Editing Standard state (section)

== Conventional standard states ==
Many standard states are non-physical states, often referred to as "hypothetical states". Nevertheless, their thermodynamic properties are well-defined, usually by an extrapolation from some limiting condition, such as zero pressure or zero concentration, to a specified condition (usually unit concentration or pressure) using an ideal extrapolating function, such as ideal solution or ideal gas behavior, or by empirical measurements. Strictly speaking, temperature is not part of the definition of a standard state. However, most tables of thermodynamic quantities are compiled at specific temperatures, most commonly [[Room temperature#Definitions in science and industry|room temperature]] ({{convert|298.15|K|C F|0|disp=comma}}), or, somewhat less commonly, the [[freezing point]] of [[water]] ({{convert|273.15|K|C F|0|disp=comma}}).<ref name="Thought2" />

=== Gases ===
The standard state for a gas is the hypothetical state it would have as a pure substance obeying the [[ideal gas equation]] at standard pressure. IUPAC recommends using a standard pressure ''p''<sup>⦵</sup> or P° equal to {{val|e=5|u=Pa}}, or 1 bar.<ref>{{GoldBookRef| file=S05921 | title = standard pressure}}</ref><ref name="libretext">{{cite web |title=Activities and their Effects on Equilibria |url=https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Physical_Properties_of_Matter/Solutions_and_Mixtures/Nonideal_Solutions/Activities_and_their_Effects_on_Equilibria |website=Chemistry LibreTexts |language=en |date=29 January 2016}}</ref> No real gas has perfectly ideal behavior, but this definition of the standard state allows corrections for non-ideality to be made consistently for all the different gases.

=== Liquids and solids ===
The standard state for liquids and solids is simply the state of the pure substance subjected to a total pressure of {{val|e=5|u=Pa}} (or 1 [[bar (unit)|bar]]). For most elements, the reference point of Δ<sub>f</sub>''H''<sup>⦵</sup>&nbsp;= 0 is defined for the most stable [[allotrope]] of the element, such as [[graphite]] in the case of [[carbon]], and the β-phase ([[white tin]]) in the case of [[tin]]. An exception is white [[phosphorus]], the most common allotrope of phosphorus, which is defined as the standard state despite the fact that it is only [[metastability|metastable]].<ref>Housecroft C.E. and Sharpe A.G., ''Inorganic Chemistry'' (2nd ed., Pearson Prentice-Hall 2005) p.392</ref> This is because the thermodynamically stable black allotrope is difficult to prepare pure.<ref>{{cite journal |last1=Rard |first1=Joseph A. |last2=Wolery |first2=Thomas J. |date=2007 |title=The Standard Chemical-Thermodynamic Properties of Phosphorus and Some of its Key Compounds and Aqueous Species: An Evaluation of Differences between the Previous Recommendations of NBS/NIST and CODATA |url=https://link.springer.com/article/10.1007/s10953-007-9205-7 |journal=Journal of Solution Chemistry |volume=36 |issue= 11–12|pages=1585–1599 |doi=10.1007/s10953-007-9205-7 |access-date=24 December 2023 |quote=Although white phosphorus is not the thermodynamically stable allotrope, the red and black forms are difficult to prepare in pure form, which makes them less suitable for quantitative thermodynamic measurements.}}</ref>

=== Solutes ===
For a substance in solution (solute), the standard state {{not a typo|C°}} is usually chosen as the hypothetical state it would have at the standard state [[molality]] or [[amount concentration]] but exhibiting infinite-dilution behavior (where there are no solute-solute interactions, but solute-solvent interactions are present).<ref name="libretext"/> The reason for this unusual definition is that the behavior of a solute at the limit of infinite dilution is described by equations which are very similar to the equations for ideal gases. Hence taking infinite-dilution behavior to be the standard state allows corrections for non-ideality to be made consistently for all the different solutes. The standard state molality is {{val|1|u=mol/kg}}, while the standard state molarity is {{val|1|u=mol/dm3}}.

Other choices are possible. For example, the use of a standard state concentration of 10<sup>−7</sup>&nbsp;mol/L for the hydrogen ion in a real, aqueous solution is common in the field of [[biochemistry]].<ref>{{cite book |last1=Chang |first1=Raymond | last2=Thoman | first2=John W. Jr. |title=Physical Chemistry for the Chemical Sciences |date=2014|publisher=University Science Books |location=New York |pages=346–347}}</ref><ref>{{cite book |last1=Sherwood |first1=Dennis |last2=Dalby |first2=Paul |title=Modern Thermodynamics for Chemists and Biochemists |date=2018 |publisher=Oxford Scholarship Online |doi=10.1093/oso/9780198782957.003.0023 |isbn=978-0-19-878295-7 |url=https://oxford.universitypressscholarship.com/view/10.1093/oso/9780198782957.001.0001/oso-9780198782957-chapter-23 |access-date=18 May 2021}}</ref> In other application areas such as [[electrochemistry]], the standard state is sometimes chosen as the actual state of the real solution at a standard concentration (often {{val|1|u=mol/dm3}}).<ref>{{cite book |last1=Chang |first1=Raymond | last2=Thoman | first2=John W. Jr. |title=Physical Chemistry for the Chemical Sciences |date=2014 |publisher=University Science Books |location=New York |pages=228–231}}</ref> The [[activity coefficients]] will not transfer from convention to convention and so it is very important to know and understand what conventions were used in the construction of tables of standard thermodynamic properties before using them to describe solutions.

=== Adsorbates ===
For molecules adsorbed on surfaces there have been various conventions proposed based on hypothetical standard states. For adsorption that occurs on specific sites ([[Langmuir adsorption model|Langmuir adsorption isotherm]]) the most common standard state is a relative coverage of {{nowrap|''θ''° {{=}} 0.5}}, as this choice results in a cancellation of the [[configuration entropy|configurational entropy]] term and is also consistent with neglecting to include the standard state (which is a common error).<ref name="Savara">{{cite journal |doi=10.1021/jp404398z |title=Standard States for Adsorption on Solid Surfaces: 2D Gases, Surface Liquids, and Langmuir Adsorbates |journal=J. Phys. Chem. C |volume=117 |pages=15710–15715  |year=2013 |last1=Savara |first1=Aditya |issue=30 |doi-access= }}</ref>  The advantage of using {{nowrap|''θ''° {{=}} 0.5}} is that the configurational term cancels and the [[entropy]] extracted from thermodynamic analyses is thus reflective of intra-molecular changes between the bulk phase (such as gas or liquid) and the adsorbed state. There may be benefit to tabulating values based on both the relative coverage based standard state and in an additional column the absolute coverage based standard state. For 2D gas states, the complication of discrete states does not arise and an absolute density base standard state has been proposed, similar for the 3D gas phase.<ref name="Savara" />