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Calculating internal energy and enthalpy changes

Calculating internal energy and enthalpy changes. Specific heat function of water vapor. Ideal gas specific heat. At low pressures, real gases behave like ideal gas

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Calculating internal energy and enthalpy changes

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  1. Calculating internal energy and enthalpy changes

  2. Specific heat function of water vapor

  3. Ideal gas specific heat • At low pressures, real gases behave like ideal gas • Ideal gas specific heat (cv0,cp0) at each given temperature is obtained by extrapolating measured specific heat data at different pressures toward zero pressure. • Under situations where the gases are nowhere close to condensation (e.g. air/nitrogen/oxygen/helium/hydrogen etc. at atmospheric pressure or lower at room temperature), • the model of “calorically imperfect perfect (ideal) gas” can represent  u and  h accurately • so that energy changes can be calculated by: du=cv0dT, dh=cpodT

  4. Specific heats of monoatomic gases are not functions of temperatures, u=cvT=3/2RT; h=cpT=5/2RT Since Cp/Cv=k=5/3 Ideal gas specific heats

  5. Ways to calculate  u and  h for calorically imperfect ideal gases • Definition of specific heat • Specific heat,  u and  h for ideal gases • Specific heat,  u and  h for solids and liquids • Use of “Ideal gas properties of” tables (Table A-17,A-22) • Use of polynomial expressions for specific heat (Table A-2c) • Use of specific heats tabulated against temperature (Table A2a-A2b).

  6. Tables A-17 and A-2

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