The  ΔfH∘  of  BaSO4(s), Ba2+(aq)  and  SO42−(aq)  are  −350.6, −128.3  and  −216.3kcal . The heat of reaction (in kcal) is minus:

The heat of reaction, also known as the enthalpy change (ΔH), can be calculated using Hess's Law, which states that the total enthalpy change for a reaction is the sum of the enthalpy changes for each step in the reaction and is independent of the path taken.

In this case, the reaction is:

Ba2+(aq) + SO42−(aq) → BaSO4(s)

The enthalpy change for this reaction (ΔH) is the sum of the enthalpy of formation of the products minus the sum of the enthalpy of formation of the reactants.

So, ΔH = [ΔfH°(BaSO4)] - [ΔfH°(Ba2+) + ΔfH°(SO42−)]

Substituting the given values:

ΔH = [−350.6 kcal] - [−128.3 kcal + (−216.3 kcal)]

ΔH = −350.6 kcal - (−344.6 kcal)

ΔH = −350.6 kcal + 344.6 kcal

ΔH = −6 kcal

So, the heat of reaction is -6 kcal.