What are the units of molar enthalpy change?

Molar enthalpy change is defined as the amount of heat energy absorbed or released during a chemical reaction per mole of substance. The units for molar enthalpy change need to reflect both energy and the amount of substance involved in a reaction.

The correct unit, kJ/mol, indicates that the energy change is measured in kilojoules (kJ) for each mole of the substance. This aligns precisely with the definition of molar enthalpy, making it essential for thermochemical calculations, where it is common to express energy changes in kilojoules for practicality, especially in reactions where large quantities are involved.

Other units provided, such as J/g or kJ/g, measure energy per gram, which rather refers to specific enthalpy changes rather than molar enthalpy changes. Similarly, cal/mol could be a valid measure of energy per mole, but in this context and under standard conventions, kilojoules are more commonly used, especially at the IGCSE level. Therefore, while cal/mol may express similar ideas, kJ/mol is the standard unit preferred in most thermochemical reactions.