Intermolecular Forces and Physical Properties in GOB Chemistry

Intermolecular Forces and Physical Properties

Introduction

Physical properties of substances are measurable characteristics that can be observed through the senses. In chemistry, these properties are often influenced by intermolecular forces, which are attractive forces that exist between molecules and affect their behavior in various states of matter.

Direct Relationships Between Intermolecular Forces and Physical Properties

When intermolecular forces are stronger, certain physical properties also become more pronounced. These direct relationships are important for understanding how substances behave under different conditions.

• Boiling Point (BP): The temperature at which a liquid becomes a gas. Stronger intermolecular forces result in a higher boiling point.

• Melting Point (MP): The temperature at which a solid becomes a liquid. Stronger intermolecular forces result in a higher melting point.

• Surface Tension: The energy required to increase the surface area of a liquid. Stronger intermolecular forces lead to higher surface tension.

• Viscosity: The resistance of a liquid to flow. Stronger intermolecular forces result in higher viscosity.

Example: Which of the following compounds would have the highest melting point?

• a) CH3OH

• b) CsI in H2O (correct answer)

• c) CH3Br

• d) CH3CH2CH3

Indirect Relationships Between Intermolecular Forces and Physical Properties

Some physical properties decrease as intermolecular forces increase. These indirect relationships are also important for predicting the behavior of substances.

• Vapor Pressure: The pressure exerted by a vapor in equilibrium with its liquid phase. Stronger intermolecular forces result in lower vapor pressure.

Intermolecular Force Strength	Vapor Pressure
Strong	Low
Weak	High

Example: Choose the substance with the highest vapor pressure.

• a) CH4

• b) CCl4

• c) CBr4

• d) CI4

Practice Questions and Applications

• Boiling Point: Which of the following will have the lowest boiling point?

  • a) CH3-O-CH3

  • b) H2 (correct answer)

  • c) C4H10

  • d) C4H9OH

  • e) C10

• Viscosity: Which molecules would most likely cause a liquid to have the lowest viscosity?

  • a) Large, polar molecules

  • b) Small, nonpolar molecules (correct answer)

  • c) Small, polar molecules

  • d) Large, nonpolar molecules

• Surface Tension: Which of the following should have the highest surface tension at a given temperature?

  • a) CH4

  • b) CF4 (correct answer)

  • c) CCl4

  • d) CBr4

  • e) CI4

Key Terms and Definitions

• Intermolecular Forces: Attractive forces between molecules, including hydrogen bonding, dipole-dipole interactions, and London dispersion forces.

• Boiling Point ($T_b$): The temperature at which a liquid's vapor pressure equals atmospheric pressure.

• Melting Point ($T_m$): The temperature at which a solid becomes a liquid.

• Surface Tension ($au$): The energy required to increase the surface area of a liquid.

• Viscosity ($u$): A measure of a liquid's resistance to flow.

• Vapor Pressure ($P_{vap}$): The pressure exerted by a vapor in equilibrium with its liquid phase.

Formulas and Equations

• Vapor pressure and temperature relationship (Clausius-Clapeyron Equation): $\ln P = -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T} \right) + C$

• Boiling point occurs when: $P_{vap} = P_{atm}$

Summary Table: Intermolecular Forces and Physical Properties

Physical Property	Effect of Stronger Intermolecular Forces
Boiling Point	Increases
Melting Point	Increases
Surface Tension	Increases
Viscosity	Increases
Vapor Pressure	Decreases

Additional info:

• Hydrogen bonding is a particularly strong type of intermolecular force, especially relevant for compounds containing O-H, N-H, or F-H bonds.

• London dispersion forces are present in all molecules but are the only forces in nonpolar substances.

• Polar molecules generally have higher boiling points and viscosities than nonpolar molecules of similar size.