What does the standard enthalpy change of formation represent?

The standard enthalpy change of formation specifically refers to the amount of energy absorbed or released when one mole of a substance is formed from its constituent elements in their standard states. This definition is fundamental in thermochemistry and underlies many applications, such as calculating the heat of reactions and understanding stability and reactivity of different compounds.

When assessing this context, it is important to recognize that the details of the elements’ states are crucial; standard states refer to the pure forms of the elements at 1 atm pressure and a specified temperature, typically 25°C. Thus, option B accurately captures the essence of standard enthalpy change of formation.

In contrast, the other options describe different processes that do not align with the definition of the standard enthalpy change of formation. For example, the formation of a gas from liquids pertains to changes in state rather than the formation from elements, and while combustion involves enthalpy changes, it specifically focuses on the reaction of substances with oxygen, not the formation of those substances from their elements. Additionally, the change when a solid is formed from a liquid relates to processes like crystallization rather than formation from elemental states.