kcalmol

Kilocalories per mole (kcal/mol) is a unit of energy per amount of substance used in chemistry to describe enthalpy changes, bond dissociations, and activation barriers. One kilocalorie equals 1000 calories, and in SI terms 1 kcal = 4184 joules. Therefore 1 kcal/mol corresponds to 4184 J per mole, or 4.184 kJ/mol. The unit is common in thermochemistry and computational chemistry because many bond energies and reaction enthalpies fall in the tens to hundreds of kcal/mol.

In practice, kcal/mol is used to express the energy change associated with breaking or forming bonds, or the energy barrier that must be overcome for a reaction to proceed. Values are obtained from experimental calorimetry or referenced from thermochemical data tables, and from quantum-chemical calculations. The exact value for a given bond or process can vary with molecular environment and measurement method.

Typical magnitudes include bond dissociation energies such as H–H about 104 kcal/mol, C–H around 98 kcal/mol, C–C near 85 kcal/mol, C=O roughly 179 kcal/mol, and O=O about 119 kcal/mol. Activation energies for many organic reactions often lie in the range of 15 to 40 kcal/mol, though larger barriers exist for slower processes.

Note that kcal/mol is a non-SI unit; it is convenient in chemistry because energy scales of molecular interactions commonly fall in this range. For SI usage, the equivalent expression is kJ/mol, with 1 kcal/mol = 4.184 kJ/mol. The unit is sometimes written as kcal mol−1.