What is a Standard State or Reference State of an element?

[marc32123]

What is a Standard State or Reference State of an element?

Can someone please give me a simple explanation of what a standard state or reference state is? I don't quite understand the way wiki describes it which is -


The standard state, also known as reference state, of an element is defined as its thermodynamically most stable state at 1 bar at a given temperature (typically at 298.15 K). In thermochemistry, an element is defined to have an enthalpy of formation of zero in its standard state. For example, the reference state for carbon is graphite, because the structure of graphite is more stable than that of the other allotropes.

 

[maajdl]

http://goldbook.iupac.org/S05925.html

 

[Chestermiller]

If you are going to be doing thermochemical calculations in which reactions are involved, you are going to want to be able to determine changes in enthalpy between reactants and products and changes in free energy between reactants and products (the latter to calculate equilibrium constants). You can't have a table of these changes for every possible reaction under all possible conditions (because it would require too much paper), but, because enthalpy and free energy are state functions, you can tabulate the enthalpy and free energy of the individual chemical species, and then use this to determine the changes in reactions. If you are doing this for individual species, it is very convenient to specify the enthalpies and free energies of the species only in a specific reference state (this requires very little data), and then determine the values of these parameters at other temperatures and pressures using, say, integrals of heat capacities over temperature. This approach allows you to calculate heat of reaction and free energies of reaction at a wide range of conditions, based on only a minimum amount of tabulated data. The reference state that is usually used is 25 C and 1 atm. The values specified for the elements under these conditions are zero. But, if a chemical compound is involved, the values specified are those required to go from the elemental species to the chemical compound at the reference state. This gives you what you need to solve thermochemical problems.

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