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Chapter 11: Problem 73

Covalent bonding occurs in both molecular and covalent-network solids. Why do these two kinds of solids differ so greatly in their hardness and melting points?

Short Answer

Expert verified
The difference in hardness and melting points between molecular and covalent-network solids arises due to the difference in the forces holding them together. Molecular solids have weaker intermolecular forces, resulting in lower hardness and melting points. In contrast, covalent-network solids have stronger covalent bonds, leading to much higher hardness and melting points.

Step by step solution

01

Molecular and Covalent-Network Solids

Molecular solids are made up of discrete molecules held together by relatively weak intermolecular forces like dipole-dipole interactions, london dispersion forces and hydrogen bonding. On the other hand, covalent-network solids consist of atoms bonded together by a network of strong covalent bonds throughout the entire structure.
02

Hardness and Melting Point Differences

Hardness and melting point properties depend on the strength of the forces holding the solid together. In molecular solids, the relatively weak intermolecular forces can be more easily broken, which results in lower hardness and lower melting points. Covalent solids, however, have strong covalent bonds that require a lot of energy to break. This results in much higher hardness and melting points in covalent-network solids compared to molecular solids.
03

Examples

For instance, sugar (sucrose) is a molecular solid with a melting point of \(186^\circ C\), while diamond, a covalent-network solid, has a melting point of approximately \(3820^\circ C\) and is one of the hardest known materials. In summary, the difference in hardness and melting points between molecular and covalent-network solids comes from the difference in the forces holding them together. Molecular solids have weaker intermolecular forces, leading to lower hardness and melting points, while covalent-network solids have stronger covalent bonds, resulting in much higher hardness and melting points.

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Most popular questions from this chapter

NaF has the same structure as \(\mathrm{NaCl}\). (a) Use ionic radii from Chapter 7 to estimate the length of the unit cell edge for NaF. (b) Use the unit cell size calculated in part (a) to estimate the density of NaF.

(a) Two pans of water are on different burners of a stove. One pan of water is boiling vigorously, while the other is boiling gently. What can be said about the temperature of the water in the two pans? (b) A large container of water and a small one are at the same temperature. What can be said about the relative vapor pressures of the water in the two containers?

Name the phase transition in each of the following situations, and indicate whether it is exothermic or endothermic: (a) When ice is heated, it turns to water. (b) Wet clothes dry on a warm summer day. (c) Frost appears on a window on a cold winter day. (d) Droplets of water appear on a cold glass of beer.

What type of intermolecular force accounts for the following differences in each case? (a) \(\mathrm{CH}_{3} \mathrm{OH}\) boils at \(65^{\circ} \mathrm{C}, \mathrm{CH}_{3} \mathrm{SH}\) boils at \(6{ }^{\circ} \mathrm{C}\). (b) Xe is liquid at atmospheric pressure and \(120 \mathrm{~K}\), whereas Ar is a gas. (c) \(\mathrm{Kr}\), atomic weight 84 , boils at \(120.9 \mathrm{~K}\), whereas \(\mathrm{Cl}_{2}\), molecular weight about 71 , boils at \(238 \mathrm{~K}\). (d) Acetone boils at \(56{ }^{\circ} \mathrm{C}\), whereas 2 -methylpropane boils at \(-12{ }^{\circ} \mathrm{C}\).

The vapor pressure of a volatile liquid can be determined by slowly bubbling a known volume of gas through it at a known temperature and pressure. In an experiment, \(5.00 \mathrm{~L}\) of \(\mathrm{N}_{2}\) gas is passed through \(7.2146 \mathrm{~g}\) of liquid benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}\), at \(26.0{ }^{\circ} \mathrm{C}\). The liquid remaining after the experiment weighs \(5.1493 \mathrm{~g}\). Assuming that the gas becomes saturated with benzene vapor and that the total gas volume and temperature remain constant, what is the vapor pressure of the benzene in torr?
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