The Enthalpy Change for a Reaction Can Be Calculated Using $\circ$

The enthalpy change for a reaction can be calculated using the equation
DH°_rxn It would be possible to use DS ^$\circ$ _f values to calculate DS ^$\circ$ _rxn in a similar manner; however, this is not done in practice.
a. Describe the procedure which is used instead.
b. Explain why this mathematical process is possible for entropy calculations but not for enthalpy calculations.