Heat capacity (usually denoted by a capital C, often with subscripts), or thermal capacity, is the measurable physical quantity that characterizes the amount of heat required to change a substance's temperature by a given amount. In the International System of Units (SI), heat capacity is expressed in units of Joules per kelvin.
Conventional thermodynamic expression predicts that the isobaric heat capacity decreases with increasing pressure. In model calculations, heat capacity increases with pressure, decreases, or remains insensitive to pressure, depending on the model applied. The expression cannot be applied to the gases, but experimental data on gases show evidently that heat capacity increases with pressure.(Journal of Thermal Analysis and Calorimetry, Vol. 95 (2009) 1, 313–317)
• heat capacity of gaseous air increases with pressure, proportional to the logarithm of pressureUsually for the ideal gas state heat capacity equation should be written as:
• the effect increases with decreasing temperature
• heat capacity of liquid air also increases with pressure.
where the superscript 'o' refers to the ideal gas state.
The ideal gas values can be used for the real gases at low pressures. At high pressures the effect of pressure on the specific heat may be appreciable.
Edmister (1948) published a generalised plot showing the isothermal pressure correction for real gases as a function of the reduced pressure and temperature.
Edmister’s chart was based on hydrocarbons, but can be used for other materials to give an indication of the likely error if the ideal gas specific heat values are used without corrections.
Effect of pressure on heat capacity of Nitrogen-Trifluoromethane System was studied by Stephan M. Balaban' and Leonard A. Wenzel and reported their obsevations.(Ind. Eng. Chem. Fundamen., 1970, 9 (4), pp 568–574)
Given the data as below,
Heat
Heat energy is transferred as a result of a temperature difference. Energy as heat passes from a warm body with higher temperature to a cold body with lower temperature.
The transfer of energy as a result of the temperature difference alone is referred to as heat flow. The Watt, which is the SI unit of power, can be defined as 1 J/s of heat flow.
Other units used to quantify heat energy are the British Thermal Unit - Btu (the amount of heat to raise 1 lb of water by 1oF) and the Calorie (the amount of heat to raise 1 gram of water by 1oC). Units of energy used may be calorie (cal), Joule (J, SI unit) or Btu. For comparing units, check the unit converter for more information!
Calorie is defined as an amount of heat required to change temperature of one gram of liquid water by one degree Celsius.
1 cal = 4.184 J
Specific Enthalpy
This is the term given to the total energy, due to both pressure and temperature, of a fluid (such as water or steam) at any given time and condition. More specifically it is the sum of the internal energy and the work done by an applied pressure.
The basic unit of measurement is the joule (J). Since one joule represents a very small amount of energy it is common to use kiloJoules (kJ) (1 000 Joules).
Specific enthalpy is a measure of the total energy of a unit mass. The unit commonly used is kJ/kg.
Heat Capacity
Heat Capacity of a system is the amount of heat required to change the temperature of the whole system by
one degree.
Specific Heat
Specific heat is the amount of heat required to change temperature of one kilogram of a substance by one degree. Specific heat may be measured in kJ/kg K or Btu/lboF. For comparing units, check the unit converter for more information!
Specific heats for different materials can be found in the Material Properties section.
Since enthalpy of a fluid is a function of its temperature and pressure, the temperature dependence of the enthalpy can be estimated by measuring the rise in temperature caused by the flow of heat at constant pressure. The constant-pressure heat capacity - cp - is a measure of the change in enthalpy at a particular temperature.
Similarly, the internal energy is a function of temperature and specific volume. The constant volume heat capacity - cv - is a measure of the change in internal energy at a particular temperature and constant volume.
Unless the pressure is extremely high the work done by applied pressure on solids and liquids can be neglected, and enthalpy can be represented by the internal energy component alone. Constant-volume and constant-pressure heats can be said to be equal.
For solids and liquids
cp == cv
The specific heat represents the amount of energy required to raise 1 kg by 1oC, and can be thought of as the ability of a substance to absorb heat. Therefore the SI units of specific heat capacity are kJ/kg K (kJ/kg oC). Water has a very large specific heat capacity (4.19 kJ/kg oC) compared with many fluids.
