How does sublimation enthalpy differ from atomisation enthalpy?

Sublimation enthalpy refers to the energy required to change a solid to a gas, while atomisation enthalpy refers to the energy needed to separate atoms in a molecule.

Sublimation enthalpy and atomisation enthalpy are both measures of energy changes in substances, but they refer to different processes. Sublimation enthalpy, also known as heat of sublimation, is the amount of energy that is required to change a substance from the solid phase directly to the gas phase without passing through the liquid phase. This is a specific type of phase transition that occurs under certain conditions of temperature and pressure. For example, dry ice (solid carbon dioxide) sublimates at room temperature, turning directly into carbon dioxide gas.

On the other hand, atomisation enthalpy, also known as heat of atomisation, is the energy required to separate all the atoms in a molecule. This is a measure of the strength of the bonds within the molecule. The higher the atomisation enthalpy, the stronger the bonds within the molecule. This process usually occurs at very high temperatures and is often measured in the gas phase.

In terms of their values, both sublimation and atomisation enthalpies are always positive because they both require energy to occur. However, the specific values will depend on the substance in question. For example, the sublimation enthalpy of iodine is much lower than its atomisation enthalpy because it is easier to turn solid iodine into gas than it is to separate the iodine atoms in a molecule.

In summary, while both sublimation and atomisation enthalpies are measures of energy changes, they refer to different processes. Sublimation enthalpy is the energy required for a solid to become a gas, while atomisation enthalpy is the energy needed to separate the atoms in a molecule. Understanding these concepts is crucial for understanding the behaviour of substances under different conditions.