Chapter 6 - Intermolecular Forces

6.1 Intramolecular Forces vs Intermolecular Forces

Intramolecular Forces: interactions within a molecule

Intermolecular Forces: forces that act between molecules and molecules and ions; weaker than intermolecular forces and act over longer distances; influence physical properties of substances

• a greater energy is needed to overcome greater intermolecular forces to cause phase changes or the breaking of bonds
  • solids have strong intermolecular forces with high melting and boiling points
  • liquids have weaker intermolecular forces
  • gases have extremely weak particle-particle interactions with low melting and boiling points

6.2 London Dispersion Forces

London Dispersion Forces: when atoms/molecules approach each other, electrostatic interactions cause a temporary unequal electron distribution, known as a temporary dipole, allowing for interactions between molecules

• strength of these interactions increases as the number of electrons increases
  • increased likelihood of unequal electron distribution or polarizability, leading to stronger temporary dipoles and therefore stronger intermolecular interactions
• larger surface area creates stronger interactions

Viscosity: measure of resistance to flow

• greatest London dispersion forces result in stronger intermolecular attractions, increasing viscosity

6.3 Interactions Involving Polar Molecules

Dipole-Dipole interactions: attractions between molecules with opposite partial charge sign

Hydrogen Bonds: When H shares electrons with a highly electronegative atom (N, O, F), the electron density on H is significantly reduced, creating a high-strength dipole

• represented by blue dotted lines and are the strongest dipole-dipole interactions
• can form between molecules of different substances

Ion-Dipole Interactions: Attractions between ion and a molecule with a permanent dipole

Sphere of Hydration: cluster of water molecules surrounding an ion in an aqueous solution

• ion dipole interactions overcome lattice energy
• inner sphere of hydration: water molecules interacting directly with ion, 4-9 molecules
  • sphere of solvation: substance dissolved in a liquid other than water
• outer sphere of hydration: molecules that form hydrogen bonds with inner sphere molecules

6.4 Solubility

Solvent: substance present in largest amount of moles

Solutes: substances dissolved in solvent

Solubility: how much solute can dissolve in a given volume of solution

• stronger intermolecular forces decrease solubility
• weaker intermolecular forces increase solubility
• polar solutes tend to dissolve in polar solvents while nonpolar solutes tend to dissolve in nonpolar solvents

Miscible: when two liquid compounds have unlimited solubility in each other

Dipole-induced diple interactions: attraction between a polar molecule and the oppositely charged pole it temporarily induces in another molecule, allowing for some nonpolar substances to dissolve in polar substances

Hydrophobic: “water fearing”, keeps compounds from dissolving in water; nonpolar interactions - London dispersion

Hydrophilic: “water loving”, promotes solubility in water; polar interactions - dipole-dipole and hydrogen bonding

6.5 Phase Diagrams

Pressure: ration of force to surface area

Atmospheric Pressure: weight of Earth’s atmosphere pressing down on its surface divided by surface area, 1.013x10^5 Pa

Phase Diagrams: graphs of pressure vs temperature used to represent which phases are most stable at different combinations of pressure and temperature

• Triple point: point in which all three phase transition lines meet - liquid, solid and vapor states of a substance exist simultaneously
• Critical point: where liquid-gas equilibrium line ends and two states are indistinguishable,
  • supercritical fluid: substances above critical point, easily diffusible and dissolves substances like a liquid

6.6 Properties of Water

Surface Tension: ability of the surface of a liquid to resist external force

Meniscus: curved surface resulting from the competing forces of cohesion (interactions between like particles) and adhesion (interactions between materials of differing substances)

Capillary Action: liquid’s ability to flow against gravity, spontaneously rising in a narrow tube or similar structure

Density of water increases as temperature decreases, but decreases as it cools towards freezing point

• ice is composed of hexagonal arrays of water molecules, decreasing density when frozen

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