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Ch6(6-7)Oct15Fa08a - CHEM 109 General Chemistry University...

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CHEM 109 General Chemistry University of Nebraska Fall 2008 15 October 2008 ANNOUNCEMENTS: Exam 2 Thursday 5:45 (check seating)
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A calorimeter contains 35.0 mL of water at 18.5 0 C. When 2.09 g of an unknown compound (Unk mm = 24.0 g/mol) is dissolved in the water, the temperature of the mixture increases to 34.9 0 C.: Unk(s) + H 2 O(l) Unk(aq) Assume no heat is lost to the calorimeter or to the surroundings and the specific heat and density of the solution are equal to that of water. Calculate the enthalpy change, ΔH, for this reaction per mole of the unknown. Solution: -29.3 kJ/mole Work this on your own !
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ΔH rxn Relationships
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Hess’s Law states that if a reaction is carried out in a series of steps, ___ for the reaction will equal the sum of the enthalpy changes for the individual steps. With Hess’s Law, we are determining ΔH using a series of _________________ reactions. Enthalpy is a state function; path independent. Hess’s Law of Heat Summation G. H. Hess
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Calculating an unknown ΔH involves three steps:
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