Intermolecular Forces: Liquids, Solids, and Phase Changes

Which forces oppose the vaporization of each substance? 

Which substance has the lower boiling point? Explain.

a.${\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CH}}_2{\mathbf{CH}}_3$ OR 

b.$\mathbf{NaBr}\mathbf{ }\mathbf{ }\mathbf{OR}\mathbf{ }{\mathbf{PBr}}_3$

c.${\mathbf{H}}_2\mathbf{O}\mathbf{ }\mathbf{ }\mathbf{ }\mathbf{OR}\mathbf{ }\mathbf{ }\mathbf{HBr}$

Which substance has the higher boiling point? Explain.

a.${\mathbf{CH}}_3\mathbf{OH}$ OR ${\mathbf{CH}}_3{\mathbf{CH}}_3$

b.$\mathbf{FNO}$  OR $\mathbf{ClNO}$

c.

For pairs of molecules in the gas phase, average H-bond dissociation energies are 17 kJ/mol for ${\mathbf{NH}}_3$, 22 kJ/mol for ${\mathbf{H}}_2\mathbf{O}$ , and 29 kJ/mol for $\mathbf{HF}$ . Explain this increase in H-bond strength.

Dispersion forces are the only intermolecular forces present in motor oil, yet it has a high boiling point. Explain.

Why does the antifreeze ingredient ethylene glycol ( ${\mathbf{HOCH}}_2{\mathbf{CH}}_2\mathbf{OH}$) have a boiling point of 197.6°C, whereas propanol (${\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CH}}_2\mathbf{OH}$ ), a compound with a similar molar mass, has a boiling point of only 97.4°C?

Before the phenomenon of surface tension was understood, physicists described the surface of the water as being covered with a “skin.” What causes this skin-like phenomenon?

In solid  ${\mathbf{I}}_2$, is the distance between the two I nuclei of one  molecule longer or shorter than the distance between two I nuclei of adjacent  molecules? Explain.

As molten iron cools to 1674 K, it adopts one type of cubic unit cell; then, as the temperature drops below 1181 K, it changes to another, as depicted below:

1. What is the change in unit cell?
2. Which crystal structure has the greater packing efficiency?

Rank the following in order of decreasing surface tension at a given temperature, and explain your ranking: (a) ${\mathbf{CH}}_3\mathbf{OH}$  (b) ${\mathbf{CH}}_3{\mathbf{CH}}_3$ (c)  ${\mathbf{CH}}_2\mathbf{O}$

For structures consisting of identical atoms, how many atoms are contained in the simple, body-centered, and face-centered cubic unit cells? Explain how you obtained the values.

Small, equal-sized drops of oil, water, and mercury lie on a waxed floor. How does each liquid behave? Explain.

Why does an aqueous solution of ethanol ( ${\mathbf{CH}}_3{\mathbf{CH}}_2\mathbf{OH}$) have a lower surface tension than water?

Why are units of energy per area $(\mathbf{J}\mathbf{/}{\mathbf{m}}^2)$  used for surface tension values?

Does the strength of the intermolecular forces in a liquid change as the liquid is heated? Explain. Why does liquid viscosity decrease with rising temperature?

Rank the following in order of increasing surface tension at a given temperature, and explain your ranking: (a) ${\mathbf{CH}}_3{\mathbf{CH}}_2{\mathbf{CH}}_2\mathbf{OH}$  (b)  ${\mathbf{OHCH}}_2\mathbf{CH}\mathbf{\left(}\mathbf{OH}\mathbf{\right)}\mathbf{C}{\mathbf{H}}_2\mathbf{OH}$  (c)  ${\mathbf{OHCH}}_2{\mathbf{CH}}_2\mathbf{OH}$

Rank the compounds in Problem 12.61 in order of decreasing viscosity at a given temperature; explain your ranking.

Rank the compounds in Problem 12.62 in order of increasing viscosity at a given temperature; explain your ranking.

Soil vapor extraction (SVE) is used to remove volatile organic pollutants, such as chlorinated solvents, from soil at hazardous waste sites. Vent wells are drilled, and a vacuum pump is applied to the subsurface.

1. How does this remove pollutants?
2. Why does heating combined with SVE speed the process?

Use Figure 12.1, p. 439 to answer the following:

(a) Does it take more heat to melt 12.0 g of ${\mathbf{CH}}_4$ or 12.0g of $\mathbf{Hg}$ ?

(b) Does it take more heat to vaporize 12.0g of ${\mathbf{CH}}_4$  or 12.0g of  ?

(c) What is the principal intermolecular force in each sample?

