122ch11a - 1 Chapter 11 - Homework Solutions 11.2) What...

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1 Chapter 11 - Homework Solutions 11.2) What types of Intermolecular Attractive Forces (IAF) are shown in the following? a) HF molecules H-F ------ H-F (This dashed line represents a H-bond.) What is being shown is a hydrogen bond. This occurs between a lone-pair of electrons on the F atom of one molecule and the H atom of another molecule. Remember, H bonding is a special type of very strong dipole-dipole interaction. It occurs when a H atom bonded directly to a an atom of high electronegativity (N, O or F) forms a strong dipole-dipole attraction to an unshared electron pair on a nearby small electronegative atom or ion (N, O or F). You need a H covalently bonded (directly connected) to a N, O or F in a molecule in order to have hydrogen bonding and a lone pair on a N, O or F. You will also have London Forces (LF) and Dipole-Dipole AF (DD) between HF molecules. Not all HF molecules in the liquid will be H-bonded to each other. The molecules have to line up just right to form H bonds. Thus, when you have liquid HF the molecules are attracted to each other by all three types of AF (LF, DD and H-bonding). In solid HF (if it is a prefect crystalline solid) the H-bonding will be maximized and on average the HF molecules form 2 H-bonds per molecule. In the liquid the HF molecules form between 1-2 H-bonds per molecule. Polar molecules with a H bonded directly to a N, O or F atom can form H-bonds in addition to LF and DD. Thus, polar molecules are attracted to each other by LF , DD and H-bonds . b) F 2 molecules F-F --- F-F (This dashed line represents LF.) This molecule is NON polar. All molecules have LF between them. NON polar molecules have ONLY LF between them.
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2 11.2) (cont.) c) Na + and H 2 O Na + *** H 2 O (This represents a Ion-Dipole AF.) Ion-Dipole AF (I-D) occur between an ion and a polar molecule. This is a very strong IAF. It occurs in solutions and we will discuss it in detail and use it in Ch. 13. In this particular case, the negative end of the polar H 2 O (the oxygen atom) is attracted to the positive Na + cation. This AF is weaker than the Ion-Ion AF forces in ionic compounds but much stronger than H-bonding. d) SO 2 molecules O = S ----- O = S (This represents LF and DD AF) \ \ O O This molecule is polar . The S-O bonds are polar and the molecule is bent (with a bond angle of about 120°). The S atom has a partial positive charge ( * + ) on it and the O atoms have a partial negative charge ( * & ). Thus, the partially negatively charged O atom of one molecule forms an attraction to the positively charged S atom of another molecule. This is in addition to LF that occur between these molecules. Thus, SO 2 molecules are attracted to each other by both LF and DD AF. Polar molecules have BOTH LF AND DD AF between them. The strengths of the AF we are talking about in chapters 11 and 13 increase in the following order (increasing in strength from left to right).
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This note was uploaded on 06/11/2011 for the course CHEM 122 taught by Professor Zellmer during the Spring '07 term at Ohio State.

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122ch11a - 1 Chapter 11 - Homework Solutions 11.2) What...

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