What is the term for the enthalpy change for a reaction that can only be measured if reactants and products are in a gaseous state?

The correct term for the enthalpy change for a reaction that can only be measured if reactants and products are in a gaseous state is average bond enthalpy. Average bond enthalpy refers to the energy required to break one mole of a given type of bond in a gaseous compound, averaged over similar compounds. This term is often relevant when discussing enthalpy changes in gaseous reactions, as the measurements typically refer specifically to reactions occurring in the gas phase where the intermolecular interactions differ significantly from those in liquids or solids.

In contrast, standard enthalpy usually applies more broadly to processes under standard conditions, which may include various phases, not specifically just gases. Reaction enthalpy is a more general term that can refer to changes in enthalpy for any reaction, irrespective of the state of the reactants or products. Heat capacity pertains to the amount of heat required to change the temperature of a unit quantity of a substance by one degree Celsius and does not directly relate to the enthalpy change of a specific reaction.