Sublimation enthalpy is the energy required to convert a solid directly into a gas, while atomisation enthalpy refers to the energy needed to separate the atoms within a molecule.

Both sublimation enthalpy and atomisation enthalpy quantify energy changes in substances, but they pertain to distinct processes. Sublimation enthalpy, also known as the heat of sublimation, represents the amount of energy necessary for a substance to transition from the solid phase to the gas phase without passing through the liquid phase. This specific phase transition occurs under certain conditions of temperature and pressure. For instance, dry ice (solid carbon dioxide) sublimates at room temperature, transforming directly into carbon dioxide gas.

In contrast, atomisation enthalpy, or heat of atomisation, measures the energy required to completely separate all the atoms in a molecule. This value indicates the strength of the bonds within the molecule: the higher the atomisation enthalpy, the stronger the atomic bonds. Typically, this process occurs at elevated temperatures and is often assessed in the gas phase.

Both sublimation and atomisation enthalpies are inherently positive values, as they both necessitate energy input to occur. However, the specific values vary depending on the substance in question. For example, the sublimation enthalpy of iodine is significantly lower than its atomisation enthalpy, indicating that it is easier to convert solid iodine into gas than to separate the iodine atoms within a molecule.

In summary, while sublimation enthalpy and atomisation enthalpy both reflect energy changes, they are associated with different processes. Sublimation enthalpy denotes the energy needed for a solid to transform into a gas, whereas atomisation enthalpy indicates the energy required to dissociate the atoms in a molecule. A solid understanding of these concepts is vital for comprehending the behavior of substances under various conditions.