Standard Entropy Section
Applicability: Cranium, Synapse (core versions 0315+)

This section contains two fields: the entropy value field; the entropy standard state field.

Entropy Calculations

The absolute entropy of a chemical substance at 298.15 K and 101.325 kPa, the standard state used by Cranium and Synaspse, can be calculated by taking the subtance's entropy at 0 K to be equal to zero and then adding contributions for the temperature change, the pressure change and phase transitions.

Cranium and Synapse use absolute entropy when calculating the Gibbs Energy of formation and entropies of reaction. For example, the is calculated in this manner, we must alwasy subtract the absolute entropy of the elements when we calculate a chemical's entropy of formation.

For example, the MKS Core Knowledge Base has the following values for ethanol: ∆Hf,298 = -235.3 kJ/mol and S°298 = 282.59 J/mol K. To calculate the Gibbs Energy of formation at 298K we need the entropy of formation which is given by:

Chemical Reaction: 3 H2 (g) + ½ O2 (g) + 2 C (s) = C2H5OH (g)

ΔS°f (J/mol K) = 282.59 - 3(130.57) - ½(205.042) - 2(5.740)

ΔS°f (J/mol K) = -223.121

The Gibbs Energy of formation can thus be calculated by:

ΔG°f = ΔH°f - T ΔS°f

ΔG°f (kJ/mol) = -235.3 - (298)(-0.223121)

ΔG°f (kJ/mol) = -168.810

Standard Entropy Values
(J/mol K)

Note that all elemental entropy values are given on an "atomic basis", e.g., the entropy value for hydrogen is for one mole of hydrogen atoms. Thus, you would need to double this value if you have H2 in your chemical reaction.

Datum Controls

The Standard Entropy Fields displays data in two datum controls. The first control displays the numerical entropy value. The second control displays the standard state associated with the datum value.

Clicking the left mouse button on either datum control activates the Standard Entropy edit dialog. (See documentation on the Entropy Datum Edit Dialog for editing details.)

The dialog enables you to enter the datum's value, units and standard state. Optionally, but recommended, you may also enter a reference and comment.

For the datum's standard state, you may enter any value you wish. Cranium and Synapse provide several common values:

Solid, cubic Solid, orthorhombic
Liquid Solid, hexagonal
Gas Solid, tetragonal
Solid Solid, rhombic
Datum Menu Commands

Clicking the right mouse button on either of the datum controls displays the command menu.

See documentation on Common Menu Commands for details on each of these menu commands.

Units Control

The Units control displays the units in which the entropy value is displayed.

Clicking the left mouse button on the field's units control activates the Set Property Units dialog.

Selecting new units from the list and press the OK button changes the current units in which the property is displayed. (Note that all data are stored in their original units - changing the displayed units does not change any stored data.) The dialog's Std Units button displays the default units used by Cranium and Synapse for the current physical property. (See documentation on the Property Units Dialog for additional details on changing property units.)

Tip: All displayed units change

The units set via the Set Property Units dialog, accessed either from the Options menu's Set Units command or clicking on the Units control in a physical property field, are assigned for all documents in the application. For example, if you have three knowledge base documents open and then change the units of the standard entropy, all documents will update their standard entropy displays using the newly selected units.

Units are also stored in the application. The next time Cranium or Synapse starts, the set of units used in your last session will be used again.

Related Documentation
Topic Description
Estimating Chemical Properties a short video demonstrating how to estimate physical properties using either Synapse or Cranium.
Chemicals Chapter the chemicals chapter contains numerous sections for entering and displaying data and estimates including the chemical structure section.
Elements Chapter the elements chapter manages information about the atoms used to construct the molecular structures of chemicals and groups.
Groups Chapter the groups chapter's main section enables you to enter molecular structure information.