Heat capacity curves for air and water vapor

Most equations for heat capacities of substances are empirical. Heat capacity at constant pressure is generally expressed in terms of temperature with a power,series type formula  

In general, heat capacity equations are valid only over a moderate range of temperatures. Table 2.1 gives constants to be used with Equation 2.28 for air and water gases. The units of heat capacity with these constants are cal/(g-mole)(°K or °C) or Btu/(lb-mole)(°R or °F). 

Another useful term is specific heat, which is the ratio of the heat capacity of one substance to the heat capacity of a reference material. The heat capacity of water is approximately unity in cgs and American engineering units. 

For estimates, a mean heat capacity is used, which is defined as the enthalpy change divided by the temperature differential for that change. This can be stated as follows  

Note also that if the heat capacity expression is a power series (e.g., Cp = a + bT + cT2 +. . .), then, 

---