Question

. The Madelung constant for ZnS is 1.638 (for the sphalerite crystal structure). Please calculate the molar cohesive energy density of ZnS from its ionic pair interaction energy.

Hints: Treat the compound as Zn+2S-2. Please assume that the ionic radius of Zn2+ is 0.074 nm and the ionic radius of S2- is 0.184 nm. Remember units!   

Answer 1

The intermolecular cohesive energy can also be defined as the energy of mutual attractive force of molecules. It can be determined simply as an energy needed for the transition of a molecule from the liquid phase, where molecules are very close to each other and interactions are strong, to the gaseous phase, where molecules are so far from one another, that considering the fall of the cohesive energy with 6 power of the distance, the interactions can be consider zero. The amount of energy needed for the transition of a mol of a material from the liquid to the gaseous phase serves then as a measure of the mutual attractiveness of molecules. This parameter is called the density of the cohesive energy that is defined as an evaporative internal energy of a volume unit of liquid.

CED = U_v/V_m = U_v/Mρ^-1

= (0.184/0.074) x 1.638

= 4.0728