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Standard state
 
 
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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, the standard state of a material (pure substance
Chemical substance

A chemical substance is a material with a specific Empirical formula. It is a concept that became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate....
, mixture or solution
Solution

In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent....
) is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry
International Union of Pure and Applied Chemistry

The International Union of Pure and Applied Chemistry is a non-governmental organization established in 1919 for the advancing of chemistry. Its members are national chemistry societies....
 (IUPAC) recommends a conventional set of standard states for general use. IUPAC recommends using a standard pressure po = 1 bar
Bar (unit)

The bar , decibar and the millibar are units of pressure. They are not SI units, nor are they cgs units, but they are accepted for use with the SI....
 (100 kilopascals).






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In chemistry
Chemistry

Chemistry is the science concerned with the composition, structure, and properties of matter, as well as the changes it undergoes during chemical reactions....
, the standard state of a material (pure substance
Chemical substance

A chemical substance is a material with a specific Empirical formula. It is a concept that became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate....
, mixture or solution
Solution

In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent....
) is a reference point used to calculate its properties under different conditions. In principle, the choice of standard state is arbitrary, although the International Union of Pure and Applied Chemistry
International Union of Pure and Applied Chemistry

The International Union of Pure and Applied Chemistry is a non-governmental organization established in 1919 for the advancing of chemistry. Its members are national chemistry societies....
 (IUPAC) recommends a conventional set of standard states for general use. IUPAC recommends using a standard pressure po = 1 bar
Bar (unit)

The bar , decibar and the millibar are units of pressure. They are not SI units, nor are they cgs units, but they are accepted for use with the SI....
 (100 kilopascals). Strictly speaking, temperature is not part of the definition of a standard state. For example, as discussed below the standard state of a gas is conventionally chosen to be unit pressure (usually in bar) ideal gas, regardless of the temperature. However, most tables of thermodynamic quantities are compiled at specific temperatures, most commonly 298.15 K or, somewhat less commonly, 273.15 K. The standard state should not be confused with standard temperature and pressure (STP) for gases, nor with the standard solution
Standard solution

Standard solution is a chemical term which describes a solution of known concentration. The concentration of the solution is normally expressed in units of Mole per Liter ....
s used in analytical chemistry
Analytical chemistry

Analytical chemistry is the study of the chemical composition of natural and artificial materials. Unlike other major sub disciplines of chemistry such as inorganic chemistry and organic chemistry, analytical chemistry is not restricted to any particular type of chemical compound or chemical reaction....
.

In the time of their development in the nineteenth century, the superscript plimsoll symbol o was adopted as a to indicate the non-zero nature of the standard state. The superscript circle º is also commonly used, not least for typographical reasons, and both are equally acceptable.

For a given material or substance, the standard state is the reference state for the material's thermodynamic state properties such as enthalpy
Enthalpy

In thermodynamics and chemistry, the enthalpy is a quotient or description of thermodynamic potential of a system, which can be used to calculate the heat transfer during a quasistatic process taking place in a closed system thermodynamic system under constant pressure....
, entropy
Entropy

In many branches of science, entropy is a measure of the disorder of a system. The concept of entropy is particularly notable as it is applied across physics, information theory and mathematics....
, Gibbs free energy
Gibbs free energy

In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating Work obtainable from an isothermal, Isobaric process thermodynamic system....
, and for many other material standards. The standard enthalpy change of formation
Standard enthalpy change of formation

The standard enthalpy of formation or "standard heat of formation" of a compound is the change of enthalpy that accompanies the formation of 1 mole of a substance in its standard state from its constituent elements in their standard states ....
 for an element in its standard state is zero, and this convention allows a wide range of other thermodynamic quantities to be calculated and tabulated. The standard state of a substance does not have to exist in nature: for example, it is possible to calculate values for steam
Steam

In physical chemistry, and in engineering, steam refers to vaporized water. It is a pure, completely invisible gaseous phase . At standard temperature and pressure, pure steam occupies about 1,600 times the volume of an equal mass of liquid water....
 at 25 °C and 1 bar, even though steam does not exist (as a gas) under these conditions. The advantage of this practice is that tables of thermodynamic properties prepared in this way are self-consistent.

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.

Gases

The standard state for a gas is the hypothetical state it would have as a pure substance obeying the ideal gas equation at 1 bar. No real gas has perfectly ideal behaviour, 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 1 bar. For elements, the reference point of ?fHo = 0 is defined for the most stable allotrope of the element, such as graphite
Graphite

The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Greek language ??afe?? : "to draw/write", for its use in pencils, where it is commonly called lead, as distinguished from the actual metallic element lead....
 in the case of carbon, and the ß-phase (white tin) in the case of tin
Tin

Tin is a chemical element with the symbol Sn and atomic number 50. Tin is obtained chiefly from the mineral cassiterite, where it occurs as an oxide, SnO2....
.

Solutes

For a substance in solution (solute), the standard state is the hypothetical state it would have at the standard state molality or amount concentration but exhibiting infinite-dilution behaviour. The reason for this unusual definition is that the behaviour 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 behaviour to be the standard state allows corrections for non-ideality to be made consistently for all the different solutes. Standard state molality is 1 mol kg−1, while standard state amount concentration is 1 mol dm−3.

See also

  • Standard conditions for temperature and pressure
    Standard conditions for temperature and pressure

    In physical sciences, standard conditions for temperature and pressure are standard sets of conditions for experimental measurements, to allow comparisons to be made between different sets of data....