This law is used to calculate a solid's lattice energy in the Born-Haber cycle. Given the heat of formation for hydrochloric acid and the bond energies of hydrogen gas and chlorine gas, it can be used to find the bond energy of hydrochloric acid. The quantity central to this statement is the sum of internal energy and pressure times volume. For 10 points, name this law of constant heat summation, which states that adding the change in enthalpy for each step of a given reaction gives the total change in enthalpy for the 
This law is used to calculate a solid's lattice energy in the Born-Haber cycle. Given the heat of formation for hydrochloric acid and the bond energies of hydrogen gas and chlorine gas, it can be used to find the bond energy of hydrochloric acid. The quantity central to this statement is the sum of internal energy and pressure times volume. For 10 points, name this law of constant heat summation, which states that adding the change in enthalpy for each step of a given reaction gives the total change in enthalpy for the whole reaction, and is named for a Swiss chemist.
This law is used to calculate a solid's lattice energy in the Born-Haber cycle.
Hess's law [or law of constant heat summation before read; or law of constant enthalpy summation before "enthalpy"]