CHEM IMF UNIT

Intermolecular Forces (IMFs)

Attractions between molecules

London Dispersion Forces (LDF)

Weakest IMF; present in all molecules; increase with molar mass and number of electrons

Dipole-Dipole Forces

Attractions between polar molecules

Hydrogen Bonding

Strong dipole attraction when H is bonded to N, O, or F

Ionic Bonding

Attraction between positive and negative ions

Network Covalent Solid

Giant network of covalent bonds; very high melting point (ex: SiO₂, diamond)

Metallic Bonding

Positive metal ions attracted to a sea of electrons

Stronger IMF causes…

Higher melting point, higher boiling point, higher viscosity, higher surface tension

Weaker IMF causes…

Lower melting point and boiling point

Surface Tension

Resistance of a liquid's surface to being broken

Viscosity

Resistance to flow

Volatility

Tendency of a liquid to evaporate

Vapor Pressure

Pressure exerted by vapor above a liquid

Fluidity

Ability of a liquid to flow

Cohesion

Attraction between molecules of the same substance

Adhesion

Attraction between different substances

Capillary Action

Movement of liquid through narrow spaces due to cohesion and adhesion

Miscible

Liquids that mix completely

Immiscible

Liquids that do not mix

Evaporation

Liquid changes to gas at the surface

Boiling

Liquid changes to gas throughout the entire liquid

Equilibrium (liquid-vapor)

Rate of evaporation equals rate of condensation

Hydrogen bonding occurs when…

H is directly bonded to N, O, or F

Examples of hydrogen bonding

H₂O, HF, NH₃, CH₃OH

Examples of molecules with only LDF

CO₂, CH₄, O₂, F₂

Examples of dipole-dipole molecules

HCl, H₂S, PCl₃

Amorphous Solid

No regular crystal pattern (glass)

Crystalline Solid

Ordered repeating structure

Difference between amorphous and crystalline solids

Crystalline solids have definite melting points; amorphous solids soften over a range

Ionic Solid Properties

High MP/BP, brittle, conduct when dissolved

Molecular Solid Properties

Low MP/BP, poor conductors

Metallic Solid Properties

Conduct electricity, malleable, ductile

Network Covalent Solid Properties

Extremely high MP, hard, poor conductor

Body-Centered Cubic (BCC)

Atom in center and corners of cube

Face-Centered Cubic (FCC)

Atoms at corners and centers of faces

Simple Cubic

Atoms only at corners

Triple Point

Temperature and pressure where solid, liquid, and gas coexist

Critical Point

Highest temperature at which a liquid can exist

Normal Melting Point

Temperature where solid and liquid are in equilibrium at 1 atm

Normal Boiling Point

Temperature where liquid and gas are in equilibrium at 1 atm

Sublimation

Solid directly to gas

Deposition

Gas directly to solid

Melting

Solid to liquid

Freezing

Liquid to solid

Condensation

Gas to liquid

Vaporization

Liquid to gas

Heating Curve: Sloped Lines

Temperature changes; kinetic energy changes

Heating Curve: Flat Lines

Phase change; potential energy changes

Melting occurs during which flat section?

First plateau

Boiling occurs during which flat section?

Second plateau

IMF Strength Order (weakest → strongest)

LDF < Dipole-Dipole < Hydrogen Bonding < Metallic ≈ Ionic < Network Covalent

How to identify hydrogen bonding quickly

Look for H attached directly to N, O, or F

How to identify ionic compounds quickly

Metal + nonmetal

How to identify nonpolar molecules quickly

Symmetrical molecules often have only LDF

How to identify polar molecules quickly

Uneven charge distribution; dipole-dipole forcesp