kilojoule per mole

The kilojoule per mole is the currency chemistry keeps its accounts in. Bond energies, lattice energies, activation energies, enthalpies of formation and the converted output of every quantum chemistry code all land in kJ/mol, and the Gibbs and Arrhenius solvers on this site expect their energies in it.

The number that gives the unit meaning is 2.5. RT, the thermal energy per mole at room temperature, is $8.314 \times 298 \approx 2480$ J/mol, and every chemical energy is implicitly measured against it: a hydrogen bond at 20 kJ/mol is eight RT, strong enough to structure water yet weak enough to break and reform constantly, while a C-C bond at 350 kJ/mol is 140 RT, which is why molecules survive being warmed.

The same comparison explains a rule biology inherited. Run the Arrhenius equation with a typical activation energy near 50 kJ/mol and a rise from 25 to 35 °C multiplies the rate by $e^{0.66} \approx 1.9$; that is the entire origin of "reaction rate doubles every ten degrees", and the rule is exactly as good as the 50 kJ/mol assumption, failing on either side of it.

The bridge to physics is worth memorising: one electronvolt per particle is 96.5 kJ/mol, so silicon's 1.1 eV band gap and a 106 kJ/mol reaction barrier are the same size of energy written in two dialects.