CHAPTER XXI 

 MOLECULAR ENERGY 



THE value of the kinetic energy of translation of all 

 the molecules in a mole has been shown to be 3RT/2. 

 To raise the temperature of a mole one degree therefore 

 requires 3R/2 ergs to supply the increases in energy of 

 translation of the molecules, that is, 12/2 ergs for each 

 degree of freedom. (If the molecule has other de- 

 grees of freedom additional energy will be required.) 

 If expansion is allowed and external work done by the 

 gas, still more energy is required to heat the mole one 

 degree. Represent this energy by A. 



In considering the amount of energy required per 

 degree of temperature per mole we therefore distin- 

 guish two cases, namely, (1) the energy required when 

 the volume is maintained constant, and (2) that re- 

 quired when the volume is allowed to increase but the 

 external pressure is maintained constant. The first 

 of these is the " specific heat at constant volume," 

 denoted by C v , and the second is the "specific heat at 

 constant pressure," C p . We may write an expression 

 for A, the external work per mole per degree of tem- 

 perature, and these specific heats as 



C P =C V +A (1) 



These magnitudes are conventionally expressed in 

 other units than the erg. The usual unit is the calorie, 



288 



