Syllabus Edition

First teaching 2014

Last exams 2024

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Deducing Intermolecular Forces (DP IB Chemistry: HL)

Revision Note

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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

Examiner 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!

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.