Question

estimate the temperature at which CuSO4 5H2O (blue crystals) undergoes dehydration and turns grayish-white.

Answer 1

Note that you have the entropy and enthalpies for CuSO4.H2O and CuSO4 reversed. ΔH_f of CuSO4 is -771.36kJ/mol, and its standard entropy is +109 J/K (see first source).

At equilibrium, the free energy change of a reaction is zero. That implies that:

0 = ΔG = ΔH - T*ΔS

At equilibrium, then:

T = ΔH/ΔS

You are given the standard entropies and enthalpies of formation for the reactants and products , so calculate the entropy and enthalpy changes for the reaction:

CuSO4.5H20 -> CuSO4 + 5H2O

If you assume that the ΔH and ΔS of the reaction are independent of temperature (at least over some temperature range near standard conditions), you can use them into the above equation to calculate the equilibrium temperature. You should get T = 397.5 K.