Calculating internal energy and enthalpy changes

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