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 (\(\Delta H_{\mathrm{f}}^{\circ}\)) values are relative to the enthalpy of the pure elements in their standard states at a specific temperature and pressure, usually 25°C (298.15 K) and 1 atm pressure. By convention, the enthalpy of formation for the standard state of an element is assigned a value of zero.

Step-by-step solution

Definition of Standard Enthalpy of Formation

The standard enthalpy of formation (\(\Delta H_{\mathrm{f}}^{\circ}\)) is the change in enthalpy during the formation of one mole of a substance in its standard state from its constituent elements in their standard states. It is a thermodynamic quantity that helps in understanding the stability of a compound and predicting the heat changes during chemical reactions.

The Reference State

The \(\Delta H_{\mathrm{f}}^{\circ}\) values are relative to the enthalpy of the pure elements in their standard states at a specific temperature and pressure, which is usually 25°C (298.15 K) and 1 atm pressure. The standard state of an element is the most stable form of the element at the specified temperature and pressure. For example, the standard state of oxygen is O2 gas, the standard state of carbon is graphite, and the standard state of hydrogen is H2 gas. By convention, the enthalpy of formation for the standard state of an element is assigned a value of zero.

Interpretation of \(\Delta H_{\mathrm{f}}^{\circ}\) Values

When comparing \(\Delta H_{\mathrm{f}}^{\circ}\) values for different compounds, a more negative value indicates a more stable compound (with respect to its constituent elements in their standard states), as more energy is released during its formation. Conversely, a more positive value indicates a less stable compound. In conclusion, the standard enthalpy of formation (\(\Delta H_{\mathrm{f}}^{\circ}\)) values are relative to the enthalpy of the pure elements in their standard states and are a useful tool for understanding compound stability and predicting heat changes in chemical reactions.