A generic model for the ionic contribution to the equation of state

Abstract

We describe an extended standard model that yields the thermodynamics of the ionic contribution for general materials, from the low temperature solid region, through melting, to the ideal gas limit. We use the Debye model for the solid. Melting is determined by the Lindemann formula with standard rules of thumb used to determine density and energy discontinuities. The model interpolates through the liquid regime to the ideal gas assuming that the specific heat drops monotonically from about 3R at melting, to 9R/4 at five times melting, and continuing to 3R/2 at high temperatures. The area under the specific heat curve is constrained in the model to reproduce the correct high temperature entropy. Thus, for a compound the extra contribution from the entropy of mixing forces into the model, in a crude way, the extra specific heat due to dissociation.

Key Words:

• Lindemann formula
• Debye model
• thermodynamics.