[Saravanan]

Why is it impossible to determine lattice enthalpy directly by experiment when we are able to determine enthapy of formation experimentally ?

[mjc123]

Because you can't take an ionic solid and convert it into a gas of ions. That's not a physically realistic process. You have to infer it using a Born-Haber cycle and other quantities that can be determined experimentally.

[Saravanan]

Thanks for your answer

[AdiDex]

The answer is hidden in the Definition Of "Lattice Enthalpy"...

Which states that the total enthalpy change if the component ions (Gaseous) were brought together from infinity.

As you can't seperate them to the infinite ..
You also know that you can't find the total enthalpy of system , you can just find enthalpy change .
The reason is similar .
Because you can't destroy it completely so you can check total enthalpy consisting in it...