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EMA4121_HW3_Solutions - 1 The heat of vaporization can be...

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1. The heat of vaporization can be used to approximate the interaction energy between molecules in a bulk condensed phase. This, in turn, can be used to calculate surface tension, approximating the molecules as cubes with one exposed side at the surface for z values and area calculations. a. Given the following information, calculate the surface tension of water: ΔH vap = 40.67 kJ/mole, d=1.00 g/cm 3 ( ) ( ) Assume z b = 6 (a cube has six sides; in the bulk, they are all in contact with adjacent cubes) Assume z s = 5 (on the surface, one side of each cube is exposed) ( ) a 0 is the area per face of the molecule, approximated as a cube. From the density and molar mass, the volume per molecule can be calculated; the 2/3 root of the volume is then the value of a 0 . [( ) ( ) ( ) ( )] ( ) b. Compare this to the experimental value: 72.8 mJ/m 2 . Why are the values different? Water has a dipole moment and thus has an entropy component related to its distribution of orientations. The theoretical calculation doesn’t take the amount of energy required to overcome the entropy contribution into account, so it is higher than the actual value.
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