Question

intermolecular forces

Rank the following substance from highest melting point to lowest melting point. My teacher gave a list of compounds: H2O, NO2,F2,CI2 and to have a high melting point means that you need a stronger IMF.

Rank the following substances from highest freezing point to lowest. Same compounds from above. What would I do in this instance?
Would I list this from lowest IMF to high IMF.

Answer 1

This must be a trick question. The melting point and the freezing point ARE THE SAME. By definition, the melting point is the temperature at which solid and liquid are in equilibrium. So whether the compound freezes or melts at that exact temperature depends only upon which way heat is flowing; i.e., in or out of the material. If it is flowing out, then the remainder of the material solidifies (freezes) and if heat is going in, the remainder of the material melts (becomes liquid). By the way, there is an error in the melting point order in your list. I don't know if the list you have is your ranking or if you are to rank them. Anyway, the highest to lowest is H2O, NO2, Cl2, F2. And you are right that the IMF determines the ranking. I looked up the melting points and they are as follows:
H2O, molar mass of 18, mp=0 degrees C.
NO2, molar mass of 46, mp=-11 degrees C.
Cl2, molar mass ~71, mp=-101
F2, molar mass 38, mp=-219