02.10 Internal Energy, Enthalpy, and Heat Capacities

Common Property Change Calculations (uakron, 11min). When we need to compute a change in energy or enthalpy, we may quickly resort to CvΔT or CpΔT, but you should also note that large changes can occur due to phase change. These considerations motivate careful consideration of the definitions of Cv and Cp, and the development of convenient equations for estimating heat of vaporization. To know when to apply the heat of vaporization, you need to know the saturation conditions, for which a quick estimate can be obtained from the short-cut vapor pressure (SCVP) equation. When the chemical of interest is H2O, these hand calculation methods can be compared to the properties given in the steam tables. Sample calculations of property changes (uakron, 21min) can be used to illustrate the precision of the quick estimates obtained from Eqs. 2.45, 2.47 and the back flap. These calculations provide practice with the steam tables at unusual conditions as well as validating your skills with the hand calculation formulas.

Comprehension Questions:
1. Develop an adaptation of props.xlsx that is most convenient for you personally to compute quick estimates of saturation temperature, saturation pressure, ideal gas enthalpy changes. You might want to view the props.xlsx and shortcut Antoine coefficients software tutorials.
2. Quickly estimate the change in enthalpy as CO2 goes from 350K, 1bar to 300K, 1bar.
3. Quickly estimate the change in internal energy as CO2 goes from 350K, 1bar to 300K, 1bar.
4. Quickly estimate the change in enthalpy as CO2 goes from 350K, 1bar to 300K, 100bar. Hint: the change in enthalpy to go from a saturated liquid to a compressed liquid can be computed from the adiabatic, reversible pump work.