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CH5 notes - Chapter 5 Thermochemistry A B C D E Definitions...

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Chapter 5 Thermochemistry A. Definitions B. Calorimetry C. Enthalpies of Formation D. Hess = s Law E. Energy Sources HW #P4 Quiz 2 and Exam 2
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Chapter 5 Thermochemistry A. Definitions Chemical reactions are accompanied by energy changes. Conservation of Energy B Energy is neither created nor destroyed. Processes that liberate heat are exothermic . Processes that absorb heat are endothermic . Enthalpy B H B is the heat content of a substance. The enthalpy change for a reaction (at constant pressure) is our focus. ΔH rxn = H products B H reactants ΔH rxn is negative for exothermic and positive for endothermic processes. 2H 2 (g) + O 2 (g) h 2H 2 O(g) ΔH = B 483.6 kJ Guidelines: (1) ΔH rxn depends on amount. 2 moles of H 2 (g) reacting with 1 mole O 2 (g) releases 483.6 kJ. 4 moles H 2 (g) reacting with 2 moles O 2 (g) gives 967.2 kJ.
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(2) For the reverse reaction the sign of ΔH rxn changes. 2H 2 O(g) 2H 2 (g) + O 2 (g) H = 483.6 kJ (3) The value of ΔH rxn depends on the state of the reactants and products.
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