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UNCW CHM 101 - Enthalpy

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Chm 101 1st Edition Lecture 12Outline of Last Lecture I. ThermochemistryII. Potential and Kinetic Energy III. Units of Energy IV. System and Surroundings V. Heat Outline of Current LectureI. State FunctionsII. Types of WorkIII. EnthalpyCurrent LectureState Functions – depends only on the present state of the system not on the path the system tookto reach that state- The internal energy of a system (E) is a state function- Work and heat are NOT state functions. These quantities describe how an internal energy state is achieved. Two Types of Work- Chemical work – done by charged ions- Pressure-volume (PV) work – done by expanding gas - Assuming that all changes occur at constant pressure (P) W = -P∆V Work being done on surroundings by a system- W < 0- ∆V > 0 Work being done on a system by surroundings - W > 0- ∆V < 0 Enthalpy (H) – the sum of the internal energy (E) and the product of the pressure (P) and the volume (V) - H = E + PV- The change in enthalpy (∆H) at constant pressure iso ∆ = ∆E + P∆V ∆E = q + w W = - P∆V -W = P∆V ∆H = (q + w) + (-w)o ∆H = q- The change in enthalpy equals the heat gained or lost for a reaction or process at constant pressure o ∆H = q- Exothermic – process that results in the loss of heat from te system to its surroundingso q < 0o ∆H < 0- Endothermic – process that results in the system gaining heat from its surroundings o q > 0o ∆H > 0- Enthalpies of Reactionso ∆Hrxn = Hproducts - Hreactants- Chemical reactions have enthalpy changes associated with themo H2 (g) + F2 (g)  2HF o ∆H = -546 kJ Exothermic reaction- Enthalpy is an extensive propertyo 2H2 (g) + 2F2 (g)  4HF o ∆H = -1072 kJ- The enthalpy change for a reaction is =, but opposite in sign to the ∆H for the reverse reactiono 2HF  H2 + F2o ∆H = 546 kJ- The enthalpy change for a reaction depends on the state of the reactants and


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