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    Example and Directions
    Words (or words that have the same definition)The definition is case sensitive(Optional) Image to display with the definition [Not displayed in Glossary, only in pop-up on pages](Optional) Caption for Image(Optional) External or Internal Link(Optional) Source for Definition
    (Eg. "Genetic, Hereditary, DNA ...")(Eg. "Relating to genes or heredity")The infamous double helix https://bio.libretexts.org/CC-BY-SA; Delmar Larsen
    Glossary Entries
    Word(s)DefinitionImageCaptionLinkSource
    entropy (S)state function that is a measure of the matter and/or energy dispersal within a system, determined by the number of system microstates; often described as a measure of the disorder of the system    
    Gibbs free energy change (G)thermodynamic property defined in terms of system enthalpy and entropy; all spontaneous processes involve a decrease in G    
    microstatepossible configuration or arrangement of matter and energy within a system    
    nonspontaneous processprocess that requires continual input of energy from an external source    
    reversible processprocess that takes place so slowly as to be capable of reversing direction in response to an infinitesimally small change in conditions; hypothetical construct that can only be approximated by real processes    
    second law of thermodynamicsall spontaneous processes involve an increase in the entropy of the universe    
    spontaneous changeprocess that takes place without a continuous input of energy from an external source    
    standard entropy (S°)entropy for one mole of a substance at 1 bar pressure; tabulated values are usually determined at 298.15 K    
    standard entropy change (ΔS°)change in entropy for a reaction calculated using the standard entropies    
    standard free energy change (ΔG°)change in free energy for a process occurring under standard conditions (1 bar pressure for gases, 1 M concentration for solutions)    
    standard free energy of formation change in free energy accompanying the formation of one mole of substance from its elements in their standard states    
    third law of thermodynamicsentropy of a perfect crystal at absolute zero (0 K) is zero    

    Key Terms is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by LibreTexts.

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