As such the intermolecular attraction is only weak

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Unformatted text preview: ................................................................................................................................... EXAMPLE 2.7–1 Use the concept of secondary bond strength to predict which member of each pair of materials below has a higher melting temperature. a. C2 H4 or C2 H2 F2 b. H2 O or H2 S c. Propane C3 H8 or dodecane C12 H26 S olution The two key factors are the type of secondary bond and the size of the molecules involved. a. C2 H4 is a symmetric molecule, so that it is not a permanent dipole. Instead C2 H4 molecules are held together by only weak van der Waals bonds. In contrast, the replacement of two of the H atoms with highly electronegative F atoms makes C2 H2 F2 a permanent dipole. Thus, we expect C2 H2 F2 to display the higher melting temperature. Indeed, the melting temperature of C2 H2 F2 is 84 C while that of C2 H4 is 169 C. b. H2 O and H2 S are both permanent dipoles; however, O is more electronegative than S, EN(O) 3.44, EN(S) 2.58 . Consequently, water forms stronger hydrogen bonds. The melting point of H2 O is 0 C, while that for H2 S is 85.5 C. c. C3 H8 and C12 H26 are organic molecules composed of carbon and hydrogen. As such, the intermolecular attraction is only weak van der Waals forces. Since C12 H26 is a larger molecule, it can form a larger number of dipoles. Thus, the melting temperature of 9.6 C for C12 H26 is higher than the 189.7 C melting temperature of C3 H8 . ....................................................................................................................................... Secondary bonds control properties such as melting point and elastic modulus in solids where primary bonds do not form a three-dimensional (3-D) network. The nondirectional nature of both ionic and metallic bonds, coupled with the high coordination numbers for these solids (CN 4), usually results in the formation of a 3-D primary bond network. Hence, there are no regions in which primary bonds are absent, and therefore the properties of metallic and ionic solids are dominated by primary bonds. In contrast, the directional nature of covalent bonds, coupled...
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This note was uploaded on 02/25/2013 for the course PHYS 2202 taught by Professor Sowell during the Spring '10 term at Georgia Tech.

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