In the case of enthalpy a reference

In the case of enthalpy a reference temperature of 25◦C and a reference pressure
of 1 atmosphere (standard temperature and pressure, STP) are assumed. It is also assumed that the reference condition for the chemically stable form of an element at 1 atmosphere and 25◦C is zero. For example, the stable form of oxygen at STP is gaseous O2, so the enthalpy for O2(g) is zero, where (g) just means it is in the gaseous state. On the other hand, since atomic oxygen (O) is not stable, its enthalpy is not zero (it is, in fact, 247.5 kJ/mol). Notice that the
state of a substance at STP matters. Mercury, for example, at 1 atmosphere and 25◦C is a liquid, so the standard enthalpy for Hg(l) is zero, where (l) means the liquid state.