Chapter 2:- Periodic properties of the elements
B.sc 1st year Book
(Page 8)
Determination of electron affinity (Born-Haber Cycle)
In 1919 Born and Haber devised a cyclic scheme that relates the lattice energy of a crystal to other thermodynamical data. In this scheme, the formation of sodium chloride crystal starts with the conversion of elements into the gaseous atom and then into ions, finally packed into the crystal lattice. During the process of sublimation, dissociation and ionization energies are supplied to the system whereas electron affinity and lattice energy are negative because they are evolved in these processes.
According to Hess’s law of constant heat summation, the overall charge in energy Depends upon initial and final states and not on the route taken during the processes. If ΔHf heat of formation then the algebraic sum of the total energy going around the cycle is represented as :
$$ \Delta H_f=S+1+\frac{1}{2}D-EA-U $$
Where ‘S’ is sublimation energy, ‘T‘ is ionization energy, ‘D’ is dissociation energy, ‘EA’ is electron affinity and ‘U’ is lattice energy,
By putting the values of S, I, D, ΔHf, and U in the above expression we can calculate the value of electron affinity.
(Note: When the lattice energy of a crystal is lower, the solid dissolves, and in the case of high: lattice energy it does not dissolve easily or becomes insoluble).