The kilocalorie per mole is a unit to measure an amount of energy per number of molecules, atoms, or other similar particles. It is defined as one kilocalorie of energy (1000 thermochemical gram calories) per one mole of substance, that is, per Avogadro's number of particles. It is abbreviated "kcal/mol" or "kcal mol-1". As typically measured, one kcal/mol represents a temperature increase of one degree Celsius in one liter of water (with a mass of 1kg) resulting from the reaction of one mole of reagents.
In SI units, one kilocalorie per mole is equal to 4.184 kilojoules per mole, or joules per molecule, or 0.043 eV per molecule. At room temperature (25 °C, 77 °F, or 298.15 K) it is equal to 1.688 units in the kT term of Boltzmann's equation.
Even though it is not an SI unit, the kilocalorie per mole is still widely used in chemistry for thermodynamical quantities such as thermodynamic free energy, heat of vaporization, heat of fusion and ionization energy, due to the ease with which it can be calculated based on the units of measure typically employed in quantifying a chemical reaction, especially in aqueous solution. Typically but not exclusively, kcal/mol are used in the United States, whereas kJ/mol are preferred elsewhere.