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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
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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
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Results and Discussion
Hydrogen bonding
Problematic statements on the nature of hydrogen bonding in Hill and Holman (1989) are highlighted in the following discussion.
“Now, if we assume that H2S, H2Se and H2Te have intermolecular forces due only to Van der Waals’bonds (negligible H-bonding) we can estimate a value for the strength of Van der Waals’forces in water.”
(Hill & Holman, 1989, p. 120)
Hill and Holman (1989) defined “H-bonding” as “extra strong intermolecular, permanent dipole-permanent dipole attractions” (p. 119) involving hydrogen atoms attached to N, O or F, so the statement that “negligible H-bonding” existed in H2S, H2Se and H2Te was misleading. Thus, permanent dipole-permanent dipole interaction should be used instead of the term “H-bonding”. Hydrogen bonding was also described as “extra strong intermolecular, permanent dipole-permanent dipole attractions”. This statement might easily mislead students into thinking that hydrogen bonding was the strongest intermolecular force, and hence whenever there was hydrogen bonding between molecules, the physical properties (e.g., melting and boiling points) of the substances involved would be the highest; indeed Chan (2003) found such results when he interviewed students and administered a free-response test on intermolecular forces. Examples such as the states of iodine and water at room temperature should be given to illustrate that other factors need also be considered.
It was noticed that the trend of the strength of the hydrogen bonding of HF>H2O>NH3 and the observed trend in boiling point were not explicitly explained. The bond polarity followed the trend, H-F>H-O>H-N, but the availability of the lone pair followed the trend, H-N>H-O>H-F. These two antagonistic trends combined to give the trend of the strength of hydrogen bonding as follows: H-F>H-O>H-N. This showed that bond polarity was a more important factor than the availability of lone pair(s) of electrons in this situation. However, the observed trend in boiling point was H2O>HF>NH3. This was due to more extensive hydrogen bonding between the water molecules because each water molecule could form two hydrogen bonds compared to hydrogen fluoride and ammonia which could form only one hydrogen bond per molecule. Again, students needed to consider additional factors, and could be at a loss to explain why water had the highest boiling point of the three substances based only on the strength of hydrogen bonding.
It was also noted that there was little discussion in both textbooks on the nature of intramolecular hydrogen bonding which could exist when functional groups capable of hydrogen bonding were in close proximity, and the effect of intramolecular hydrogen bonding in limiting the sites available for intermolecular hydrogen bonding.
Copyright (C) 2004 HKIEd APFSLT. Volume 5, Issue 1, Article 3 (Apr., 2004). All Rights Reserved.
