Enthalpy of Formation

Every element in its standard state, like O2(g), N2(g), or solid graphite, is assigned a formation enthalpy of exactly zero. That is not a measurement, it is a reference point, the same way sea level is defined as zero elevation. Everything else is measured relative to it. A compound's standard formation enthalpy, delta Hf, is the energy change when one mole of that compound forms from its elements in their standard states. If delta Hf is negative, the compound is more stable, lower in energy, than the separate elements it came from.

Because enthalpy is a state function, the delta H of any reaction equals the sum of the formation enthalpies of the products minus the sum of the formation enthalpies of the reactants, each multiplied by its coefficient. This works even for reactions nobody has run in a calorimeter, because you can imagine breaking every reactant down into its elements and then rebuilding the products from those same elements. The path does not matter, only the starting and ending points, so tabulated values for thousands of compounds let you predict delta H for a reaction just by looking up numbers and subtracting.

This sits next to Hess's law and bond enthalpies as another route to the same answer, delta H for a reaction, and it connects back to calorimetry, which is how those tabulated formation values were originally measured. A negative delta Hf tells you a compound is thermodynamically stable relative to its elements, but says nothing about how fast it forms or decomposes. That speed question belongs to kinetics and activation energy, a completely separate idea covered elsewhere.