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CHEMISTRY 121 1 Department of Chemistry University of British Columbia Problem Set 5 Answer Consequences of Molecular Polarity 1. What is the predominant intermolecular force holding the Br 2 molecules together in the liquid state of bromine? Br 2 is a non-polar molecule, therefore London forces are the predominant intermolecular forces involved. 2. Which of the following compounds is expected to have the least hydrogen bonding: CH 4 , H 2 O, HF or NH 3 ? Hydrogen bonding results when a H atom bonded to a highly electronegative atom (e.g. F, O, N) is simultaneously attracted to a highly electronegative atom in a neighboring molecule. Therefore hydrogen bonding occurs in H 2 O, HF and NH 3 molecules, but not in CH 4 . 3. What are the forces of attraction between I 2 molecules? I 2 is non-polar, therefore London forces are the predominant intermolecular forces involved. 4. Arrange the following in order of increasing boiling point: RbCl, CH 3 Cl, CH 3 OH, CH 4 . The stronger the intermolecular forces, the higher the boiling point. In the table below the intermolecular forces involved in each of the four substances are given. Based on this the intermolecular forces increase in the following order: CH 4 < CH 3 Cl < CH 3 OH < RbCl (refer to the following table). The boiling points increase in the same order. Compound Polarity Intermolecular force CH 4 Non polar London force CH 3 Cl polar Dipole-dipole interaction, London force CH 3 OH polar Hydrogen bonding, dipole-dipole interaction, London force RbCl polar (ionic) Ionic bonds 5. C 60 and C 84 are two of the fullerene allotropes of carbon pictured below. The boiling point of a liquid is determined by the temperature at which the vapour pressure of the liquid just equals the external atmospheric pressure. From what you know about what determines the boiling point of a liquid, which fullerene solid would you expect to have the higher vapour pressure? Explain your reasoning.
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CHEMISTRY 121 2 Department of Chemistry University of British Columbia C 60 C 84 C 60 is expected to have a lower boiling point due to its lower molecular weight, the intermolecular dispersion forces (London forces) are therefore weaker than in C 84 . The higher the vapour pressure, the lower the boiling point, thus C 60 is expected to have a higher vapour pressure than C 84 . 6. Provide a rationale for the following observation. The boiling points of the Group 14 hydrides increase smoothly on descending the group, whilst for the Group 15 (also known as the pnictogens), ammonia shows an anomalously high boiling point. A smooth increase in boiling point is expected as we descend the group and the polarizability and molecular weight of the molecules increases. Hydrogen bonding, a relatively strong intermolecular force, occurs between ammonia (NH 3 ) molecules, but not in the other hydrides. For this reason ammonia shows an anomalously high boiling point. 7.
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This note was uploaded on 11/03/2009 for the course ECON 210 taught by Professor James during the Spring '09 term at The University of British Columbia.

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