Lesson 9: Determining Bond Type

We can use information about the shape and polarity of molecules to determine what kind of intermolecular bonds govern their behavior.  Nonpolar molecules use van der Waal’s bonds, polar molecules use dipole-dipole bonds, and polar molecules with hydrogen bonded to N, O, or F use hydrogen bonds.

Sometimes it is difficult to determine simply from the molecular formula when a compound that contains both O and H has O-H bonds, or when one that contains both N and H has N-H bonds. Until you gain some familiarity with a variety of common compounds, it is best to at least begin the process of drawing the Lewis structure each time you encounter a molecule like this which you have not seen before.

 

Intermolecular Bonds (Molecular Materials) | Interatomic Bonds (Network Materials) | Practice

Intermolecular Bonds (Molecular Materials)

Of these three intermolecular bond types, the strongest are hydrogen bonds.  For a given size of molecule, chemicals that use hydrogen bonds will tend to have higher melting points and boiling points than chemicals that use the weaker dipole-dipole or van der Waals bonds.

Compared to other molecules its size, water has a remarkably high melting point and boiling point.  If we replace oxygen, for example with tellurium, which is the heaviest stable element in the same group (column) of the periodic table as oxygen, to make H₂Te, the melting point of the new compound is only -49^oC and its boiling point is 2^oC.

The next strongest intermolecular bonds are dipole-dipole.  As you can see, chemicals with similar size molecules that use dipole-dipole bonds have lower melting points and boiling points than chemicals that use hydrogen bonds.

The difference is actually quite dramatic.  Formaldehyde melts at a temperature 92^oC lower than water and boils at a temperature 79^oC lower than water, in spite of the fact that water has a molecular weight of only 18 g/mole compared to 30 g/mole for formaldehyde.

The weakest of the intermolecular bond types is the van der Waals bond.  For a given size molecule, chemicals that display this type of intermolecular bonding have the lowest melting and boiling points. For very large molecules, van der Waals forces can be quite strong.

Dodecane (pronounced: doe-DECK-ane), for example, is about six times larger than ethane and ten times larger than water.  It melts at -10^oC and boils at 216^oC.

 

Top of page

 

Interatomic Bonds (Network Materials)

By comparison, network materials use much stronger interatomic bonds, which give them much higher melting points and boiling points, than any of the chemicals that use intermolecular bonds.

The strength of metallic, ionic, and covalent bonds varies widely from chemical to chemical.  There are many metals that have higher melting and boiling points than many ionic compounds.

Titanium metal, for example, melts at 1675^oC and boils at 3260^oC.  Barium oxide, an ionic material, melts at 1923^oC, well above the melting point of quartz.

 

Top of page

 

Practice

Given a series of chemicals, you should now be able to list them in order from strongest to weakest bond types, from highest to lowest melting points, and from highest to lowest boiling points.  One method you might use is shown below.

In making such comparisons, we will compare molecules of similar size, where we can reasonably assume that the order of melting points and boiling points matches the order of strength of intermolecular bonds.

 

Practice this by placing in the third column of the table in Exercise 7 the intermolecular bond type you would expect for each of the chemicals listed.  Do the same for the chemicals listed in Exercise 8.

Exercise 7 answers:

1. CH₄: van der Waals
2. NH₃: hydrogen
3. H₂O: hydrogen
4. SO₃: van der Waals
5. SO₂: dipole-dipole
6. CH₃Cl: dipole-dipole
7. HCN: dipole-dipole (the H is not bonded directly to the N, so it can't be hydrogen bonding)
8. CH₂O: dipole-dipole (the H is not bonded directly to the O)
9. O₃: dipole-dipole
10. BH₃: van der Waals
11. PCl₃: dipole-dipole

 

Exercise 8 answers:

1. hydrogen peroxide: hydrogen bonding
2. ethanol: hydrogen bonding
3. acetic acid: hydrogen bonding
4. van der Waals
5. van der Waals

Now do Exercise 9.  For each set of three chemicals, put them in order from strongest to weakest intermolecular bond type, and from highest to lowest melting and boiling points.

Answers to Exercise 9:

(The strongest bonding will lead to the highest melting and boiling points. In each case, the first compound has intermolecular hydrogen bonding, the middle compound has dipole-dipole bonding, and the final molecule has van der Waals bonding.)

1. H₂O > HCl > CH₄
2. NH₃ > Cl₂O > Cl₂
3. CH₃OH > CH₂O > CO₂

Exercises 7, 8, and 9 dealt only with intermolecular bonds.  Exercise 10 includes some network materials whose phase, melting points, and boiling points are determined by much stronger interatomic bonds.  As before, determine the bond type used by each chemical.

Exercise 10 answers:

a. Solids
	diamond	covalent bonding
	quartz	covalent bonding
	iron	metallic bonding
	aluminum	metallic bonding
	sodium chloride	ionic bonding
b. Liquids
	water	hydrogen bonding
	octane	van der Waals bonding
c. Gases
	oxygen	van der Waals bonding
	nitrogen	van der Waals bonding
	carbon dioxide	van der Waals bonding

Finally, in Exercise 11, determine the bond type used by each of the six chemicals listed and rank them in order from strongest to weakest bond type.

Exercise 11 answers:

From strongest to weakest:

 

 

Top of page