Intermolecular Forces: Boiling Point, Viscosity, and Vapor Pressure Study Guide

Administrative and Course Information

• Homework Chapters: Students should currently be working on or have completed Chapters 12 and 15 in the Alex platform.
  • Chapter 15 was covered first.
  • Chapter 12 was covered second.
• Slide Accuracy: While slide titles are generally correct, occasional numbering errors may persist from previous versions of the material.

Comparative Analysis of Boiling Points

Boiling points are determined by the strength of Intermolecular Forces (IMFs). The general rule is: the stronger the IMF, the higher the boiling point. If IMFs are similar, secondary factors like molar mass, size, and surface area (polarizability) are considered.

• $CO_2$ vs. $OCS$:
  • $CO_2$ is a nonpolar molecule and only exhibits dispersion forces.
  • $OCS$ is a polar molecule and exhibits dipole-dipole forces.
  • Higher Boiling Point: $OCS$ due to stronger dipole-dipole interactions.
• $NaNO_3$ vs. $NO$:
  • $NaNO_3$ (Sodium Nitrate) is a salt held together by ionic bonding.
  • $NO$ (Nitric Oxide) is a polar molecule with dipole-dipole forces.
  • Higher Boiling Point: $NaNO_3$ because ionic bonds are significantly stronger than any intermolecular force.
• $NH_3$ vs. $PH_3$:
  • $NH_3$ (Ammonia) exhibiting hydrogen bonding (Hydrogen bonded to Nitrogen).
  • $PH_3$ (Phosphine) exhibits much weaker interactions; it is often debated as nonpolar or slightly polar, but lacks hydrogen bonding.
  • Higher Boiling Point: $NH_3$ due to the relative strength of hydrogen bonding.
• $PCl_3$ vs. $AsCl_3$:
  • Both are polar molecules with an $\text{AX}_3\text{E}$ molecular geometry (trigonal pyramidal).
  • Since both have dipole-dipole forces, the tie-breaker is polarizability.
  • Higher Boiling Point: $AsCl_3$ because Arsenic ($As$) is larger, heavier, and more polarizable than Phosphorus ($P$).
• $CH_2Cl_2$ vs. $CHCl_3$:
  • Both are highly polar molecules exhibiting dipole-dipole forces.
  • Higher Boiling Point: $CHCl_3$ (trichloromethane) because it has three chlorine atoms, making it heavier and more polarizable.
• Propane vs. Butane:
  • Both are nonpolar alkanes with dispersion forces.
  • Higher Boiling Point: Butane because it has a longer carbon chain, providing more surface area and higher molar mass.
• Hexane vs. 2,3-dimethylbutane:
  • These are constitutional isomers (same formula) with dispersion forces.
  • Higher Boiling Point: Hexane (linear) has a higher boiling point than 2,3-dimethylbutane (branched) because the linear structure allows for more surface area contact between molecules.
• $CH_3I$ vs. $CH_3Cl$:
  • Both are tetrahedral and polar.
  • The electronegativity difference ($\Delta EN$) for $C-Cl$ is $0.5$, and for $C-I$ is $0.3$.
  • Higher Boiling Point: $CH_3I$ ($42^\text{ }\text{C}$) despite the smaller dipole, because the Iodine atom is significantly larger and more polarizable (nearly $100\text{ }g/mol$ heavier than Chlorine).
• Diol ($\text{two -OH groups}$) vs. Butanol ($\text{one -OH group}$):
  • Higher Boiling Point: The diol due to having two hydrogen bonding sites per molecule compared to one.

Viscosity

Viscosity is defined as a liquid's resistance to flow. It can be visualized as the ease with which molecules move past one another.

• Intermolecular Forces: There is a direct correlation between IMF strength and viscosity. As IMF increases, the "drag" between molecules increases, raising viscosity.
• Molecular Shape and Entanglement: Flexible, long-chain molecules (like alkanes) can become entangled like spaghetti noodles, making them more viscous than smaller, compact molecules.
• Examples of Viscous Liquids: Molasses, motor oil, vegetable oil, lidocaine syrups, and honey.
• Temperature Effects: Viscosity decreases as temperature increases. Heat provides kinetic energy to overcome IMFs, allowing the liquid to flow more easily.
  • Example: Crystallized honey can be liquefied by running it under hot water.
  • Example: Motor oil is significantly less viscous at an operating temperature of $100^\text{ }\text{C}$ than at room temperature.
• Measuring Units: Viscosity is often measured in centipoise ($cP$), a metric unit of dynamic viscosity.
  • Hexane: Low viscosity similar to gasoline.
  • Decane ($C_{10}$): Nearly twice the viscosity of Octane ($C_8$).
  • Dodecane ($C_{12}$): $1.34\text{ }cP$.
  • Hexadecane ($C_{16}$): $3.34\text{ }cP$.

