Sciencemadness Discussion Board

Polar Bond Confusion!

Backyard Chemist - 14-1-2015 at 17:23

Can the following compounds have a polar covalent bond?

1. AgCl

2. FeS

The electronegativities of these compounds are between 0.1 and 1.7, so therefore there would be a polar covalent bond? If so, how? If not, how?

Brain&Force - 14-1-2015 at 17:30

"Ionic" and "covalent" are a spectrum, there are no purely ionic bonds. You can only get purely covalent bonds between atoms with the same electronegativity.

The bonds in this case have both ionic and covalent characteristics.

diddi - 14-1-2015 at 18:47

In the case of FeS, the compound displays typical transition metal to sulphide bonding which is highly covalent. very few of the transition metal sulphides are soluble I water Many metal sulfides are often found in nature in that form (eg galena, pyrite, orpiment etc) demonstrating their stability and lack of ionic bonding present. A number of the sulphides also form covalent lattice structures.

and from :

I have looked at this problem in detail before, and the result is quite surprising. I thought that a major contribution to the insolubility of AgCl was that the solvation energy was much smaller than something like K^+ because Ag^+ is soft and water is hard, so the attraction of H2O for Ag^+ is less than to the harder K^+. But this is not the case: the hydration energy of Ag^+ is as I recall comparable or slightly greater than the hydration energies of the Group 1 metal cations.
It turns out that when you do the theoretical calculation of the lattice energies of the Group 1 halides (e.g., KCl) using the Born-Lande equation the agreement with experimental (Born-Haber cycle) is remarkably good. However when you do the same procedure for AgCl (and AgBr, AgI) the experimental lattice energy is as I recall some 150 kJ mol-1 higher than that calculated by the B-L eqn. In other words AgCl has an added energy component that of course is attributed to partial covalent character. Besides the Coulombic attraction of Ag^+ ion for the Cl^- ion there is also some sharing of electrons that further strengthens the lattice. A larger lattice energy means more energy to break up the lattice in the solvation process. There is a delicate balance between U the lattice energy, and the ΔH(hyd) for M^+ and X^- for Group 1 halides; the ΔS term in ΔG also plays a major role. For AgCl the larger U(AgCl) dominates and the stuff is completely insoluble. So AgCl is mainly ionic but with significant covalent character.

Bert - 14-1-2015 at 19:35

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