RE: What are the applications of Hess’s law ?
(a) Determination of heat of formation; Compounds whose heats of formation cannot be measured directly using calorimetric methods because they cannot be synthesised from their elements easily e.g. methane, carbon monoxide, benzene etc are determined using Hess’s Law.
For example, the heat of formation of carbon monoxide can be calculated from the heat of combustion data for carbon and carbon monoxide as shown above.
(b) Determination of heat of transition; The heats of transition of allotropic modification of compounds such as diamond to graphite, rhombic sulphur to monoclinic sulphur, yellow phosphorous to red phosphorous etc. can be determined using Hess’s Law.
For example, the heat of transition of diamond to graphite can be calculated from the heat of combustion data for diamond and graphite, which is -395.4 kJ and -393.5 kJ respectively.
(c) Determination of heat of hydration
The heats of hydration of substances is calculated using Hess’s law.beor example the heats of hydration of copper sulphate can be calculated from the heats of solution of anhydrous and hydrated salts of copper. The heats of solution of CuSO4 and CuSO4.5H2O are -66.5 and -11.7 kJ mol-1. The corresponding thermochemical equations are: DH1 = DH + DH2
(d) Determination of heats of various reactions; Hess’s law is useful in calculating the enthalpies of many reactions whose direct measurement is difficult or impossible.