Gen Chem Destroyer #87 = possible mistake?

[lezdoitlezgo]

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okay soo

"consider : MgO, fructose, NaCl, HF"

rank them i nthe order of melting point (hence bp):

Answer is: MgO>NaCl>fructose>HF

okay I understand that HF has a low bp/mp because it is a gas, but I don't understand why MgO and NaCl (which are IONIC) have a higher bp than fructose. Fructose is COVALENTLY bonded molecules. Chad says in his video that covalent bonds are the strongest hence has the highest bp/mp.

Can someone explain why this answer is the case?😡

 

[J777]

The atoms of one fructose molecule are covalently bonded to each other. However, melting does not look at these intramolecular forces but rather intermolecular forces.

Generally, stronger IMFs = higher MP/BPs

Ionic>Hydrogen Bond>Dipole-Dipole>London Dispersion

MgO and NaCl both have ionic bonding between molecules so they will have highest MP. MgO has higher MP because the electrostatic attraction between them is greater (since two electrons are transferred between Mg2+ and O2-) compared to the attraction in an NaCl molecule (which only involves transfer of one electron). Fructose comes next because it has extensive hydrogen bonding (lots of oxygens and hydrogens). The small HF is last, but still does exhibit H-bonding as well. It just doesn't H-bond nearly as much as the much larger fructose molecule.

In short, we don't care about the covalent bonding within a fructose molecule. What we do care about is the hydrogen bonding between fructose molecules. These are still not as strong as the ionic bonds between salt molecules in a crystal lattice.

 

[lezdoitlezgo]

may I worship you?

THANKS