ES 106 Spring 2007 Week 2: April 9-13 Water Properties I. Covalent bonds A. Covalent bonds share electrons 1. Cl+Cl-->Cl2 2. octet rule—have eight electrons surrounding each element a. may lead to need for multiple bonds for some atom pairs b. stronger than single shared-pair bonds! 3. Naming covalent compounds a. Use prefixes for each element, determined by how many b. Mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca- B. Polarity of covalent bonds 1. identical nonmetals share electrons equally—non-polar: ‘dipole’ 2. different nonmetals a. one element holds electrons more closely b. polar molecule 3. electronegativity a. fundamental property of elements b. those further to right are more electronegative c. those further to right dominate the shared electrons II. Poly-atomic molecules A. Form several covalent bonds, predicted by the formula: 8-(group number) 1. draw the most electrically negative in the center of diagram 2. draw dot structure around it, determined by its group 3. put atoms it bonds with around it, where there are missing dots B. poly-atomic ions 1. made of several covalent bonds 2. charge does not cancel to zero, so lacking octet of electrons 3. involved in ionic bonding as a coherent group, not broken up C. formulas of poly-atomic molecules 1. subscripts attached to element only apply to that element 2. parentheses indicate poly-atomic ion a. subscripts attached to parentheses apply to all atoms in the group b. be sure to multiply element subscript by group subscript 3. coefficients in reaction formulas tell how many of that molecule are needed for reaction to proceed III. Shapes of molecules A. Depends upon how octet of electrons is obtained B. Draw electron-dot structure, showing bonds and unbonded pairs 1. count how many sets of bonds plus unbonded pairs 2. multiple bond is a single set! C. Shapes 1. linear—two sets of bonds 2. triangular—three sets 3. tetrahedral—four sets 4. bent, pyramidal—created with unbonded pairs a. count in initial shape b. erased out, and only actual bonds considered c. H2O, H2S, SCl2, SO2, O3D. Properties of substances depend upon shape and polarity of molecules 1. melting temperature 2. temperature of vaporization 3. bonding with other substances a. hydrogen bonds—where hydrogen is bonded to small atoms with large electronegativity b. hydrogen exhibits stronger attractive forces than would be expected by the strength of its dipole 4. fluid substances with same type of bonds are soluble to one another a. oil to gasoline b. alcohol to water IV. Water A. Bent molecule B. Covalent bonds C. Has polarity—acts as a ‘dipole’ 1. oxygen end is negative 2. hydrogen ‘end’ is positive D. dissolves ionic substances because dipole is favorable to the bond with its ionic mate E. Unusual properties 1. Depend on the polar nature of water molecule, and hydrogen bonding 2. Solid state less dense than liquid state 3. High specific heat (or heat capacity) 4. High heat of vaporization 5. Readily dissolves ionic substances F. Distribution 1. Covers 71% of Earth’s surface 2. 97%+ of water is salt—sea water 3. 2% of water is ice-cap 4. Less than 1% remains as fresh surface water V. Water cycle A. evaporates 1. 84% from sea surface 2. Evapotranspiration is a. transpiration is plants releasing moisture to atmosphere b. “Evapotranspiration” is combined effect 3. Becomes atmospheric moisture moved by winds B. condensation into clouds allows precipitation 1. 75% over sea, 25% over land 2. Concentrated in tropical and midlatitudes 3. Much falls as snowfall, a. ‘storage’ of solid water on land surfaces b. Glaciers hold over 2% of Earth’s water 1) Most of fresh water on land 2) If it melted, sea level would rise 75 meters+/- C. runoff, infiltration 1. back to the sea—about 1/3 of land precipitation runs off 2. (most of the other 2/3 of land precipitation is returned to atmosphere by evapotranspiration) 3. groundwater storage removes water from cycle for long timesVI. Natural water composition A. Rainwater contains impurities 1. dust from natural and man-made sources 2. gases dissolved from atmosphere a. nitrogen b. oxygen c. carbon dioxide d. nitric acid B. runoff and groundwater contain dissolved mineral material 1. Cations a. sodium b. potassium c. calcium d. magnesium e. iron 2. Anions a. bicarbonate b. chloride 3. Silt, etc. 4. Radon 5. hard water contains ionic compounds of calcium, magnesium and iron C. Water contamination 1. micro-organisms: typhoid, cholera, dysentery 2. sewage—adds organic material: bacterial decay depletes dissolved oxygen, killing fish a. aerobic decay—uses oxygen present b. anaerobic decay—after all oxygen used up 1) methane 2) sulfur→hydrogen sulfide 3) fertilizers add plant nutrients: lead to additional organic material to decay 3. industrial pollutants a. Fertilizers and detergents 1) Nitrates replace oxygen on hemoglobin—blue-baby syndrome 2) Algal bloom increases BOD b. petroleum and its derivatives 1) gasoline, diesel leak from ~10% of underground tanks 2) volatile organic compounds: a) solvents, cleaners—carcinogenic b) persistent because of lack of reactivity c) benzene, toluene, carbon tetrachloride c. acids—directly, and acid rain 1) acid mine drainage from sulfur compounds in ore minerals 2) air pollutants cause rainwater to become acidic a) sulfur from eastern coals burned in power plants b) nitrogen from auto and industrial emissions 3) neutralized naturally by limestone, not by granite d. radio-isotopes, pesticides, toxic metal, chemicals 1) water requirement to produce steel, paper, plastic 2) cleaning water to be released to streams major expense 4. measured in parts per million, and parts per billion a. some are dangerous at minute levels b. some are over-sensationalized in media presentationHeat and Energy VII. Heat A. Moves from substance that has more into substance that has less 1. heat is the motion of the energy from one substance to the other 2. molecules have energy of motion, not heat 3. Cold is not a quantity, it is the absence of heat a. Temperature—related to the random molecular motion of the substance b. Heat transferred from substance with higher temperature to one with lower temperature c. As heat is transferred, molecular energy changes 1) May not change temperature if a phase change is occurring 2)
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