Application: Modern Lubrication Technology

• Polymeric Viscosity Improvers: Engineered polymers are added to motor oils to manage viscosity across temperature ranges.
  • At low temperatures, these polymers are spherical/curled up.
  • At high temperatures (represented by $\Delta$), they uncurl into linear chains, creating drag to maintain the necessary viscosity for engine protection.
• Oil Nomenclature:
  • The number before the 'W' (e.g., $0W$ in $0W-20$) indicates cold weather startability. A zero rating indicates synthetic oil that remains fluid even at extremely low temperatures ($-35^\text{ }\text{C}$).
  • The number after the 'W' indicates viscosity performance at full operating temperatures.

Surface Interactions and Contact Angle

• Contact Angle Experiment: A method to analyze how a liquid interacts with a surface using a microscope and a droplet.
  • If water spreads out (low contact angle), it has a strong interaction with the surface.
  • If water beads up (high contact angle), it indicates a hydrophobic or dispersion-only surface with weak interactions.

Vapor Pressure

Vapor pressure is the pressure exerted by a gas that is in dynamic equilibrium with its liquid phase in a closed container.

• Dynamic Equilibrium: A state where the rate of evaporation equals the rate of condensation. There is no net change in the amount of liquid or gas.
• IMF Relationship: Inversely proportional. The stronger the IMF, the lower the vapor pressure.
  • Strong IMFs act like "strings" holding molecules in the liquid phase.
  • Weak IMFs allow more molecules to escape into the vapor phase, resulting in more collisions and higher pressure.
• Temperature Dependence: As temperature increases, vapor pressure increases non-linearly. Molecules gain kinetic energy, allowing more of them to overcome IMFs and enter the gas phase.
• Boiling Point Definition: The temperature at which the vapor pressure of a liquid equals the external atmospheric pressure.
  • Normal Boiling Point: Measured at standard atmospheric pressure ($1\text{ }atm$ or $760\text{ }torr$).
• Data for Water:
  • $0^\text{ }\text{C}$: Very low vapor pressure (even ice has a vapor above it).
  • $25^\text{ }\text{C}$: $23.8\text{ }torr$.
  • $50^\text{ }\text{C}$: $92.5\text{ }torr$.
  • $100^\text{ }\text{C}$: $760\text{ }torr$ (Boiling point).

Demonstrations and Examples

• Flammability and Vapor Pressure Demo: Using Ethanol and Ethyl Ether.
  • Ethyl Ether: Has weak dipole-dipole forces and high vapor pressure. When lit, the flames are very high because of the dense vapor screaming off the liquid.
  • Ethanol: Has strong hydrogen bonding and lower vapor pressure. The flame is smaller and more contained because fewer molecules are in the gas phase.
  • Note: Liquids themselves do not burn; only the vapors above the liquid ignite.
• The Rocky Mountain Pasta Riddle:
  • Scenario: Cooking pasta at sea level versus a mountaintop.
  • Physics: At high altitudes, atmospheric pressure is lower, so water boils at a lower temperature ($85^\text{ }\text{C}$ instead of $100^\text{ }\text{C}$).
  • Conclusion: Pasta cooks slower on the mountain because the boiling water is at a lower temperature, and thermal energy (heat) is what drives the cooking process.

Questions & Discussion

• Student Question on Aircraft Oil: A student mentioned that aviation-grade oil must be between "four and six" to fly.
  • Instructor Clarification: The student clarified they were referring to oil levels on a dipstick (marked between 4 and 8).
  • Anecdote: The student shared a story about performing an emergency landing on a hot day because the oil level was at the minimum (4). Upon landing, the dipstick appeared empty because the heat had made the oil so light/low-viscosity that it didn't register correctly or had burned off.
• Discussion on Engine Care: Tips included allowing the engine to warm up and circulate oil before driving, especially in cold weather, and the importance of regular fluid changes and tire rotations to protect the investment in a vehicle.