What does the standard enthalpy change refer to?

The standard enthalpy change specifically refers to the change in enthalpy for substances when they are in their standard states, which is a defined reference point for measuring thermodynamic properties. Standard states typically are considered as 1 bar (or 1 atm) of pressure and a specified temperature, usually 25°C (298 K). This concept allows for the comparison of enthalpy changes for different reactions under uniform conditions, facilitating a consistent basis for thermodynamic calculations.

The standard state provides a way to express the energy changes associated with chemical reactions, making it essential for calculating reaction enthalpies, formation enthalpies, and other thermodynamic properties. By focusing on the standard states, it also removes variations that can arise due to different physical states or environmental variables, enabling clearer insights into the nature of the energy changes that occur during reactions.