A material’s standard state is a reference point used to determine its properties under various conditions. A thermodynamic quantity in the standard state is designated by a superscript circle or Plimsoll, such as a change in enthalpy (ΔH°), entropy (ΔS°), or Gibbs free energy (ΔG°). Since thermodynamic quantities are frequently expressed in terms of standard state conditions, it’s important to know all the basics of standard state.

Standard State

A thermodynamic quantity under standard state conditions is denoted by a superscript circle:

ΔH = ΔH°

ΔS = ΔS°

ΔG = ΔG°

A substance’s standard state does not have to exist in nature: for example, values for steam can be calculated even though steam does not exist under these conditions. The benefit of this method is that the tables of thermodynamic properties produced are self-consistent.

Rules of Convention

The following are the rules of the convention:

1. The standard state of any liquid or solid substance at any given temperature is the most stable form of that substance at one bar of pressure. Water at 5 C, for example, has a standard condition of ice at one bar of pressure; at +5 C, it has a standard state of liquid water at one bar of pressure. 

2.   The ideal gas standard state at a certain temperature is defined as the standard condition of gas at that temperature. A finite low pressure at which the real gas acts as an ideal gas is referred to as the ideal gas standard state. A material in its ideal gas standard state is a hypothetical substance with a pressure of one bar, but its molar enthalpy is that of the real gas at arbitrarily low pressure. This is because the enthalpy of an ideal gas is independent of pressure.
3. The standard enthalpy of formation is the enthalpy change for a reaction in which the product is one mole of the substance, and the reactants are the compound’s constituent elements in their standard states for any substance at a given temperature.
4.   The symbol for the standard enthalpy of formation is ΔfHo. In this symbol, the superscript degree sign shows that all the reactants and products are in their standard states. The enthalpy change for creating the particular compound from its constituents is denoted by the subscript f. 

The compound and additional criteria are frequently stated in parentheses after the symbol. The letters “s”, “l” (or “liq”), and “g” are used to represent the solid, liquid, and gas states, respectively. When a substance is in a crystalline condition, the letter “c” is occasionally used to show it. The ideal gas standard state is usually meant by the specification of the gas state in this context.

Tables of thermochemical data with standard formation enthalpies can be available in a variety of publications or internet sites. Values are available for various compounds at a variety of temperatures. These tables normally indicate the value of the standard enthalpy of formation at 298.15 K for compounds for which there is fewer data available. For example, 298.15 K is commonly shortened as 298 K.

5.   The standard enthalpy of formation is defined as zero for any element at any temperature. We choose the elements in their standard states with a common reference state for the enthalpies of all substances at a given temperature when we determine standard enthalpies of formation. While we might choose any arbitrary value for an element’s enthalpy in its normal state, zero is a particularly convenient choice.

The Ideal Gas Standard State

The ideal gas standard state is a helpful invention with further benefits that will become apparent as our research progresses. The difference between the enthalpy in the ideal gas state and the enthalpy at 1 bar is insignificant for permanent gases, for example, gases whose behaviour is roughly ideal regardless. The ideal gas standard state becomes a second standard state for volatile chemicals that are ordinarily liquid or solid at 1 bar. 

The term “standard state” is frequently used in the chemistry of solutions at extreme temperatures and pressures to refer to a hypothetical standard concentration, typically 1 mol/kg for solutes assuming optimal behaviour (i.e., an infinite dilution) or a unity molar fraction (for pure substances). It does not imply a specific temperature or pressure since, contrary to IUPAC advice, it is more convenient for discussing solutions over a wide range of temperatures and pressures.

Conclusion

The standard state is the reference state for thermodynamic state parameters such as enthalpy, entropy, Gibbs free energy, and many other material standards for a certain substance. The standard enthalpy change of formation for a material in its standard state is zero. This convention allows for the calculation and tabulation of a wide range of other thermodynamic parameters.