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KThur18 - Thermodynamics Cascade du Sallanches 1 Standard...

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1 Thermodynamics Cascade du Sallanches
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2 Standard Enthalpies and Stoichiometry The enthalpy changes associated with a reaction make use of a quantity called the extent of a reaction . The extent of a reaction is given the symbol ξ (Greek lower case xi). Consider the following reaction aA + bB cC + dD
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3 Stoichiometry The reaction is sometimes written: v 1 A 1 + v 2 A 2 v 3 A 3 + v 3 A 3 The letters v are called the stoichiometric coefficients. These terms are negative for reactants and positive for products. The importance of the extent is that ξ is the same for all the reactants and the products. The extent is calculated from: i i i v n n 0 - = ξ
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4 Example 2A + B 3C + 2D When the reaction is carried out, 1 mole of A and three moles of B are reacted. At the end of the reaction there will be 1.5 moles of C and 1 mole of D and there will be 2.5 moles of B left over.
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5 Extent 5 . 0 2 1 0 = - - = A ξ If we specify the enthalpy of a reaction then the stoichiometry of the reaction must also be specified. 5 . 0 3 0 5 . 1 = - = C 5 . 0 1 3 5 . 2 = - - = B 5 . 0 2 0 1 = - = D At the end of the reaction there will be 1.5 moles of C and 1 mole of D and there will be 2.5 moles of B left over.
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6 Standard States Enthalpy is a state function. The changes that occur are dependent on the states of the reactants and products. Therefore H values are quoted for a precise set of conditions, that is the standard state of a substance at a specified temperature in its pure form at 1 bar. (1 bar is 10 5 Pascals). Enthalpy under these conditions is given the symbol H o
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7 Measuring Enthalpies The heat supplied to a system at constant pressure is equal to the increase in enthalpy. The classic method to measure heat change in a chemical process is calorimetry. One of the most common reactions to study by this method is oxidation (combustion) and this is done using a bomb calorimeter. P q H =
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8 Bomb Calorimeter http://www.colby.edu/chemistry/PChem/lab/EnthalpyofFormation.pdf
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9 Bomb Calorimeter
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10 Experiment The calorimeter is calibrated by measuring the rise in temperature caused by the complete oxidation of a sample of known heat of combustion. A common standard is benzoic acid (-3227 kJ mol -1 ). These values can be very large but the apparatus
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This note was uploaded on 03/17/2010 for the course CH 3530 taught by Professor Consors during the Fall '10 term at WPI.

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KThur18 - Thermodynamics Cascade du Sallanches 1 Standard...

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