Nitrogen enthalpy-pressure

The measurements of the nitrogen dissociation pressure of Th3N4(cr) have been discussed in Section X. 1.1.2, where they are used to calculate the enthalpy of formation of ThN(cr). The selected value for the Gibbs energy of formation is ... 

Figure D-2 Nitrogen pressure-enthalpy diagram. (From VM Tejada et al. Thermo properties of non-hydrocarbons. Hydrocarbon Proc Petrol Refiner, March 1966. Reprinted by permission.)... 

When the Al-N2 system is raised from 0.1 to 10 atm, the Al(g) product condenses and the species of decomposition are no longer in their standard states. When the assigned enthalpy is increased further so that the A1 returns to the gaseous state, an enthalpy of volatilization AH°ol based on this new assigned enthalpy can be defined. The values of AH°ol specify these new values based on the condition that the elements are in their gaseous states at the pressure and volatilization temperature. When values of AH°ol for the metal-nitrogen systems are calculated for the condition in which the decomposition products are completely in the gaseous state, the pressure variation of AH°ol, unlike AH%, is minimal. 

By definition, the enthalpy of formation of an element in its standard state is zero. The standard state of an element is usually its most stable form under standard conditions. Recall, from section 5.1, that standard conditions are 25°C and 100 kPa (close to room temperature and pressure). Therefore, the standard state of nitrogen is N2(g). The standard state of magnesium is Mg(s). 

Figure 12. Specific enthalpy of nitrogen for different pressures.

Figure 7. Molar enthalpy of nitrogen and iso-butane for different pressures.

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