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GWedSep10 - Thermodynamics 20,469 feet, in the Himalayas,...

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1 Thermodynamics 20,469 feet, in the Himalayas, Pakistan
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2 Special Cases Consider the work done on some special cases, i.e. constant volume and constant pressure.
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3 Constant Volume If a system is held at constant volume and expansion was the only sort of work that the system could do then we can write: The subscript V indicates constant volume. The change in internal energy is simply given by the heat supplied. V q U =
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4 Constant Pressure If the expansion of a gas occurs at constant pressure: (Remember the symbol for an inexact differential has the strikethrough going through the stem of the d) Rearrange this equation and specify a constant pressure and a reversible change in volume. w d q d dU + = w d dU q d - = PdV dU dq P + =
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5 Enthalpy The subscript P indicates a constant pressure. dq P is now a state function, provided that only PV work is carried out. If the gas undergoes a change in volume and we integrate this equation then: PdV dU dq P + = ) ( ) ( ) ( ) ( 1 1 2 2 1 2 1 2 2 1 2 1 PV U PV U q V V P U U q dV P dU q P P V V V V P + - + = - + - = + =
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6 Enthalpy The term (U+PV) is a state function since U, P and V are all state functions. The term (U+PV) is given a name … Enthalpy The increase in enthalpy of a system is equal to the heat absorbed at constant pressure (assuming the system only does PV work). Just as it is only possible to measure changes in internal energy it is also only possible to measure changes in enthalpy PV U H + =
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Summary So far everything we have considered is PV work, i.e. work that arises from a change in pressure – volume. For a system that is at constant volume no work is done since V is 0. For a system at constant pressure, such as a gas generated in an open beaker, work is done in pushing back the atmosphere. If heat is added to a system then q is +ve. For an endothermic process (that is a process that takes heat in from the surroundings) H is +ve. H
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GWedSep10 - Thermodynamics 20,469 feet, in the Himalayas,...

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