What is the standard enthalpy change of formation?

The standard enthalpy change of formation is defined as the enthalpy change that occurs when one mole of a compound is formed from its elements in their standard states. This means that for this term to be applicable, the reactants must be the pure elements, and their physical states (solid, liquid, gas) must be at standard conditions (usually 1 atmosphere of pressure and a specific temperature, typically 25 degrees Celsius).

This definition highlights the importance of considering both the identity and the condition of the reactants when measuring the enthalpy change associated with the formation of compounds. The standard enthalpy change of formation is a critical value in thermodynamics as it allows for the calculation of enthalpy changes in various chemical reactions using Hess's law and is essential in understanding energy changes during chemical processes.

Understanding this concept is fundamental for studying thermodynamics in chemistry, as it links the formation of chemical substances to energy changes in a quantifiable way.