How is the heat of formation of a substance defined?

The heat of formation of a substance, also known as standard enthalpy of formation, is defined specifically as the energy change that occurs when one mole of a compound is formed from its constituent elements in their standard states. This definition is significant because it provides a standardized way to measure the energy associated with the formation of various compounds, making it easier to compare and calculate reaction enthalpies.

When referring to "standard states," we are talking about the most stable physical forms of the elements at a defined set of conditions, usually at a pressure of 1 atmosphere and a specified temperature, typically 25 degrees Celsius (298 K). This focus on elements in their standard states is crucial, as it ensures consistency in measurements and allows for accurate calculations in thermodynamic processes.

Understanding this definition is essential for various applications in chemistry, including predicting reaction behavior, calculating energy changes in reactions, and optimizing synthetic pathways in chemical processes.