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# chm2 - Hesss Law Problem done in class on Tuesday One way...

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Hess’s Law Problem done in class on Tuesday, September 18, 2007 One way to find the heat of reaction for a “target reaction” is to take combustion analysis data for products and reactants within the target reaction, and then manipulate those separate reactions and their respective heats of reaction, so that by virtue of Hess’s Law we can derive the heat of reaction for the final target reaction: Here is the target reaction we are concerned with: 3C(g) + 4H 2 (g) CH 3 CH 2 CH 3 (g) ΔH rxn = ??? Now let’s consider the combustion reactions of the individual reactants and product: C(s) + O 2 (g) CO 2 (g) ΔH comb = –393.5 kJ/mol (this refers to the combustion of ONE mole of C(s), as per the reaction to the left) H 2 (g) + ½O 2 (g) H 2 O(g) ΔH comb = –285.8 kJ/mol (this refers to the combustion of ONE mole of H 2 (g), as per the reaction to the left) CH 3 CH 2 CH 3 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O(g) ΔH comb = –2220.1 kJ/mol (this refers to the combustion of ONE mole of CH 3 CH 2 CH 3 (g) as per the reaction to the left)

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