Question

Considering the substances' boiling points, which statement is true?
-The boiling point of CO is expected to be higher than NaCl.
-The boiling point of H2 is expected to be higher than NH3.
-The boiling point of HF is expected to be higher than HCl.
-The boiling point of O2 is expected to be higher than CO.

Answer 1

The boiling point is largely based on the molecular weight of the compound. The higher the molecular wight, the higher the boiling point. That is because the compound is so heavy it will not be able to float around in the air easily unless a lot of energy is put in.

CO: 28.01 g/mol
NaCl: 58.44 g/mol

Since NaCl has a higher molar mass, it will therefore have a higher boiling point, not lower. That makes the first answer false. Why don't you try the next ones and I can check your answer :D

Answer 2

You need to understand the forces holding atoms and molecules together in the liquid or solid state. These forces must be overcome to boil a substance, and the stronger the force, the higher the amount of energy needed in the form of high temperature.

These forces, and their characteristic boiling points are, from strongest to weakest:

(1) Metallic bonding forces, Tb = 2500C or so.

(2) Ionic bonding forces, Tb = 1000C or so.

(2) Hydrogen bonding forces, Tb = 100C or so.

(3) Dipole forces, Tb = -50C or so.

(4) Dispersion (London) forces, Tb = -200C or so.

An additional complication which I haven't mentioned is that dispersion forces grow with the size of the molecule, and for large molecules (more than 10 atoms or so) they come to dominate all other forces in classes (2)-(4). So when you are talking *big* molecules (which you are not, here) the important question is the size of the molecule.

Furthermore, if you identify two compounds with exactly the same kind of major forces holding them together, there are variations in the size of the force you can use to figure out which one boils higher. For example, among ionic compounds those with higher charges on the ions (e.g. +2 and -2 instead of +1 and -1) will have stronger ionic bonding forces. Compounds that experience dipole forces will have stronger forces if the dipole moment is larger, which usually means the electronegativity difference between bonded atoms is larger. And so on.

In each case here, you need to identify the forces holding each compound together. Then decide which compound experiences the greater force. That will be the one that boils higher.

To start you off, I'll note that both HCl and HF have dipole moments, because the F or Cl is a lot more electronegative than the H. The dipole moment of HF would be larger, and, more importantly, HF can experience hydrogen bonding, while HCl cannot. Therefore we expect HF to boil at a higher temperature than HCl, and in fact the bp are about +20C and -85C, respectively.