Asia-Pacific Forum on Science Learning and Teaching, Volume 5, Issue 1, Article 3 (Apr., 2004) Daniel Kim Chwee TAN and Kim Seng CHAN An analysis of two textbooks on the topic of intermolecular forces Previous Contents Next

Results and Discussion

Instantaneous dipole-induced dipole interactions

Hill and Holman (1989) had not attempted to indicate that instantaneous dipole-induced dipole interactions did exist in polar molecules as given below.

The existence of dipole-dipole attractions will explain the forces holding together polar molecules in liquids such as trichloromethane (CHCl₃), propanone (acetone, CH₃COCH₃) and nitrobenzene (C₆H₅NO₂).

(Hill & Holman, 1989, p. 114)

There are instances when instantaneous dipole-induced dipole interactions are the more prominent intermolecular force in play rather than permanent dipole-permanent dipole interactions. For example, the higher boiling point of hydrogen iodide as compared to hydrogen chloride could be explained by the stronger instantaneous dipole-induced dipole interactions between hydrogen iodide molecules.

There was also a lack of discussion on the factors that governed the strength of instantaneous dipole-induced dipole interaction, for example, the number of electrons a molecule had and its surface area. Iodine had a higher boiling point than nitrogen because it had more electrons than nitrogen, but methane also had a higher boiling point than nitrogen even though the number of electrons present in methane was less than nitrogen –in this situation, the surface area factor was more important. As mentioned in an earlier section, the states of iodine and water at room temperature should also be discussed to show that there were cases when instantaneous dipole-induced dipole interactions were stronger than hydrogen bonding. These examples, which showed the interplay of different factors in different situations, should be introduced to students to help them realise that they needed to carefully analyse the given physical data from various perspectives and consider all relevant factors. A summary of the major inconsistencies between the propositional knowledge identified by Tan and Chan (2003) and the content on the topic of ionisation energy in the two textbooks is given in Table 2.

 

Table 2. Summary of major inconsistencies between the propositional knowledge identified and the content of the two textbooks on the topic of intermolecular forces

	Major inconsistencies identified	Textbooks
1.	Intermolecular forces (IMF) were not discussed as electrostatic attractions which are very much weaker than conventional bonds.	HH, R
2.	Non-ideality of real gases was not related to IMF.	R
3.	Net dipole moment was not used to predict the molecular polarity.	HH, R
4.	Electronegativity was not related to nuclear charge, shielding by inner shell electrons and distance from the nucleus.	HH, R
5.	Hydrogen bonding was not mentioned as a type of permanent dipole-permanent dipole interaction.	R
6.	Trend of hydrogen bonding strength was not discussed.	HH, R
7.	The reason why water had a higher boiling point than hydrogen fluoride though its hydrogen bonding was weaker than that of hydrogen fluoride was not discussed.	R
8.	Intramolecular hydrogen bonding was not discussed in detail.	HH, R
9.	Presence of instantaneous dipole-induced dipole interactions for polar molecule was not emphasized.	R
10.	The number of electrons affecting the strength of instantaneous dipole-induced dipole interactions was not discussed.	HH
11.	The surface area of a molecule affecting the strength of instantaneous dipole-induced dipole interactions was not discussed.	HH, R
12.	Instantaneous dipole-induced dipole interactions could be stronger than hydrogen bonding was not discussed.	HH, R

 

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