Pentanol ( ${\mathbf{C}}_5{\mathbf{H}}_{11}\mathbf{OH}$,$\mathbf{\mu }\mathbf{=}\mathbf{88}\mathbf{.}\mathbf{15}\mathbf{g}\mathbf{/}\mathbf{mol}$  ) has nearly the same molar mass as hexane ( ${\mathbf{C}}_6{\mathbf{H}}_{14}$,$\mathbf{\mu }\mathbf{=}\mathbf{86}\mathbf{.}\mathbf{17}\mathbf{g}\mathbf{/}\mathbf{mol}$  ) but is more than 12 times as viscous at  $\mathbf{20}\mathbf{°}\mathbf{C}$. Explain.

For what types of substances are water a good solvent? For what types is it a poor solvent? Explain.

A water molecule can engage in as many as four H bonds. Explain.

Warm-blooded animals have a narrow range of body temperature because their bodies have a high-water content. Explain.

What property of water keeps plant debris on the surface of lakes and ponds? What is the ecological significance of this?

A drooping plant can be made upright by watering the ground around it. Explain.

Describe the molecular basis of the property of water responsible for the presence of ice on the surface of a frozen lake.

Describe in molecular terms what occurs when the ice melts.

What is the difference between an amorphous solid and a crystalline solid on the macroscopic and molecular levels? Give an example of each.

How are a solid’s unit cell and crystal structure related?

An element has a crystal structure in which the width of the cubic unit cell equals the diameter of an atom. What type of unit cell does it have?

What specific difference in the positioning of spheres gives a crystal structure based on the face-centered cubic unit cell less empty space than one based on the body-centered cubic unit cell?

Both solid Kr and solid Cu consist of individual atoms. Why do their physical properties differ so much?

Predict the effect (if any) of an increase in temperature on the electrical conductivity of

1. a conductor;
2. a semiconductor;
3. an insulator.

Besides the type of unit cell, what information is needed to find the density of a solid consisting of identical atoms?

What type of crystal lattice does each metal form? (The number of atoms per unit cell is given in parentheses.)

(a) Ni (4)

(b) Cr (2)

(c) Ca (4)

What is the number of atoms per unit cell for each metal?

   (a) Polonium, Po              (b) Manganese, Mn (c) Silver, Ag

When cadmium oxide reacts to form cadmium selenide, a change in unit cell occurs, as depicted below: (a) What is the change in unit cell? (b) Does the coordination number of cadmium change? Explain.

Of the five major types of crystalline solid, which does each of the following forms, and why: (a) Ni; (b) F₂; (c) CH₃OH; (d) Sn; (e) Si; (f) Xe?

Of the five major types of crystalline solid, which does each of the following forms, and why: (a) SiC; (b) Na₂SO₄; (c) SF₆; (d) cholesterol (C₂₇H₄₅OH) ; (e) KCl; (f) BN?

Zinc oxide adopts the zinc blende crystal structure (Figure). How many ${\mathbf{Zn}}^{\mathbf{2}\mathbf{+}}$ ions are in the ZnO unit cell?

Calcium sulfide adopts the sodium chloride crystal structure (Figure). How many ${\mathbf{S}}^{\mathbf{2}\mathbf{-}}$ ions are in the CaS unit cell?

Zinc selenide (ZnSe) crystallizes in the zinc blende structure (see Figure) and has a density of 5.42 g/cm³.

(a) How many Zn and Se ions are in each unit cell?

(b) What is the mass of a unit cell?

(c) What is the volume of a unit cell?

(d) What is the edge length of a unit cell?

An element crystallizes in a face-centered cubic lattice and has a density of $\mathbf{1}\mathbf{.}\mathbf{45}\mathbf{ }\mathbf{g}\mathbf{/}{\mathbf{cm}}^{\mathbf{3}}$. The edge of its unit cell is 4.52108 cm.

(a) How many atoms are in each unit cell?

(b) What is the volume of a unit cell?

(c) What is the mass of a unit cell?

(d) Calculate an approximate atomic mass for the element.

Classify each of the following as a conductor, insulator, or semiconductor: 

1. Phosphorus
2. Mercury
3. germanium.

Classify each of the following as a conductor, insulator, or semiconductor: 

1. carbon (graphite)
2. sulfur
3. platinum.

Predict the effect (if any) of an increase in temperature on the electrical conductivity of (a) antimony; (b) tellurium; (c) bismuth.

Predict the effect (if any) of a decrease in temperature on the electrical conductivity of (a) silicon; (b) lead; (c) germanium.

Polonium, the Period 6 member of Group 6A (16), is a rare radioactive metal that is the only element with a crystal structure based on the simple cubic unit cell. If its density is $\mathbf{9}\mathbf{.}\mathbf{142}\mathbf{ }\mathbf{g}\mathbf{/}{\mathbf{cm}}^{\mathbf{3}}$, calculate an approximate atomic radius for polonium.

The coinage metals—copper, silver, and gold—crystallize in a cubic closest packed structure. Use the density of copper (8.95 g/cm3 ) and its molar mass (63.55 g/mol) to calculate an approximate atomic radius for copper.