Problem 115
X rays of wavelength \(0.154 \mathrm{nm}\) strike an aluminum crystal; the rays are reflected at an angle of \(19.3^{\circ} .\) Assuming that \(n=1,\) calculate the spacing between the planes of aluminum atoms (in pm) that is responsible for this angle of reflection.
Problem 116
The properties of gases, liquids, and solids differ in a number of respects. How would you use the kinetic molecular theory (see Section 10.6 ) to explain the following observations? (a) Ease of compressibility decreases from gas to liquid to solid. (b) Solids retain a definite shape, but gases and liquids do not. (c) For most substances, the volume of a given amount of material increases as it changes from solid to liquid to gas.
Problem 117
Select the substance in each pair that should have the higher boiling point. In each case identify the principal intermolecular forces involved and account briefly for your choice: (a) \(\mathrm{K}_{2} \mathrm{~S}\) or \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N},\) (b) \(\mathrm{Br}_{2}\) -or \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} .\)
Problem 118
A small drop of oil in water assumes a spherical shape. Explain. (Hint: Oil is made up of nonpolar molecules, which tend to avoid contact with water.)
Problem 119
Under the same conditions of temperature and density, which of the following gases would you expect to behave less ideally: \(\mathrm{CH}_{4}\) or \(\mathrm{SO}_{2}\) ? Explain.
Problem 120
The fluorides of the second-period elements and their melting points are: \(\mathrm{LiF}, 845^{\circ} \mathrm{C} ; \mathrm{BeF}_{2}, 800^{\circ} \mathrm{C} ; \mathrm{BF}_{3}\), \(-126.7^{\circ} \mathrm{C} ; \mathrm{CF}_{4},-184^{\circ} \mathrm{C} ; \mathrm{NF}_{3},-206.6^{\circ} \mathrm{C} ; \mathrm{OF}_{2},\) \(-223.8^{\circ} \mathrm{C} ; \mathrm{F}_{2},-219.6^{\circ} \mathrm{C}\). Classify the type(s) of intermolecular forces present in each compound.
Problem 121
The distance between \(\mathrm{Li}^{+}\) and \(\mathrm{Cl}^{-}\) is \(257 \mathrm{pm}\) in solid \(\mathrm{LiCl}\) and \(203 \mathrm{pm}\) in an \(\mathrm{LiCl}\) unit in the gas phase. Explain the difference in the bond lengths.
Problem 122
The distance between \(\mathrm{Li}^{+}\) and \(\mathrm{Cl}^{-}\) is \(257 \mathrm{pm}\) in solid \(\mathrm{LiCl}\) and \(203 \mathrm{pm}\) in an \(\mathrm{LiCl}\) unit in the gas phase. Explain the difference in the bond lengths.
Problem 123
Heat of hydration, that is, the heat change that occurs when ions become hydrated in solution, is largely due to ion-dipole interactions. The heats of hydration for the alkali metal ions are \(\mathrm{Li}^{+},-520 \mathrm{~kJ} / \mathrm{mol} ; \mathrm{Na}^{+}\) \(-405 \mathrm{~kJ} / \mathrm{mol} ; \mathrm{K}^{+},-321 \mathrm{~kJ} / \mathrm{mol}\). Account for the trend in these values.
Problem 124
If water were a linear molecule, (a) would it still be polar, and (b) would the water molecules still be able to form hydrogen bonds with one another?
