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

• 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

Ideal gas specific heats

Specific heats of monoatomicgases are not functions oftemperatures,u=cvT=3/2RT; h=cpT=5/2RTSince Cp/Cv=k=5/3

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).

Tables A-17 and A-2