ENTHALPY, ENTROPY, AND SPECIFIC HEAT BASIC AND TUTORIALS


Enthalpy is not a directly measurable property. It is a synthetic combination of the internal energy (U) and the flow energy (pV) exchanged with the surroundings.

Enthalpy and specific enthalpy are symbolized by H and h, and are defined by
H = U + pV
h = u + pv


Entropy is a microscopic property associated with the microscopic energy transfer called heat (Q), between the system and its surroundings. Entropy is also called displacement of heat.

It is not directly measurable, but can be related to other properties. Entropy is a measure of the level of irreversibility associated with any process.

Unlike energy, it is nonconservative. It is a very important property in thermodynamics


Specific Heat (c, cp and cv)
The quantity c = δq/dT is called the specific heat or heat capacity. It is a measure of the heat added to a mass of a system to produce a unit increase in temperature.

For example, the specific heat of water at 25ºC and 101.3 kPa is 4.18 kJ/[kg(K)], which means 4.18 kJ of heat added is required to a kg mass of water in order to raise its temperature by 1 K.

The most commonly used specific heats are specific heat at constant pressure (cp) and specific heat at constant volume (cv); cp and cv are defined in the following equations:
cp = (∂h/∂T)P
cv = (∂u/∂T)v


The specific heat of a substance at constant pressure is the rate of change of specific enthalpy of the substance with respect to a change in the temperature of the substance while maintaining a constant pressure.

The specific heat of a substance at constant volume is the rate of change of specific internal energy of the substance with respect to a change in the temperature of the substance while maintaining a constant volume.

Both cp and cv are measurable properties and are measured on a constant pressure process and a constant volume process for a closed system, respectively. Values of cp and cv can be obtained by measuring the heat transfer required to raise the temperature of a unit mass of substance by one degree, while holding the pressure and volume constant, respectively.

The unit of c (cp or cv) is kJ/[kg(K)] in SI system and Btu/[lbm(ºR)] in English system. The heat capacities of gases other than cp and cv for an arbitrary process can also be defined (Reference: Chen and Wu, The heat capacities of gases in arbitrary process, The International Journal of Mechanical Engineering Education, 29(3), 227-232, 2001).

Since cp and cv are measurable properties, we therefore have a method of calculating the internal energy and enthalpy for any process if we know the end states.



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