Notes on Standard State

Introduction

A material’s standard state serves as a reference point for identifying its thermodynamic state properties, such as enthalpy, entropy, Gibbs free energy. The standard state is used to calculate the properties of a material under various conditions and is denoted as Hf⊖. The hypothetical state at 1 bar is the standard state for a gas; the pure substance at 1 bar is the standard state for liquids and solids; the most stable allotrope is the standard state for elements; and the concentration at 1 M and 1 bar is the standard state for a substance in solution (solute).

Body

Gases

The hypothetical state of a gas as a pure substance, obeying the ideal gas equation at standard pressure, is referred to as its standard state (105 Pa, or 1 bar). There is no real gas that shows perfectly ideal behaviour. However, this definition of the standard state allows for consistent corrections for non-ideality across all gases.

Liquids and Solids

The standard state for liquids and solids is simply the pure state under a total pressure of 105 Pa (or 1 bar). The reference point of Hf = ⊖ is defined for most elements for their most stable allotropes, such as graphite in the case of carbon, and the -phase (white tin) in the case of tin. White phosphorus, the most common allotrope of phosphorus, is an exception because it is defined as the standard state despite being only metastable.

Solutes

For comparing free energies of chemical processes, it is convenient to normalise free energy changes. It helps to eliminate the differences in reaction volumes. This is achieved by using  Go, or the molar free energy change, and a standard state for reactants in solution.

Under normal temperature and pressure (NTP) of 25oC and 1 atm pressure,

• solutes are in their standard states when they have an activity of 1 M;
• gases are in their standard states when they are at a pressure of 1 atm.

Adsorbates

The adsorbate molecules are mainly constituted by linear paraffin hydrocarbons—the remainder from saturated fatty acids, and/or other active substances in the base liquid. Adsorbate diffusion is the process by which adsorbates bonded to a solid surface can change their adsorption positions by thermal fluctuations. 

Elements in their standard states are assumed to have chemical potentials and enthalpies of 0, or µoelement = 0. The standard state of an element is its natural state at 1 atm pressure and 25oC.

To define the free energy of elements in this manner, assume that the compounds have a chemical potential (partial free energy) or an enthalpy of formation. The enthalpy of formation is the sum of all changes in chemical potential (or enthalpy) of a reaction, leading to product formation by a convenient path. 

Free energy and enthalpy are state variables, so the value is a unique function of the state. It shows that this method can be used to define the relative energy content of any chemical system by referring to the work required to get there, starting from the elements.

Standard state conditions for compounds

• A gas is said to be under standard state conditions if it is at 1 atm pressure.

• A solute is said to be under standard state conditions if its concentration is 1 M.

• Pure solids and liquids are in their standard states.

Standard state conditions for elements

• An element is said to be under standard state conditions if it exists at 1 atm pressure and 25 °C temperature. (By the way, standard state conditions are different from STP.)

Enthalpy of formation

The enthalpy of formation is the standard reaction enthalpy for the formation of the compound from its elements (atoms or molecules) in their most stable reference states at 298.15 K temperature and 1 bar pressure. The standard conditions for which most thermochemical data are tabulated are a pressure of 1 atmosphere (atm) for all gases and a concentration of 1 M for all species in solution (1 mol/L).

Standard conditions serve as a reference point for measuring differences in enthalpy. The standard enthalpy of formation is the enthalpy change for the formation of 1 mole of a compound from its component elements in standard states. The standard enthalpy of formation of any element in its most stable form is zero, by definition.

Conclusion

The standard state is the reference state for thermodynamic state parameters of substances. The change in the standard enthalpy formation for a material in its standard state is zero. This convention allows for the calculation and tabulation of a wide range of thermodynamic parameters.