PS 23 2022 key~

Problem Set 23 – Intermolecular forces Chem 105 1. Why do real gases behave nonideally at very low temperatures and very high pressures? At low temperatures the gas particles move more slowly, and their collisions become inelastic; they stick together because of the weak attractive forces between them. The particles, therefore, do not act separately to contribute to the pressure in the container, and the pressure is lower than would be expected from the ideal gas law. Also, the gas particles take up real volume in the container and as the pressure increases, the volume of the particles takes up a greater percentage of the free space in the container. That has the effect of raising the pressure above what we would expect from the ideal gas law for that same volume (in a plot of PV / RT vs. P ). In other words, it makes it seem like the container has a smaller volume than it actually has. 2. Calculate the pressure exerted by 5.00 mol of CO 2 in a 1.00 L vessel at 300 K a. Assuming the gas behaves ideally P = nRT V = 5.00 mol ´ 0.08206 L × atm/mol × K ´ 300 K 1.00 L = 123 atm b. Using the van der Waals equation P + (5.00 mol) 2 ´ 3.59 L 2 × atm/mol 2 (1.00 L) 2 æ è ç ö ø ÷ 1.00 L – 5.00 mol ´ 0.0427 L/mol ( ) = 5.00 mol ´ 0.08206 L × atm/mol × K ´ 300 K P + 89.75 atm ( ) = 156.5 atm P = 66.8 atm c. Explain any differences in the results. The van der Waals equation always gives more accurate values for pressure and volume because it takes into account the actual volume of the gas particles and the real interactions between them (i.e. the intermolecular forces both attractive and repulsive). Because the pressure is lower than the ideal gas law would predict, the intermolecular forces have a more significant contribution than the volume of the particles themselves. 3. Acetone (2-propanone) is the primary component in most nail polish removers. It is miscible (completely soluble in any proportion) in water due to hydrogen bonding. Draw the hydrogen bond between acetone and water. Put a box around the 3 atoms that are important for the hydrogen bond. 4. Which of the following will exhibit the strongest London Dispersion forces? Explain your reasoning. a) Helium (He) b) Nitrogen (N 2 ) c) Oxygen (O 2 ) d) Iodine (I 2 ) e) Fluorine (F 2 ) Iodine is the largest of the molecules, so its electron cloud is the largest and most polarizable; a bigger dipole can be temporarily induced, thus I 2 has the strongest dispersion forces. 5. State the dominant intermolecular forces for the following pure substances:

For ALL problems involving intermolecular forces, it is useful to draw the Lewis structure of the molecule first so you know how the atoms are connected. Once you know the connectivity, you can infer the shape of the molecule. By looking for the distribution of polar bonds and lone pairs, you can infer whether the molecule is polar or not. Polarity governs which IM forces are involved. a) BeH 2 London dispersion b) H 2 O Hydrogen bond c) XeF 2 London dispersion d) PH 3 dipole dipole interaction e) CrCl 3 Ionic bond f) SF 2 dipole dipole interaction g) CS 2 London dispersion 6. State the dominant intermolecular forces for the following substances: a) MgF 2 Ionic b) BrF 3 polar, dipole dipole c) CH 4 London dispersion force d) XeF 4 London dispersion e) PF 5 London dispersion f) SF 4 Dipole Dipole g) SF 6 London Dispersion 7. Assuming each of the following are pure liquids, for which is hydrogen bonding the dominant intermolecular force? a) propanol b) difluoromethane c) phosphine (PH 3 ) d) hydrogen peroxide (HOOH) e) acetone (2-propanone) f) pentanal 8. State the dominant intermolecular forces between the following substances: a) Gasoline (a mixture of hydrocarbons with 4-12 carbons) London dispersion b) Vinegar (a mixture of acetic acid and water) Hydrogen-bonding c) Crayon (colored wax, or paraffin, which is a mixture of hydrocarbons with 20-40 carbons) London dispersion d) Pure phosphorous trichloride dipole dipole interaction e) Pure boron trichloride London dispersion 9. Ammonia, alcohols, and water are the main ingredients of Windex. What is the dominant intermolecular force in Windex? Since all molecules listed above are polar, all the molecules undergo dipole-dipole interactions. Furthermore, these molecules engage in hydrogen bonding. Since all the molecules are relatively small, the dominant intermolecular force will be hydrogen bonding. 10. In an aqueous solution containing Na + , Mg 2+ , K + , and Ca 2+ salts, which do you expect to experience the strongest ion-dipole interactions? Why? Magnesium will experience the strongest ion-dipole interactions because it has a 2+ charge and a small atomic size. 11. Tetrafluoroethylene (C 2 F 4 ) is the molecule from which teflon is made.