What are the units of standard enthalpy change of formation?

• A. kJ/mol
• B. J/mol
• C. J
• D. kcal/mol

Answer: (A)

The key idea is that standard enthalpy change of formation is the energy change that occurs when one mole of a compound is formed from its elements in their standard states. Since it’s an energy change associated with a specific amount of substance, the units must express energy per amount of substance. Energy in chemistry is routinely reported in kilojoules, and the amount is per mole, so the standard units are kilojoules per mole (kJ/mol). While joules per mole (J/mol) would be dimensionally correct, the convention is to use kJ/mol to keep numbers practical; joules alone would ignore the per-mole aspect, and kcal/mol is a non-SI unit not typically used in standard tables. Hence, the correct units are kJ/mol.