Chapter2 - CIVL1005 - Environmental Engineering...

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1 CIVL1005 - Environmental Engineering Environmental Chemistry Environmental Chemistry Office: Rm. 6-30, Haking Wong Building Phone: 2859-1973 E-mail: kshih@hku.hk Dr. Kaimin Shih DEPARTMENT OF CIVIL ENGINEERING THE UNIVERSITY OF HONG KONG • Stoichiometry • Enthalpy in Chemical Systems • Chemical Equilibria • Organic Chemistry • Nuclear Chemistry - OUTLINES -
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2 Stoichiometry A chemical reaction provides both qualitative ( what are those elements participating the reaction? ) and quantitative ( how many of them? ) information. For example, burning methane: Unbalanced (qualitatively only): CH 4 + O 2 CO 2 + H 2 O Balanced (both): CH 4 + 2 O 2 CO 2 + 2 H 2 O Atomic weight: C = 12 amu (atomic mass units); O = 16 amu Molecule weight (MW): CH 4 = 12 + 4 1 = 16 amu One mole (“mol”; 6.02 10 23 molecules ) of CH 4 is 16 g Molarity ( M ) is a concentration unit: 1 mole of molecules in 1 liter solution 1 molar ( M ) of NaCl (aq) (MW= 23 + 35.5) is 58.5 g/L 58.5 mg/g
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3 The expression of methane oxidation can be: CH 4 + 2 O 2 CO 2 + 2 H 2 O 1 molecule of methane 2 molecules of oxygen 1 molecule of carbon dioxide 2 molecules of water 1 mole of methane 2 mole of oxygen 1 mole of carbon dioxide 2 mole of water 16 g of methane 64 g of oxygen 44 g of carbon dioxide 36 g of water Reactants 16 + 64 = 80 g Products 44 + 36 = 80 g The expression of glucose oxidized can be: C 6 H 12 O 6 + 6 O 2 6 CO 2 + 6 H 2 O 192 g 264 g 108 g Theoretical Oxygen Demand (OD): Need 192 / 180 = 1.07g of O 2 for 1 g of glucose If the reaction is carried out by bacteria (using glucose for food), the amount of oxygen demand is biochemical oxygen demand ( BOD ) 180 g Theoretical Oxygen Demand BOD ( Why? ) Some carbon will be used for bacteria cells
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4 Enthalpy in Chemical Systems U 1 + Q = U 2 + W U 1 = internal energy at the beginning U 2 = internal energy at the end Q = heat absorbed during the reaction W = work done by the system during reaction U 1 + Q = U 2 + P(V 2 –V 1 ) (U 2 + PV 2 ) – (U 1 + PV 1 ) = Q H 2 –H 1 = H = Q H is “ heat of reaction H (Q) 0 , endothermic H (Q) 0 , exothermic The First Law of Thermodynamics Conservation of energy:
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5 Standard enthalpy = H 0 298 = enthalpy at 1 atm & 298 K (kJ/mol) The reference condition for pure elements is the stable state of the substance at 1 atm & 25 o C . For example, the stable state of oxygen at 1 atm & 25 o C is gaseous O 2 , so H 0 298 for O 2 is defined to be 0. But we are usually more interested at the change of enthalpy ( H): H 0 298 of products - H 0 298 of reactants = H Example: Modern Combustion CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (g) (-74.9) 2 (0) (-393.5) 2 ( -241.8 ) H = [(-393.5) + 2 (-241.8)] – [(-74.9) + 2 (0)] = - 802.2 kJ/mol of CH 4(g) Lower Heating Value ( LHV ) or net heat of combustion CH 4(g) + 2 O 2(g) CO 2(g) + 2 H 2 O (l) (-74.9) 2 (0) (-393.5) 2 ( -285.8 ) H = [(-393.5) + 2 (-285.8)] – [(-74.9) + 2 (0)] = - 890.2 kJ/mol of CH 4(g)
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This note was uploaded on 10/02/2010 for the course CIVIL ENGI 111 taught by Professor 222 during the Spring '10 term at SUNY Rockland.

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Chapter2 - CIVL1005 - Environmental Engineering...

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