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Name: Scott B.
Status: educator
Age: 50s
Location: N/A
Country: N/A
Date: Friday, April 26, 2002


Question:
While observing a chemistry class on chemical bonding the teacher ended the lesson with the statement "Ionic bonds are generally stronger than covalent bonds that is why ionic materials melt at higher temperatures".

The statement has two parts,
First "Ionic bonds are stronger generally than covalent bonds"
Second "that is why ionic materials melt at higher temperatures"

What is the truth here?

Can any such generalizations be made about Ionic and Covalent bonds and their melting points?

I can think of many contradictory examples. I have several texts on mineralogy that specifically say that covalent bonds are stronger than ionic because they are directional and most of the bond energy in concentrated in the area of the shared electrons where as ionic bonds are a general attraction of + and - charged atoms due to the almost complete transfer of the electron. Is this true for all materials like say CO2 or O2 or just covalently bonded solids?


Replies:
Bond dissociation energies (the energy required to separate the bonded atoms to an infinite distance apart) for ionic bonds are generally higher for ionic compounds than for covalent compounds. In fact, as a good general, rule, the more ionic character a covalent bond has, the stronger it is. This observation by Linus Pauling led to his electronegativity scale.

However, this says nothing about melting points. Ionic solids are arranged into lattices in which each charged species is surrounded by species of opposite charge. To disrupt such a lattice generally means introducing unfavorable like-like charge interactions and disrupting favorable opposite charge interactions. When covalent solids melt, usually the covalent bonds themselves can stay intact. For instance, when water melts, the water molecule H-O-H keeps its connectivity and shape. It's the weaker, non-covalent interactions between different water molecules that are disrupted when diamond melts.

Some covalent solids, however, consist of essentially enormous covalent molecules. Such compounds include quartz, diamond, and silicon carbide. These solids cannot be divided into discrete molecular units. The chains of covalent bonds continues in all directions without end. To melt such substances requires quite high temperatures.

To further illustrate ho you cannot make ionic/covalent generalizations about melting points, I need to mention that there is a very large class of compounds known as room temperature ionic liquids, or room-temperature molten salts. These consist of compounds in which the anion or cation (sometimes both) are molecules in which the charge is spread out over several atoms. The cation and anion do not readily form a neat crystalline lattice, so they are actually liquid down to room temperature or below. Now, these salts do no vaporize at accessible temperatures (unless they decompose), but they most definitely do not have high melting points.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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