Question

Standard enthalpies of formation are relative values. What are \(\Delta H_{\mathrm{f}}^{\circ}\) values relative to?

Short answer

The standard enthalpies of formation values (\(\Delta H_{\mathrm{f}}^{\circ}\)) are relative to the constituent elements in their most stable forms (standard states) at specific reference conditions (typically 298 K and 1 atm pressure).

Step-by-step solution

Understanding Standard Enthalpy of Formation

Standard enthalpy of formation, denoted as \(\Delta H_{\mathrm{f}}^{\circ}\), is the change in enthalpy that occurs when one mole of a compound is formed from its constituent elements in their standard states at a particular reference temperature, usually 298 K and 1 atm pressure. It is a measure of the energy released or absorbed during the formation of the compound.

Reference point for Standard Enthalpy of Formation

The reference point for standard enthalpies of formation is the state of the constituent elements in their most stable forms (standard states) at the specified reference conditions (typically 298 K and 1 atm pressure). The standard state is the most stable physical state of an element at the specific reference conditions. For example, the standard state of oxygen is O2 gas and the standard state of carbon is graphite.

Relation to Standard Enthalpies of Formation

Since the enthalpy of formation of a compound is the energy change when one mole of the compound is formed from its constituent elements, the \(\Delta H_{\mathrm{f}}^{\circ}\) values are relative to the energies of the constituent elements in their standard states. Thus, the standard enthalpies of formation values (\(\Delta H_{\mathrm{f}}^{\circ}\)) are relative to the constituent elements in their most stable forms (standard states) at specific reference conditions (typically 298 K and 1 atm pressure).