Final answer:
To determine the enthalpy change of a reversible reaction from known entropy change, use ΔS = qrev / T and solve for qrev which is equal to ΔH in cases involving only solids at constant pressure. Hess's Law can also be used if standard enthalpies of formation are available.
Step-by-step explanation:
To determine the change in enthalpy (ΔH) of a reversible reaction when the change in entropy (ΔS) is known, one can use the thermodynamics equation for a reversible process at constant pressure where ΔS is the heat transferred (qrev) divided by temperature (T). If the reaction involves only solids, ΔH and ΔU (change in internal energy) are essentially equal, meaning we can use ΔH in place of qrev. By substituting the known values of ΔS and T into ΔS = qrev / T, and rearranging it to qrev = ΔS × T, we can calculate the ΔH as the qrev value. It's important to check if the calculated ΔH makes sense; for example, in a reversible process, the total entropy should not decrease.
Using Hess's Law, the ΔH for a reaction can also be calculated from enthalpies of formation if the substances involved have known standard enthalpies. However, determining ΔH from ΔS directly applies to reversible processes at constant pressure where there is a direct relationship between the two quantities.