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sect4 - ABE 201 Biological Thermodynamics 1 Calculating...

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11/15/2011 1 ABE 201 Biological Thermodynamics 1 Section 4 Calculating Heats of Reaction ABE 201 Heats of Reaction Definition: Enthalpy change where stoichiometic quantities of reactants react completely in a single reaction to form products at same T & P Heat of Reaction Ĥ r (T,P) Dependent upon temperature and pressure At low pressures (near 1atm), ideal gas law is true, thus Ĥ r (T) and P can be ignored Heats of Reaction Heats of Reaction may be measured directly Calorimetry Heats of Reaction may be calculated Hess’s Law
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11/15/2011 2 Hess’s Law If 1 reaction can be obtained by algebraic operations (+, -, x, ÷ ) of two or more reactions, Ĥ r for reaction 1 can be obtained through the same algebraic operations on the Ĥ r for the two or more reactions. Hess’s Law - Example kJ/mol 1000 - H ˆ 2C, B 2A r = + kJ/mol -2000 H ˆ 3E, C D A r = + + ??? H ˆ 2D, 6E B r = + Heats of Formation Standard Heat of Formation Ĥ ° f (T,P) The heat of reaction (enthalpy) to form a compound from its elemental constituents By definition, elements (as found in nature) have a Ĥ f = 0 As found in nature: O 2 , N 2 , H 2 , C, Fe, etc. Standard Heats of Formation Ĥ ° f (25 C, 1atm) are tabulated in back of text (table B.1) Using Standard Heats of Formation What is the heat of reaction for the combustion of ethanol (assume all reactants and products are gaseous)? ??? H ˆ O, 3H 2CO 3O OH H C r 2 2 2 5 2 = + +
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11/15/2011 3 Standard Heats of Combustion Standard Heat of Combustion Ĥ ° c (T,P) The heat of reaction (enthalpy) to combust (oxidize) a compound with oxygen Standard Heats of Combustion Ĥ ° c (25 C, 1atm) are tabulated in back of text (table B.1) Note that standard states of the products are all gaseous except water , which is liquid. To calculate with gaseous water as a product, add 44.01n w kJ/mol where n w is the number of moles of water formed Standard Heat of Combustion What is the heat of combustions for ethanol? How does it compare to our calculate value? Energy Balances with Chemical Reactions ABE 201 Energy Balances Same governing equations Enthalpy must contain thermodynamic state changes (T,P) due to both chemical reactions and traditional changes (heat, work, elevation changes, velocity changes) Two methods for solving Heat of Reaction ( Ĥ r ) Method Heat of Formation ( Ĥ f ) Method s p k W Q - = + + E E H W Q E E U p k - = + +
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11/15/2011 4 Energy Balances In this class we are not going to worry about the rate of reaction, we will only solve problems for given reactions as completed. Kinetics (you will study next year) will give you the tools to work with transient effects (rate of reaction) Heats of Reaction Method 1. Draw process diagram 2. Prepare table of masses, composition, and enthalpy values for all process streams 3. Complete Mass Balance (if needed/possible) 4. Chose reference state for specific enthalpy (25 C, 1 atm tabulated values) 5. Calculate extent of reaction ( ξ ) 6. Calculated unknown enthalpies from reference state (e.g. T
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