CHEM 105 Problem Set 23 PDF

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Problem Set 23 – Intermolecular forces Chem 105 1. Why do real gases behave nonideally at very low temperatures and very high pressures? At low temps the gas particles move more slowly, and their collisions become inelastic; they stick together bc 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 volume of the free space in the container. That has the effect of raising the pressure – volume above what we would expect from the ideal gas law 2. Calculate the pressure exerted by 5.00 mol of CO2 in a 1.00 L vessel at 300 K a. Assuming the gas behaves ideally P = nRT/ V = 5.00 ml x 0.08206 L x atm/ mol x K x 300 K / 1.00 L = 123 atm b. Using the van der Waals equation (P + (5.00 mol)^2 x 3.59 l 2 atm/mol^2 / 1.00 L ^2) (1.00 l – 5.00 mol x 0.0427 l/mol) = 5.00 mol x 0.08206 l atm / mol x k x 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) 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 (N2) c) Oxygen (O2) d) Iodine (I2) e) Fluorine (F2) 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 I2 has the strongest dispersion forces 5. State the dominant intermolecular forces for the following pure substances: a) BeH2 London dispersion b) H2O Hydrogen bond c) XeF2 London dispersion d) PH3 Dipole dipole interaction e) CrCl3 Ionic bond f) SF2 dipole dipole interaction g) CS2 London dispersion For ALL problems involving intermolecular forces, it is useful to draw lewis structure first to know how atoms are connected. Once you know connectivity you can infer the shape of the molecule. By looking for distribution of polar bond and lone pairs, you can infer whether the molecule is polar or not. Polarity governs which IM forces are involved 6. State the dominant intermolecular forces for the following pure substances: a) MgF2 ionic b) BrF3 polar, dipole dipole c) CH4 London dispersion force d) XeF4 London dispersion

e) PF5 London dispersion f) SF4 dipole dipole g) SF6 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 (PH3)

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 are polar, all the molecules undergo dipole-dipole interactions. These molecules have hydrogen bonding. Since all of the molecules are relatively small, the dominant intermolecular force will be hydrogen bonding 10. In an aqueous solution containing Na+, Mg2+, K+, and Ca2+ 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 (C2F4) is the molecule from which teflon is made. a. Draw the Lewis Structure for C2F4 b. Does C2F4 contain polar bonds? If so, draw the dipole moment(s). Polar bond due to the unequal electronegativity between carbon and fluorine c. Is C2F4 Polar or Nonpolar? Non-polar because it is symmetric d. What is the steric number of each C? e. Give the bond angles in C2F4 120 degrees f. State the hybridizations of the two C atoms in C2F4 Sp2, electron geometries of the c atoms = trigonal planar g. Teflon is formed when C2F2 is polymerized, or in other words, when the pi bonding electrons are used to form sigma bonds between molecules, linking many C2F4 molecules together in a long chain (a polymer) as shown to the right. What intermolecular forces do you think dominate in Teflon? London dispersion force because it is non-polar h. Do you think Teflon should be hydrophobic or hydrophilic? Why? We would expect Teflon to be hydrophobic. The bonds between c and f are polar, but the dipoles cancel out because of the symmetry of the molecule would be nonpolar and hydrophobic i. Why do you think Teflon forms such an effective ‘non-stick’ surface for cooking food? It forms an effective non-stick surface between it sticks better to itself than to anything else. Assuming it is hydrophobic, water would not stick very well to it. Understanding why nonpolar things like oil do not stick to it is a bit more complicated, though the essence of it is still that it forms stronger interactions with itself than with anything else. One possible reason is that the strong electronegativity of the fluorine atoms on the chain pull all the electrons to the surface and form a big continuous negatively charges electron cloud around the polymer. Other Teflon molecules are the only things that can induce a dipole effectively enough to undergo dispersion force attractions. Therefore, it has very low friction with other substances; they just can’t interac...