kJmol

kJ/mol, or kilojoules per mole, is a unit of energy normalized by the amount of substance. It expresses the amount of energy associated with a chemical process, such as a reaction, phase change, or formation step, per mole of reacting or formed substance. In thermochemistry, many tabulated values are given in kJ/mol, including standard enthalpies of formation (ΔHf°), reaction enthalpies (ΔHrxn), lattice energies, and often bond dissociation energies when stated per mole of bonds involved.

Interpretation and usage:

A negative value in kJ/mol indicates that the process releases energy (exothermic) per mole, while a positive value indicates energy absorption (endothermic). The unit is convenient because chemical quantities are usually given per mole of substance, aligning with how reactions are described in stoichiometry.

Relationship to other units:

1 kJ/mol equals 1000 J per mole. To convert to energy per molecule, divide by Avogadro’s constant (NA ≈ 6.022×10^23): 1 kJ/mol ≈ 1.66×10^-21 J per molecule. This reflects the difference between per-mole and per-molecule energy scales commonly used in chemistry.

Context and examples:

Standard enthalpies of formation for compounds are frequently reported in kJ/mol (for example, ΔHf°(H2O, l) ≈ -285.8 kJ/mol). Activation energies are also often expressed in kJ/mol, representing the energy barrier per mole of reacting species. It is important to note that kJ/mol is an intensive way to express energy changes linked to the amount of substance involved and is not a single intrinsic property of a molecule alone.