A Closer Look at Trends in Boiling Points of Hydrides: Using an Inquiry-Based Approach To Teach Intermolecular Forces of Attraction

Abstract

Boiling-point data are commonly used in introductory chemistry classes to illustrate the effects of intermolecular forces of attraction (IMFs) on familiar physical properties. In this article, we describe how we use boiling-point trends of group IV-VII hydrides to introduce intermolecular forces in our first-year general chemistry classes. Our approach grew out of our experience teaching with the ACS general chemistry textbook (1), which emphasizes conceptual understanding and uses an inquiry-based approach to help students understand chemical principles.A number of parameters can be used as a measure of intermolecular forces, and it could be argued that the enthalpy of vaporization is a more direct, and therefore better, indicator of the strength of these attractive forces. Boiling point is the temperature at which Gibbs energy, ΔG vap , equals zero, so that the boiling temperature, T BP , is the ratio of the enthalpy of vaporization, ΔH vap , to the entropy of vaporization, ΔS vap :Because ΔH vap depends directly on the strength of intermolecular forces, boiling points also incorporate a similar dependence. However, the boiling point also depends on ΔS vap , which theoretically could change the overall dependence of boiling point on IMFs. We explore this question and explain why we believe that it is legitimate to look at boiling-point trends, at least for the group IV-VII hydrides. We also examine the choice of variable to use on the x axis when graphing boiling point or ΔH vap and discuss why the number of electrons, although an imperfect indicator of the total molecular interaction energy, is conceptually more sound than other reasonable alternatives, especially in an introductory chemistry class. Finally, we address the common misconception that dispersion forces are the weakest of the IMFs and then use London dispersion forces to explore the nonlinearity of the boiling-point data for the group IV compounds. Discussion: Trends in Boiling Points of HydridesThe Effects of Intermolecular Forces of Attraction on Boiling PointAll general chemistry courses introduce the different types of intermolecular forces and how they help to explain a variety of physical properties. A graph of the boiling points of the group IV-VII hydrides is often used to illustrate periodic trends that relate to intermolecular forces because it is generally assumed that a higher boiling point is the result of stronger intermolecular attractions. Because the enthalpy of vaporization is a more direct measure of the strength of IMFs in a liquid and differences in IMF strength will change the magnitude of ΔH vap , there is an argument for using ΔH vap rather than boiling point to examine the role of IMFs. In our courses, IMFs are introduced early in the chemistry course, when students have not yet learned about enthalpy, and thus, it makes sense to talk instead about a property (boiling point) with which the students are familiar. To test the validity of using boiling point as an indicator of IMF strength, we compare graphs of boiling points...

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