Energy and Heat Units

In the SI system energy is given in joules (J) or kilojoules (kJ). Energy is also expressed in btu (British thermal unit) or cal (calorie). The g calorie (abbreviated, cal) is defined as the amount of heat needed to heat 1.0 g water 1.0°C (from 14.5°C to 15.5°C). Also, 1 kcal (kilocalorie) = 1000 cal. The btu is defined as the amount of heat needed to raise 1.0 lb water 1°F. Hence, from Appendix A. 1, 

It can be shown that the actual numerical value of a heat capacity is the same in mass units or in molar units. That is, 

The heat capacities of gases (sometimes called specific heat) at constant pressure are functions of temperature and for engineering purposes can be assumed to be independent of pressure up to several atmospheres. In most process engineering calculations, one is usually interested in the amount of heat needed to heat a gas from one temperature f, to another at t2- Since the varies with temperature, an integration must be performed or a suitable mean used. These mean values for gases have been obtained for Ti of 298 K or 25°C (77°F) and various Tj values, and are tabulated in Table 1.6-1 at 101.325 kPa pressure or less as in kJ/kg mol K at various values of Tj in K or °C. 

The gas N2 at 1 atm pressure absolute is being heated in a heat exchanger. 

Mean Molar Heat Capacities of Gases Between 25 and T°C at I atm Pressure or Less  

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