How do you find the expected melting points of compounds?

Question

When compounds are larger, does that mean they have higher melting points? Or is it inversely related?

Also, what about if its an ionic compound? How does size make a difference in the melting point?

Answer 1

Melting point has little to do with size. Melting point can be lower or higher depending on the chemical bonding of the compound.
Compounds with stronger bonds have higher boiling points but lower melting points.

Ionic compounds have the strongest bond,hence,they have the lowest melting points.
The size is significant in determining the bond strength.Two compounds can have dispersion forces of attraction but the one with strongest bond is the largest,hence,it's melting point will be lower than the other compound.

Answer 2

Melting Points Of Compounds

Answer 3

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All of the compounds have very polar covalent bonds. In fact, all bonds are covalent. (But that's a topic for another time.) There is no such thing as a 100% ionic bond. All bonds lie along a continuum, and have characters of both kinds of bonds. The compounds in your list have bonds with high ionic character, in other words, they are more ionic, than they are covalent. But that's not the main thing. The main thing is that the atoms in these compounds arrange themselves in networks. It is the formation of a network, regardless of whether the bonds are predominately ionic or predominately covalent, that make for high melting points. Diamond, with 100% covalent bonds, has the highest of all melting points. And the reason is the network arrangement where each C atom is bonded to adjacent atoms. The same is true of the compound in your list. Mg and Cl are able to form more and stronger bonds within the network. Since Mg has two valence electrons it can form more bonds than Na or K, and that makes for a stronger network. Since these atoms do not form discrete molecules, then dispersion forces are way down the list in what makes for high melting points. Since the K and Br are larger than Na and Cl, then the nuclei in K and Br are farther away from the overlapping valence electrons, the ones that are both being attracted to the nuclei, and the attraction is less strong and the bonds are less strong. (It follows Coulomb's law, F = kq1q2/r2, where r is the distance between the charged particles.)

Answer 4

How To Find Melting Point

Answer 5

CRC tables for most common chemicals (pure). Industry uses DSC and DTA (differential scanning calorimeter, Differential thermal analysis) equipment to show what happens when a compound is slowly heated. A TGA/ DTA combination piece of equipment (Thermal Gravametric analysis) will show weight changes along with energy uptake and releasing to give much information on water(s) of hydration, specific heat (calories per gram to raise the temperature) melting point and decomposition temperatures. Ionic compounds are not generally done on the DSC equipment because their melting points are generally too high.

Answer 6

There are a lot of rough rules. If organic compounds have higher molecular weights ("larger") they tend to melt higher.

If organic compounds are highly symmetrical (p-xylene), they melt higher than low-symmetry compounds (o-xylene).

Ionic compounds melt higher than covalent ones. Some high-molecular weight ionic organic compounds (ionic liquids) melt lower LOL.

What chemists actually do is measure melting points and publish them. Or they look them up if they need them.

Answer 7

Hope this helps!