Deducing Intermolecular Forces
- In order to deduce the types of intermolecular forces present in molecules you need information about the structure and chemical formula of the molecules
- The chemical formula will tell you about the presence of electronegative elements present in the molecule
- Any potential polar bonds can be identified
- If N, O or F is present then hydrogen bonds are potentially possible
- The structure and symmetry of the molecule will enable you to determine if the molecule is polar following the principles laid out in 4.1.10 Molecular Polarity
Worked example
Which of the compounds below can form intermolecular hydrogen bonds in the liquid state?
A. (CH3CH2)3N
B. CH3OCH3
C. CCl4
D. C2H5OH
Answer:
The correct option is D.
- Draw the displayed structures of the molecules:
- Molecules A and B form dipole-dipole attractions as they are polar molecules
- Molecule C forms dispersion forces as the dipoles cancel out so there is no overall polarity
- Molecule D is the only one capable of forming hydrogen bonds
Exam Tip
Sometimes a question will ask you to name all the IMFs present in molecules and students frequently forget to include dispersion forces which are present in all molecules, since everything contain electrons!
