- Standard state is a reference state for a substance that is set by convention.
Like internal energy (U), chemists are usually interested in the change in enthalpy (ΔH) that occurs during physical and chemical processes, and not necessarily the absolute value of enthalpy (H). This is complicated by the fact that a pure sample of a particular chemical can have different amounts of enthalpy depending on the state (e.g. phase, temperature, pressure). To help standardize the measurement of enthalpy, chemists use a reference state called standard state.
The state of aggregation (phase) of a substance at standard state is the phase that you would expect that substance to have at P = 1 bar and your temperature of interest. For example, at T = 298 K the phases of some elements in their standard states are given in the table below.
Some elements can exist in multiple forms (allotropes). For example, carbon has several allotropes including diamond, graphite, C60, and carbon nanotubes. Since each allotrope can have a different enthalpy, for consistency we must select just one of these allotropes as the reference standard state. For elements with multiple allotropes, the standard state is selected (by IUPAC consensus) as either the most abundant or most stable form at P = 1 bar. For carbon, the standard state is solid graphite, C(s), graphite, because this is the most thermodynamically stable form at P = 1 bar. The other allotropes of carbon are less stable. Like carbon, sulfur has a large number of allotropes. The standard state of sulfur is the cyclic form S8(s), which is the most abundant form of elemental sulfur. The standard state of phosphorus is white phosphorus, P4(s), because it is more abundant at P = 1 bar than the more stable black phosphorus.
In addition to specifying the phase and allotrope (when relevant) to define the standard state, a few more pieces of information must be established before we can start quantifying changes in enthalpy, ΔH. Standard state specifies a pressure of 1 bar, as well as extra conditions for each state of aggregation (solid, liquid, gas, or solution), as shown in the table below.
Avoid this common error of confusing standard state with STP
Standard state specifies a pressure (P = 1 bar) and a purity. It is different from standard temperature and pressure (STP: T = 273 K and P = 1 bar) and standard ambient temperature and pressure (SATP: T = 298 K and P = 1 bar). Unlike STP and SATP, standard state does not specify a particular temperature. For standard state, the temperature is indicated separately. When consulting tables with data collected at standard state, you will normally find the relevant temperature indicated in the chart heading or caption. Note that some tabulated standard state data is reported for P = 1 atm instead of P = 1 bar because standard state pressure was defined as P = 1 atm until 1982.
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