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In chemistry, the standard state of a material (pure substance, mixture or solution) 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 (IUPAC) recommends a conventional set of standard states for general use.[1] IUPAC recommends using a standard pressure po = 1 bar (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,[2] nor with the standard solutions used in analytical chemistry.[3]

In the time of their development in the nineteenth century, the superscript plimsoll symbol o was adopted 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.[4]

For a given material or substance, the standard state is the reference state for the material's thermodynamic state properties such as enthalpy, entropy, Gibbs free energy, and for many other material standards. The standard enthalpy change of formation 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 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.

Contents

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.

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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 ΔfHo = 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.

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

References

  1. ^ International Union of Pure and Applied Chemistry. "standard state". Compendium of Chemical Terminology Internet edition.
  2. ^ International Union of Pure and Applied Chemistry. "standard conditions for gases". Compendium of Chemical Terminology Internet edition.
  3. ^ International Union of Pure and Applied Chemistry. "standard solution". Compendium of Chemical Terminology Internet edition.
  4. ^ International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. pp. 48–54. Electronic version.

Simple English

The standard state is a common way for measuring properties of chemicals. Standard state normally is room temperature (25°C) and normal atmospheric pressure (14.7 psi). Normally when no temperature or pressure is given, standard state is implied. If an article says "bromine is a liquid" it means bromine is a liquid in the standard state.


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