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LMonSep22

LMonSep22 - Thermodynamics Britton Hill 1 Bomb Calorimeter...

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1 Thermodynamics Britton Hill

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2 Bomb Calorimeter Calculation If ethanol is combusted in a bomb calorimeter C 2 H 5 OH (l) = 3O 2 (g) 2CO 2(g) + 3H 2 O (l) The amount of heat produced in the bomb calorimeter is 1364.67 kJ mol -1 at 25 °C. Calculate H. The conversion for U to H is given by H = U + (n 2 -n 1 )RT Therefore in this example ( 29 -1 mol kJ 1367 298 314 . 8 3 2 67 . 1364 - = × × - + - = H
3 Enthalpies of Formation The enthalpy for any reaction may be determined from the standard enthalpies of formation of the reactants and products from their elemental components. H° = Σ∆ f (products) - Σ∆ f (reactants) The enthalpy of formation is the change in enthalpy accompanying the production of 1 mole of a substance from its elements if the products and reactants are in their standard states.

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4 Standard States The standard state of a substance is the physical state that is stable at 1 bar and a specified temperature. This temperature is typically 298 K. To be able to calculate the enthalpy of formation from the elements the heat of formation of the element has to be known … which is impossible. We can only measure the change in enthalpy. The enthalpy of an element in its standard state at 298 K is defined to be zero JK -1 mol -1.
5 Standard Enthalpies for Solutions There are a number of processes that occur in solution for which an enthalpy is measured: Integral enthalpy of solution: The change in enthalpy accompanying the dissolving of one mole of a substance in a specified quantity of solvent. Enthalpy of dilution: The difference between the integral enthalpies of solution of the original and final solutions.

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