Chapter 11 - Conceptual Questions

Question 1

Why, for covalent molecular compounds, is the heat of vaporization typically 3-7 times larger than the heat of fusion for the same mass of a substance?

Answer 1

Vaporization breaks nearly all IMFs, but melting breaks a small proportion of them (~10-20%)

Question 2

Why does density decrease with increasing temperature within a given phase?

Answer 2

As temperature increases, the average KE increases, and so the average speed increases. The faster moving molecules occupy slightly more space, and so the volume increases slightly. Thus density increases slightly.

Question 3

Why does density decrease with increasing temperature more for gases than for liquids, and more for liquids than for solids?

Answer 3

This occurs because there are weaker IMF’s in gases compared to liquids, therefore the change in temperature (KE) has a bigger effect on the volume.

Question 4

What keeps atoms apart in a crystalline solid?

Answer 4

Repulsion of like charges will maintain atomic separation (e.g., cations or nuclei repel each other)

Question 5

What is the basis of the force holding substances together?

Answer 5

Electrostatic attraction of opposite charges is the basis of cohesive forces between atoms and molecules.

Question 6

When camping at high altitude, you need to pay particular attention to changes in cooking times for foods that are boiled in water. If you like eggs that are boiled for 10 minutes near sea level, would you have to cook them for a longer or a shorter time at 3200 m to get the egg you like?

Answer 6

You would have to cook the egg for a longer time. Since there is lower atmospheric pressure at high altitude, water boils at a lower temperature than near sea level (boiling when vapor pressure = atmospheric pressure). Since the temperature is lower, it would take longer to transfer an equivalent amount of heat to the egg.

Question 7

Describe the molecular basis of the phenomenon of vapor pressure.

Answer 7

In molecular terms, vapor pressure involves molecules of a liquid vaporizing from the liquid phase, colliding with any surface above the liquid, and thus exerting pressure. The equilibrium is a dynamic one because molecules of the liquid are continually leaving the liquid phase and returning to it from the vapor phase (via condensation)

Question 8

The pressure in a cylinder of nitrogen continuously decreases as gas is released from it. Conversely, a cylinder of propane maintains a constant pressure as propane is released. Explain this difference.

Answer 8

• The pressure in the nitrogen cylinder at room temp (above its critical temp -147 C) decreases continuously as gas is released because the number of molecules in the vapor phase, which governs the pressure, decreases continuously. - The pressure of propane at room temp (below its critical temp) is constant because liquid propane and gaseous propane exist at equilibrium in the cylinder. The pressure will remain constant at the vapor pressure of propane until only gaseous propane remains. At that point, the pressure will decrease until the propane is gone.

Question 9

How is vapor pressure related to intermolecular forces in the liquid?

Answer 9

If such molecules attract each other strongly, the vapor pressure will be low. If they attract each other weakly, the vapor pressure will be relatively high.

Question 10

Why do molecular solids melt at relatively low temperatures?

Answer 10

Molecular substances have relatively low melting points because the forces broken by melting are weak intermolecular attractions in the solid state, not strong bonding attractions as in ionic compounds.

Question 11

What does the covalent network for diamond look like?

Answer 11

Diamond

Question 12

What does the covalent network for fullerenes look like?

Answer 12

Fullerene

Question 13

What does the covalent network for SiO₂/Quartz look like?

Answer 13

Quartz

Question 14

What does the covalent network for graphite look like?

Answer 14

Graphite

Question 15

What is the difference between covalent bonding and covalent network bonding?

Answer 15

Covalent bonding refers to, for example, the bond between carbon and carbon in ethanol. Network covalent bonding is when the covalent bonds extend in all directions to form a crystalline structure.

Question 16

What do the variables stand for in Coulombs Law?

Answer 16

Question 17

According to Coulombs law, what effect will a higher lattice energy have on melting point?

Answer 17

A higher lattice energy will lead to a higher melting point.

Question 18

What does the heating curve for water look like?

Answer 18

Question 19

Where are phase transitions shown on a heating curve?

Answer 19

They are shown on the flat lines.

Question 20

Why is temperature constant during phase transitions?

Answer 20

All of the energy that would normally increase the temperature of the object, is being used to convert the object from one phase to another (breaking IMF’s/forming IMF’s)

Question 21

When are kinetic or potential energy changing in phase transitions/heating curves?

Answer 21

• Kinetic energy only changes when temperature changes (when the line has a slope in heating curves)
• Potential energy changes when IMF’s are being broken/formed during actual phase transitions (when the line is flat in heating curves)

Question 22

What are the “normal” melting and boiling points?

Answer 22

The normal melting and boiling points are located at 1 atm on the phase diagram

Question 23

What are critical points?

Answer 23

Critical point is a term used in thermodynamics to describe a pressure and temperature condition beyond which distinctions between phases, particularly between gas and liquid, cease to exist. Beyond such a point, a substance is neither completely liquid nor completely gaseous; it displays properties of both the liquid phase and the gas phase and is referred to as a supercritical fluid.

Question 24

What are triple points?

Answer 24

In thermodynamics, the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium.

Question 25

What do phase boundaries represent?

Answer 25

Phase boundaries which refer to lines that mark conditions under which multiple phases can coexist at equilibrium. Phase transitions occur along lines of equilibrium.

Question 26

Where do you expect to find (s), (l), and (g) phases represented on the phase diagram?

Answer 26

Typically like this:

Question 27

Do ionic solids or covalent network solids have a higher melting point?

Answer 27

Covalent Network solids have higher IMF’s and higher melting points

Question 28

What is the relative strength of these intramolecular forces: covalent & ionic bonds.

Answer 28

Question 29

What is the relative strengths of these intermolecular forces: H-bonding, dipole-dipole & London/dispersion

Answer 29