standard enthalpy of formation
The standard enthalpy of formation is the heat change when one mole of a compound is built up from its raw elements, each in its most stable everyday form, under standard conditions. Think of it as the energy price tag for assembling a substance from scratch: how much heat is released or required to make one mole of water, or carbon dioxide, or table salt, starting from pure hydrogen, oxygen, carbon, sodium, and so on.
It is written ΔH_f° and serves as a universal reference point. The elements in their standard states are assigned a formation enthalpy of exactly zero — they are the agreed-upon "sea level" from which everything else is measured. A compound with a large negative ΔH_f°, like water, sits in a deep energy valley: it releases a lot of heat when it forms and is correspondingly stable.
These tabulated values are the workhorses of thermochemistry. Because enthalpy is a state function, the enthalpy of almost any reaction can be found by subtracting the summed formation enthalpies of the reactants from those of the products. One table of formation enthalpies unlocks the heat of countless reactions you never have to run.
The standard formation enthalpy of liquid water is about ΔH_f° = −286 kJ/mol: making one mole of water from hydrogen and oxygen gas releases 286 kilojoules. By definition, pure oxygen gas and pure hydrogen gas each have ΔH_f° = 0.
Elements in their standard form are the zero mark; compounds are measured from there.
"Most stable form" is precise and occasionally surprising: for carbon it is graphite, not diamond; for oxygen it is O₂ gas, not ozone. The little degree symbol (°) marks the standard state — a specified pressure (now 1 bar) and a stated temperature, usually 298 K. Change the state and the value changes.