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 (), or, somewhat less commonly, the
freezing point of
water (). 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 (or 1
bar). For most elements, the reference point of Δf
H⦵ = 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
metastable. This is because the thermodynamically stable black allotrope is difficult to prepare pure.
Solutes For a substance in solution (solute), the standard state 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). 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 ). 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 isotherm) the most common standard state is a relative coverage of , as this choice results in a cancellation of the
configurational entropy term and is also consistent with neglecting to include the standard state (which is a common error). The advantage of using 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. == Typesetting ==