Hess's Law
The total enthalpy change of a reaction is the same regardless of the route taken, provided the initial and final states are identical.
Formula / equation
ΔH_total = ΣΔH_steps
Unit
kJ/mol
In depth
Hess's law follows directly from enthalpy being a state function and from the first law. It allows enthalpy changes that cannot be measured directly — such as the formation of carbon monoxide, which is always contaminated by CO₂ — to be calculated from reactions that can.
Examples in the real world
Combining the combustion enthalpies of carbon and CO gives the formation enthalpy of CO as −110.5 kJ/mol.