Chapter 5

For which of the following molecules will dipoledipole interactions be important: (a) \(\mathrm{CH}_{4} ;\) (b) \(\mathrm{CH}_{3} \mathrm{Cl}\); (c) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\); (d) \(\mathrm{CHCl}_{3}\); (e) \(\mathrm{CCl}_{4}\) ?

For which of the following molecules and ions will dipole-dipole interactions be important: (a) \(\mathrm{SF}_{4}\); (b) \(\mathrm{PF}_{5} ;\) (c) \(\mathrm{PF}_{3} ;\) (d) \(\mathrm{SF}_{4}\) ?

Place the following types of molecular and ion interactions in order of increasing magnitude: (a) iondipole; (b) induced dipole-induced dipole; (c) dipoledipole in the gas phase; (d) ion-ion; (e) dipole-dipole in the solid phase.

Explain why ionic solids like \(\mathrm{NaCl}\) have high melting points yet dissolve readily in water, whereas network solids like diamond have very high melting points and do not dissolve.

Which of the following moiecules are likely to form hydrogen bonds: (a) \(\mathrm{HF}\); (b) \(\mathrm{CH}_{4}\); (c) \(\mathrm{NH}_{3}\); (d) \(\mathrm{CH}_{3} \mathrm{OH}\) ?

Which of the following molecules are likely to form hydrogen bonds: (a) \(\mathrm{D}_{2} \mathrm{O}\); (b) \(\mathrm{CH}_{3} \mathrm{COOH}\); (c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\); (d) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) ?

Suggest, giving reasons, which substance in each pair is likely to have the higher melting point (Lewis structures may help your arguments): (a) HCl or NaCl; (b) \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OC}_{2} \mathrm{H}_{3}\) (diethyl ether) or \(\mathrm{C}_{4} \mathrm{H}_{9} \mathrm{OH}\) (butanol); (c) \(\mathrm{HF}\) or \(\mathrm{HCl}\); (d) \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{CH}_{3} \mathrm{OH}\).

Suggest, giving reasons, which substance in each pair is likely to have the higher boiling point: (a) \(\mathrm{H}_{2} \mathrm{~S}\) or \(\mathrm{H}_{2} \mathrm{O}\); (b) \(\mathrm{NH}_{3}\) or \(\mathrm{PH}_{3}\); (c) \(\mathrm{KBr}\) or \(\mathrm{CH}_{3} \mathrm{Br}\); (d) \(\mathrm{CH}_{4}\) or \(\mathrm{SiH}_{4}\) *

Predict which substanoe has the greater viscosity in its liquid form at \(0^{\circ} \mathrm{C}\) : (a) ethanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\), or dimethyl ether, \(\mathrm{CH}_{3} \mathrm{OCH}_{3}\); (b) butane, \(\mathrm{C}_{4} \mathrm{H}_{10}\) or propanone, \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\).

Potassium crystallizes in a bce structure. The atomic radius of potassium is \(235 \mathrm{pm}\). Determine (a) the number of atoms per unit cell; (b) the coordination number of the lattice; (c) the length of the side of the unit cell.

The metal polonium (which was named by Maric Curie after her homeland, Poland) crystallizes in a primitive cubic strucrure, with an atom at each corner of a cubic unit cell. The atomic radius of polonium is \(167 \mathrm{pm}\). Sketch the unit cell and determine (a) the number of atoms per unit cell; (b) the coordination number of an atom of polonium; (c) the length of the side of the unit cell.

Silicon can be doped with small amounts of phosphorus to create a semiconductor used in transistors. (a) Is the alloy interstitial or substitutional? Justify your answer. (b) How do you expect the properties of the doped material to differ from those of pure silicon?

Name two compounds that exist as network solids at room temperature and pressure.

Name at least four clements that, in at least one allotropic form, are network solids.

Account for the following observations in terms of the type and strength of intermolecular forces. (a) The melting point of xenon is \(-112^{\circ} \mathrm{C}\) and that of argon is \(-189^{\circ} \mathrm{C}\). (b) The vapor pressure of diethyl ether \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OC}_{2} \mathrm{H}_{3}\right)\) is greater than that of water. (c) The boiling point of pentane, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{3} \mathrm{CH}_{3}\), is \(36.1^{\circ} \mathrm{C}\), whereas that of 2,2 -dimethylpropane (also known as neopentane) is \(9.5^{\circ} \mathrm{C}\).

Classify the following solids as ionic, network, or molecular: (a) quartz, \(\mathrm{SiO}_{2}\); (b) limestone, \(\mathrm{CaCO}_{3}\); (c) dry ice, \(\mathrm{CO}_{2}\); (d) sucrose, \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}\); (c) polyethylene, a polymer of repeating \(-\mathrm{CH}_{2} \mathrm{CH}_{2}-\) units.

If the edge length of a foc unit cell of \(\mathrm{Rbl}\) is \(732.6 \mathrm{pm}\), how large must the edge length of a cubic single crystal of Rbl be in order for it to contain \(1.00 \mathrm{~mol}\) RbI?

Aluminum metal has a density of \(2.70 \mathrm{~g} \cdot \mathrm{cm}\) and crystallizes in a lattice with a unit cell edge of \(404 \mathrm{pm}\). (a) What type of cubic unit cell is formed by aluminum? (b) What is the coordination number of aluminum?

An oxide of niobium has a unit cell in which there are oxide ions at the middle of each cdge and niobium atoms at the center of each face. What is the empirical formula of this oxide?

Indicate whether the following statements are true or false: (a) If there is an atom present at the comer of a unit cell, there must be the same type of atom at all the comers of the unit cell. (b) A unit cell must be defined so that there are atoms at the corners. (c) If one face of a unit cell has an atom in its center, then the face opposite that face must also have an atom at its center. (d) If one face of a unit cell has an atom in its center, all the faces of the unit cell must also have atoms at their centers.

As can be seen in Fig. \(5.33\), not all unit cells are cubic. Other types of unit cells have different restrictions placed on the lattice parameters (edge lengths and angles). Unit cell properties such as cell volume, density, and distances between atoms are calculated just as the calculations are done for cubic unit cells, except the geometry is more complex. (a) With this in mind, calculate the distance between a corner atom and the atom at the body center of a tetragonal unit cell that has \(a=b=549 \mathrm{pm}\) and \(c=769 \mathrm{pm}\). (b) What is the volume of this unit cell?

Salts can be prepared from organic molecules such as acetic acid and methanol. For example, it is possible to prepare sodium acetate, \(\mathrm{NaCH}_{3} \mathrm{CO}_{2}\), and sodium methoxide, \(\mathrm{NaOCH}_{3}\). How do you expect the forces that hold these compounds together in the solid state to differ from those that hold together salts like sodium chloride or sodium bromide?

A commonly occurring mineral has a cubic unit cell in which the metal cations \(\mathrm{M}\) occupy the comers and face centers. Inside the unit cell, there are anions \(\mathrm{A}\) that occupy all the tetrahedral holes created by the cations. What is the chemical formula of the \(M_{x} A_{y}\) compound?

Tetrahedral and octahedral interstitial holes are formed by the spaces left when anions pack in a cubic close-packed array. (a) Which hole can accommodate the larger ions? (b) What is the size ratio of the largest metal cation that can occupy an octahedral hole to the largest that can occupy the tetrahedral hole while maintaining the close-packed nature of the anion lattice? (c) If half the tetrahedral holes are occupied, what will the chemical formula of the compound \(\mathrm{M}_{2} \mathrm{~A}_{2}\) be, where \(\mathrm{M}\) represents the cations and \(\mathrm{A}\) the anions?