Thermodynamic databases for pure substances

A thermodynamic datafile is a set of equation parameters from which the numerical data values can be calculated.

The normal standard state is commonly defined as the most stable physical form of the substance at the specified temperature and a pressure of 1 bar or 1 atm.

If a substance can exist but is not thermodynamically stable (for example, a supercooled liquid), it is called a metastable state.

Thermodynamic functions that refer to conditions in the normal standard state are designated with a small superscript °.

The heat content has been measured and tabulated for virtually all known substances, and is commonly expressed as a polynomial function of temperature.

At the Curie temperature, Cp shows a sharp discontinuity while the enthalpy has a change in slope.

For the special case of the formation of a compound from the elements, the change is designated ΔHform and is a weak function of temperature.

The absolute value of entropy for a substance in its standard state at the reference temperature of 298.15 K is designated S°298.

Hence, the main functional application of Gibbs energy from a thermodynamic database is its change in value during the formation of a compound from the standard-state elements, or for any standard chemical reaction (ΔG°form or ΔG°rx).

For a compound: Similarly, the absolute Gibbs energy G(T) is defined by the absolute enthalpy and entropy of a substance: For a compound: Some tables may also contain the Gibbs energy function (H°298.15 – G°T)/T which is defined in terms of the entropy and heat content.

More recently, computerized databases are used which consist of the equation parameters and subroutines to calculate specific values at any temperature and prepare tables for printing.

Computerized databases often include subroutines for calculating reaction properties and displaying the data as charts.

Depending on the accuracy of the data and the length of the temperature span, the heat content equation may require more or fewer terms.

The values are organized in a format that makes them readable by a thermodynamic calculation program or for use in a spreadsheet.

For example, the Excel-based thermodynamic database FREED [1] creates the following type of datafile, here for a standard pressure of 1 atm.

For MgCl2(c,l,g) at 1 atm pressure: The table format is a common way to display thermodynamic data.

The FREED table gives additional information in the top rows, such as the mass and amount composition and transition temperatures of the constituent elements.

Transition temperatures for the constituent elements have dashes ------- in the first column in a blank row, such as at 922 K, the melting point of Mg.

Transition temperatures for the substance have two blank rows with dashes, and a center row with the defined transition and the enthalpy change, such as the melting point of MgCl2 at 980 K. The datafile equations are at the bottom of the table, and the entire table is in an Excel worksheet.

Molar heat content of four substances in their designated states above 298.15 K and at 1 atm pressure. CaO(c) and Rh(c) are in their normal standard state of crystalline solid at all temperatures. S 2 (g) is a non-physical state below about 882 K and NiO(g) is a non-physical state at all temperatures.
Molar heat capacity of four substances in their designated states at 1 atm pressure. CaO(c) and Rh(c) are in their normal standard state of crystalline solid at all temperatures. S 2 (g) is a non-physical state below about 882 K and NiO(g) is a non-physical state at all temperatures.
Molar enthalpy of zinc above 298.15 K and at 1 atm pressure, showing discontinuities at the melting and boiling points. The Δ H °m of zinc is 7323 J/mol, and the Δ H °v is 115 330 J/mol.
Standard molar heat of formation of ZnBr 2 (c,l) from the elements, showing discontinuities at transition temperatures of the elements and the compound.
Absolute entropy of strontium . The solid line refers to the entropy of strontium in its normal standard state at 1 atm pressure. The dashed line refers to the entropy of strontium vapor in a non-physical state.
Standard heat and Gibbs energy change for the reaction:
The Δ H ° rx shows discontinuities at the melting points of Pb (600.65 K) and PbCl 2 (771 K). Δ G ° rx is not discontinuous at these phase transition temperatures, but does undergo a change in slope, which is almost imperceptible on the chart.
Thermodynamic datafile for MgCl 2 (c,l,g) from FREED. Some values have truncated significant figures for display purposes. The explanation for the values is shown below.
Thermodynamic properties table for MgCl 2 (c,l,g), from the FREED datafile. Some values have truncated significant figures for display purposes.