[Corribus]

No - what is the definition of enthalpy of formation? You either need to look up the enthalpy of dissolution, or calculate it from the enthalpies of formation of the solid sulfate and the dissolved ions.

[BHAVESH]

you mean to say based on Lattice & Hydration Energy should I calculate? As mentioned below

[1ΔHf(Ca+2 (aq)) + 1ΔHf(SO4-2 (aq))] - [1ΔHf(CaSO4 (s))]
[1(-542.83) + 1(-909.27)] - [1(-1434.5)] = -17.5999999999999 kJ
-17.60 kJ     (exothermic)
And use this as ΔH° KJ mol^1-

[mjc123]

Yes. I have told you before - the ΔH you should use is that of the equilibrium reaction under consideration.. In this case it is
CaSO₄(s)  Ca²⁺(aq) + SO₄^2-(aq)
This is not the formation reaction of CaSO₄, so ΔH°_f(CaSO₄) is not the thing to use.
Generally ΔH°_reaction = ΣΔH°_f(products) - ΣΔH°_f(reactants).
Next time you have a van't Hoff equation problem, write down the equilibrium reaction and determine what ΔH you need to use.

[BHAVESH]

Dear mjc123,
Thank you very much for your guidance. However, the reaction ΔH°reaction = ΣΔH°f(products) - ΣΔH°f(reactants) you have given but the product is CaSO₄ and reactants are Ca²⁺ & SO₄^2- in saturated water.
when it is calculated it comes to 17.6 KJ/mol (Endothermic)??

[mjc123]

No, in the equilibrium for the dissolution of CaSO₄, which is what Ksp refers to, CaSO₄(s) is the reactant and Ca²⁺(aq) + SO₄^2-(aq) are the products, as I wrote the equation above.

[BHAVESH]

Thank you very much dear for making me understand (sometimes even silly things!!!) the conceptual thing.

[BHAVESH]

How to calculate Lattice Enthalpy of Formation of individual element like Ca²⁺, Ba²⁺ etc...
Normally, that we use in enthalpy calculation. For an example we use -537.6 kj mol^-1 for Ba²⁺. So how do we calculate this???

probably, the reaction is Ba_(s)<----> Ba²⁺ + 2e^-

[mjc123]

There is no such thing as lattice enthalpy of formation. Lattice energy and enthalpy of formation are different things.
-537.6 kJ/mol is the enthalpy of formation of Ba²⁺(aq). This is not calculated; it is derived from experiment. It can't be directly measured, but it can be derived from things that can be measured, by Hess's law, and some necessary conventions (e.g. that ΔH_f° of H⁺(aq) ≡ 0).

[BHAVESH]

Thank you very much. One Lesson over!!!