Quiz 3 - Properties of Liquids | Liquids and Solids

Propane (CH₃CH₂CH₃) has the lowest boiling point because it is nonpolar and only experiences weak London dispersion forces. In contrast, the other substances have stronger intermolecular forces, such as dipole-dipole interactions or hydrogen bonding, leading to higher boiling points.

At the critical point on a phase diagram, the liquid and vapor phases become indistinguishable from each other. Beyond this point, the substance exists as a supercritical fluid, where the distinctions between the liquid and gas phases disappear.

Carbon tetrachloride (CCl₄) is a nonpolar molecule, while methanol (CH₃OH) is a polar molecule with hydrogen bonding. Because polar and nonpolar substances do not mix well, CCl₄ and methanol are likely to separate into two layers when mixed.

In contrast, ethanol and methanol are both polar and miscible, hexane and pentane are both nonpolar and miscible, and carbon tetrachloride and hexane are both nonpolar and miscible.

As intermolecular forces increase, the molecules in a liquid are held together more strongly, making it harder for them to escape into the vapor phase. This results in a decrease in vapor pressure. In contrast, the boiling point, enthalpy of vaporization, and viscosity all increase with stronger intermolecular forces.

To find the boiling point of water at the summit of Mt. Everest, where the atmospheric pressure is 0.333 atmospheres, we can use the Clausius-Clapeyron equation:

Therefore,

ln $\left(\frac{0.333}{1}\right)$ = - $\frac{40700}{8.314}$ $\left(\frac{1}{{\mathrm{T}}_2} - \frac{1}{373}\right)$

⇒  - 1.100 = - 4895 $\left(\frac{1}{{\mathrm{T}}_2} - \frac{1}{373}\right)$

⇒  2.247 x 10^-4 = $\frac{1}{{\mathrm{T}}_2} - \frac{1}{373}$

⇒ $\frac{1}{{\mathrm{T}}_2}$ = 2.906 x 10^-3

⇒ T₂ = 344 K = 71 ^oC

London dispersion forces are present in all molecules, whether polar or nonpolar. They arise due to temporary fluctuations in the electron distribution within molecules, which induce temporary dipoles.

• CH₃CH₃ is a nonpolar molecule, so its intermolecular forces are primarily London dispersion forces.
• CH₃OH is a polar molecule and primarily experiences hydrogen bonding. However, it also experiences London dispersion forces in addition to hydrogen bonding.

Therefore, both substances experience London dispersion forces.

Boiling point is influenced by the strength of intermolecular forces:

• CH₄ experiences only London dispersion forces, which are weak.
• SiH₄ also experiences only London dispersion forces, similar to CH₄.
• PH₃ has weak dipole-dipole interactions, but they are not as strong as hydrogen bonding.
• NH₃ has strong hydrogen bonding due to the presence of nitrogen bonded to hydrogen, which significantly increases its boiling point.

Hydrogen bonding is much stronger than the other intermolecular forces present in CH₄, SiH₄, and PH₃, leading to NH₃ having the highest normal boiling point among the listed substances.

• Option A is incorrect because, typically, solids are denser than liquids for most substances, including nitrogen (except in anomalous cases like water). Solid nitrogen is denser than liquid nitrogen.
• Option B is incorrect because nitrogen cannot be liquefied at 150 K since its critical temperature is 126.0 K. Above this temperature, nitrogen cannot be liquefied regardless of the pressure.
• Option C is incorrect because at 63.1 K and 1 atm, nitrogen would be in the solid state, not a liquid-gas equilibrium. The triple point occurs at 63.1 K and 0.127 atm, where solid, liquid, and gas phases coexist.
• Option D is correct. Below the triple point (63.1 K, 0.127 atm), nitrogen sublimates, meaning it transitions directly from solid to gas without passing through the liquid phase. At 0.100 atm, which is below the triple point pressure, nitrogen will sublime when heated from 60 K to 70 K.

The enthalpy of fusion (ΔH_fusion​) is the amount of heat required to change a substance from the solid phase to the liquid phase at its melting point. In the heating curve shown, the segment BC represents the phase change from solid to liquid (melting). During this phase change, temperature remains constant while the substance absorbs heat to overcome the forces holding the solid together.

The length of the horizontal segment BC is related to the enthalpy of fusion. A longer BC segment indicates a higher enthalpy of fusion because more heat is required to melt the substance.

Freezing is the process of a liquid turning into a solid, which is exothermic because it releases heat. At –10 ^oC, which is below the freezing point of water (0 ^oC), the process is spontaneous because water naturally freezes at temperatures below 0 ^oC under 1 atm pressure.