonds The hydrogen bonds joining water molecules are weak, about 1/20 th as strong as covalent bonds. They form, break, and reform with great frequency Extraordinary Properties that are a result of hydrogen bonds. Cohesive behavior Resists changes in temperature High heat of vaporization Expands when it freezes Versatile solvent UNIVERSAL SOLVENT Solution o Solute o Solvent Aqueous solution Hydrophilic o Ionic compounds dissolve in water o Polar molecules (generally) are water soluble Hydrophobic o Nonpolar compounds Distribution of Water within Human Body Most biochemical reactions involve solutes dissolved in water. Most biochemical reactions involve solutes dissolved in water There are two important quantitative proprieties of aqueous solutions. o Concentration o pH DISSOCIATION OF WATER MOLECULES The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H+). The water molecule that has lost a proton is now a hydroxide ion (OH-). The water molecule with the extra proton is a hydrogen/hydronium ion (H+/H3O+). Water Molecule Dissociates Water Molecule Dissociates cont…. 1 mole of water= 18 G 1 Liter= 1000G = ? mole 18 Gram = 1 mole 1000 Gram = 1000/18 = 55.56 mole Molar conc. Of H2O = 1.8 × 109 × 55.56 = Probability × [H2O] = 1.0 × 10-7 Dissociation of Water Molecules Water molecule dissociates into a hydrogen ion and a hydroxide ion: H2O <=> H+ + OH- This reaction is reversible. At equilibrium the concentration of water molecules greatly exceeds that of H+ and OH-. In pure water only one water molecule in every 554 million is dissociated. At equilibrium, the concentration of H+ or OH- is 10-7M (25°C) . Water Molecule Dissociates cont…. pH SCALE The pH scale in any aqueous solution : [ H+ ] [OH-] = 10-14 Measures the degree of acidity (0 – 14) Most biologic fluids are in the pH range from 6 – 8 Each pH unit represents a tenfold difference (scale is logarithmic) A small change in pH actually indicates a substantial change in H+ and OHconcentrations. ACIDS AND BASES An acid is a substance that increases the hydrogen ion concentration in a solution. Any substance that reduces the hydrogen ion concentration in a solution is a base. Some bases reduce H+ directly by accepting hydrogen ions. Strong acids and bases completely dissociate in water. Weak acids and bases dissociate only partially and reversibly. Problem REFRENCES Harper chapter 1 Thank you
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