Standard State and Enthalpy of Formation, Gibbs Free Energy of Formation, Entropy and Heat Capacity

Definition and explanation of the terms standard state and standard enthalpy of formation, with listing of values for standard enthalpy and Gibbs free energy of formation, as well as standard entropy and molar heat capacity, of 370 inorganic compounds.

The term standard state is used to describe a reference state for substances, and is a help in thermodynamical calculations (as enthalpy, entropy and Gibbs free energy calculations). The superscript degree symbol (°) indicates that substances are in their standard states. (ΔH°, ΔG°, S°. )

Definitions of standard states:

• For a gas, the standard state is as a pure gaseous substance as a (hypothetical) ideal gas at a pressure of exactly 1 bar.
• For a substance present in a solution, the standard state is a concentration of exactly 1 M at an applied pressure of 1 bar, but exhibiting infinite-dilution behavior. (Hence taking infinite-dilution behavior to be the standard state allows corrections for non-ideality to be made consistently for all the different solutes.)
• For a pure substance in a condensed state (liquid or solid), the standard state is the pure liquid or solid under 1 bar pressure.
• For an element the standard state is the form in which the element exists (is more stable) under condition of 1 bar and at the temperature of interest (usually 25°C).
  • ΔH 0 _f for an element in its standard state is zero. Thus, elements in their standard states are not included in the ΔH_reaction calculations.

  Note! Standard state is NOT the same as standard temperature and pressure (STP) for a gas, and must not be confused with this term.

  Enthalpy is a state function, defined by the internal energy (E), the pressure (P) and volume (V) of a system:

  H = E + PV and ΔH = ΔE + Δ(PV)

  For enthalpy, there are no method to determine absolute values, only enthalpy changes (ΔH values) can be measured. Then it is important to have a common and well defined reference state. Since enthalpy is a state function, a change in enthalpy does not depend on the pathway between two states.

  Hess's law: In going from a particular set of reactants to a particular set of products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps.

  At constant pressure: ΔH = q_p (q_p = heat from or to the chemical system at constant pressure, q is also called heat of reaction)

  Exothermic reaction: negative ΔH (heat transferred to the surroundings from the system)

  Endothermic reaction: positive ΔH (heat adsorbed by the system from the surroundings)

  The standard enthalpy of formation (ΔH 0 _f) of a compound is the change in enthalpy that accompanies the formation of 1 mole of a compound from its elements with all substances in their standard states.

  The table below shows the standard enthalpy of formation, the standard Gibbs free energy of formation, standard entropy and molar heat capacity at constant pressure of several inorganic compounds.

  The specific heat capacity can be calculated from the molar heat capacity, and vise versa:

  c_p = specific heat capacity

  M = molar weight of the actual substance (g/mol).