Unit II – Intermolecular Forces: Vocabulary Review

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A set of vocabulary flashcards covering key terms and definitions from the Unit II lecture on intermolecular forces, including types of forces, their effects on physical properties, and related molecular concepts.

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30 Terms

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Intermolecular Forces (IMF)

Attractive forces that act between separate molecules or ions and influence physical properties such as boiling point, melting point, and solubility.

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Kinetic Energy (of particles)

Energy of motion that tends to keep particles apart in gases, liquids, and solids.

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Attractive Forces (between particles)

Intermolecular attractions that draw particles together, opposing their kinetic energy.

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Gas – Characteristic Properties

Widely separated particles in rapid, random motion that expand to fill their container.

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Liquid – Characteristic Properties

Particles held close by attractive forces; definite volume, takes the shape of its container, more dense than gases.

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Solid – Characteristic Properties

Particles packed in a fixed, ordered arrangement with extremely strong intermolecular forces and minimal space between particles.

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Covalent Bond

Intramolecular bond where atoms share valence electrons; much stronger than any intermolecular force.

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Polar Covalent Bond

Bond in which two atoms share electrons unequally, producing partial charges (commonly when bonding involves N, O, F, Cl, or Br).

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Non-polar Covalent Bond

Bond in which two identical or similarly electronegative atoms share electrons equally, resulting in no permanent dipole.

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Instantaneous (Temporary) Dipole

Momentary uneven electron distribution in an atom or molecule that can induce attraction with neighboring particles.

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London Dispersion Forces

Weak attractions arising from synchronized instantaneous dipoles in all atoms and molecules; only intermolecular force present in non-polar substances.

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Polarizability

Ease with which an electron cloud is distorted; increases with molar mass and leads to stronger dispersion forces.

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Dipole-Induced Dipole Force (Debye Force)

Attractive interaction where a polar molecule induces a dipole in a nearby non-polar molecule.

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Dipole-Dipole Force

Attraction between the positive end of one polar molecule and the negative end of another polar molecule.

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Hydrogen Bond

Special dipole-dipole attraction between H bonded to highly electronegative F, O, or N and a lone pair on F, O, or N in another molecule.

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Tetrahedral Hydrogen-Bond Network (Water)

Structure allowing each H₂O molecule to form four hydrogen bonds, giving water unusually high BP, MP, Cp, and ΔH°vap.

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Molar Mass Effect on Dispersion

Within similar molecules, larger molar mass → larger electron cloud → stronger dispersion forces.

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Enthalpy of Vaporization (ΔH°vap)

Heat required to convert one mole of liquid to gas; increases as intermolecular forces strengthen.

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Vapor Pressure

Pressure exerted by a vapor in dynamic equilibrium with its liquid; decreases as intermolecular forces strengthen.

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Dynamic Equilibrium (Liquid ↔ Gas)

State where the rate of evaporation equals the rate of condensation in a closed system.

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Boiling Point

Temperature at which a liquid’s vapor pressure equals external (atmospheric) pressure.

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Normal Boiling Point

Boiling point measured when external pressure is exactly 1 atm.

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Relationship: IMF vs Vapor Pressure

Stronger intermolecular forces → lower vapor pressure → higher boiling point.

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Relationship: IMF vs Boiling Point

Substances with stronger intermolecular forces require more energy to boil, thus have higher boiling points.

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Hierarchy of IMF Strength

Hydrogen bonding > Dipole-dipole > Dipole-induced dipole > London dispersion (for comparable molecules).

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Dispersion-Only Molecules

Non-polar molecules like C₈H₁₈ (octane) or C₅H₁₂ (pentane) that rely solely on London dispersion forces.

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Polar Molecule

Molecule with a permanent dipole moment due to uneven charge distribution (e.g., isopropyl alcohol, H₂O).

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Non-polar Molecule

Molecule with symmetrical charge distribution and no permanent dipole (e.g., pentane, octane).

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Heat Capacity (Cp) of Water

Amount of heat required to raise the temperature of water, unusually high because of extensive hydrogen bonding.

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Debye Interaction

Synonym for dipole-induced dipole attraction described by Peter Debye.