Exam+3_Solution-final

Exam+3_Solution-final - #1 (15 points) a) (5 pts) Many...

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#1 (15 points) a) (5 pts) Many synthesis procedures in chemistry involve heating the reactant in a boiling solvent under reflux (i.e. using a reflux condenser). Explain in 2-3 complete sentences why this method is an effective way to control the temperature of the reaction mixture. A boiling solvent will be at its boiling point temperature. The temperature will not increase until all the solvent has evaporated. However, the reflux condenser continuously returns the evaporated solvent to the mixture to balance the amount of solvent evaporated. b) (5 pts) State the general form of an open system energy balance. Provide examples for systems in which four of the terms can be eliminated. For example: term A is eliminated in system 1, term B is eliminated in system 2 etc. ∆ܪ ൅ ∆ܧ ൅ ∆ܧ ൌ ܳ ൅ ܹ Δ H can be eliminated for a bouncing ball. Δ E k can be eliminated for a fermentation vessel. Δ E p can be eliminated for a calorimeter. Q can be eliminated for a traditional mill. Ws can be eliminated for heating a room by burning wood in a fireplace. c) (5 pts) Why are reference states and hypothetical process paths used in energy balance calculations? Reference states are necessary because we cannot measure the absolute values of U and H. Energy balance calculations always involve differences in energy. Therefore, these calculations are not affected by an arbitrarily chosen reference point. Hypothetical process paths allow us to break a specific transformation into several steps, for which we can assess the change in enthalpy or internal energy.
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#2 (15 pts) Hypothetical process path Toluene has been used in the process to remove cocaine from coca leaves to produce Coca-Cola syrup. We have pure toluene that needs to be condensed from 778 ° C to 65 ° C for use as a solvent in this process. Define a hypothetical process path and write the equations (with the appropriate numbers) to find the change in specific enthalpy. (Note: You are not being asked to calculate a number! Set up the equations, but you do not have to solve them.) Process path: Equations: ∆ܪ ൌන ܥ݌ ݀ܶ ଵଵ଴.଺ଶ°஼ ଻଻଼°஼ ൌ න 94.18 ൅ ሺ38 ൈ 10 ିଶ ሻܶെ ሺ27.86 ൈ 10 ିହ ሻܶ ൅ ሺ80.33 ൈ 10 ିଽ ሻܶ ݀ܶ ଵଵ଴.଺ଶ°஼ ଻଻଼°஼ ∆ܪ ൌെ∆ܪ ൌ െ33.47 ݇ܬ/݉݋݈ ∆ܪ ൌන ܥ݌ ݀ܶ ଺ହ°஼ ଵଵ଴.଺ଶ°஼ ൌ න 148.8 ൅ ሺ32.4 ൈ 10
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This note was uploaded on 02/22/2012 for the course CHBE 2100 taught by Professor Staff during the Summer '08 term at Georgia Tech.

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Exam+3_Solution-final - #1 (15 points) a) (5 pts) Many...

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