Topic 12:Intermolecular Forces

IMF

Non covalent interaction between atoms, ions, molecules that determine macroscopic properties (boiling point, viscosity solubility)

How do solids, liquids, and gases differ in IMFs, distance and motion

Solids: strong IMF fixed shape and volume, limited motion

Liquid: medium IMF, fixed volume but shale can vary. Molecules slide around

Gases: weakest IMF, variable shape and volume, molecules move freely

Normal boiling point

Temperature at which a liquid becomes a gas at 1 atm pressure

Relationship of boiling point and IMF

Stronger IMF= higher boiling point (more energy needed to break bonds)

Types of IMF

1. Ion-ion

2. Dipole dipole

3. Hydrogen bonds

4. London dispersion forces

5. Ion-dipole interactions

What are ion - ion interactions

Attraction between FULLY charged ions (governed by Coulomb’s law)

Where do ion-ion interactions occur

Ionic compounds that from crystal lattices of alternating cations or anions

Lattice energy

Energy required to separate one mole of an ionic solid into gas ions (pretty much measures ionic bond strength)

2 factors that determine lattice energy

Charge magnitude: greater charges = stronger attraction

Ion size: smaller ions = shorter distance and stronger lattice energy

How do ion size and charge vary across the periodic table

Down a group of= ion size increasing

Across a group of= cation charge increases, size decreases

Anion size changes little overall

How does lattice energy explain melting or boiling points?

Higher lattice energy causes stronger ionic bonds and higher melting boiling points

• mgO > NaCl because Mg2+ and O 2- have higher charges

Dipole dipole interactions

Attraction between molecules with permanent partial charges (polar molecules)

Give an example of dipole dipole effects on boiling points

CH4 (non polar, BP -161*C) vs CH3Cl (polar, BP = -24*C)

• the polar molecule has stronger IMFs and higher boiling point

London dispersion forces

Weakest attraction formed by electron fluctuations, present in all compounds

What facets the strength of dispersion forces

• molecular size and mass (more electrons = more polarizability)

• Molecular shape (longer, less branched molecules = more surface contact and stronger dispersion)

Two examples of dispersion force trends

• boiling point increases with molecular mass in noble gases or alkanes

• Straight chain alkanes have higher boiling points than branched isomers

Hydrogen bond

Strong dipole dipole interaction between a hydrogen afom that is covalentlh. Landed to N, O, or F( the donor) and a lone pair on another electronegative atom (the acceptor)

Define hydrogen bond donor vs acceptor

Donor: atom where H is covalently bonded (N-H)

Acceptor: atom with lone pair accepting the H bond (N, O, F)

Difference between intermolecular and intra molecular hydrogen bonds

Intermolecular: between different molecules

Intramolecular: within the same molecule

Why are hydrogen bonds directional

they align linearly between the donors H and the acceptors lone pair orbital which gives the most overlap

Give examples of substances with hydrogen bonding

Water, HF, NH3, DNA base pairs, Kevlar fibers

How do hydrogen bonds explain why ice floats

hydrogen bonds in ice make hexagon lattices which lower the density of the ice compared to water

What are ion dipole interactions

Attractions between a charged ion and polar molecule (NA+ and H2O)

How do ion dipole interactions cause ionic compounds to dissolve

Polar water molecules surround ions, stabilize them and break ionic lattices

What is a sphere of hydration

A shell of oriented water molecules surrounding a dissolved ion, reducing electrostatic attraction between ions

Solvent vs solute vs solubility

Solvent : component in greatest amount

Solute: component dissolved in solvent

Solubility: maximum solute that dissolves in a given solvent

How is solubility determined by IMFs

Solubility increases when solute/solvent IMFs are similar in strength and type to those within the solute and solvent

Miscible

Two liquids that mix in all proportions without separating into layers

Like dissolves like

Polar solvents dissolve polar/ionic solutes, non polar solvents dissolve non polar solutes because similar IMFs are compatible

Hydrophobic vs hydrophilic

Hydrophobic: repels water

Hydrophilic attracts water

How do you identify the main IMF in a compound?

Examine molecular polarity, presence of H in N/O/F bonds, and ion presence

What is viscosity

Resistance of a liquid to flow or deformation, a measure internal friction between molecules

How does IMF strengthen viscosity

Stronger IMF = higher viscosity because it has a higher bond and can hold together better

Surface tension

The resistance of a liquid surface to external force due to cohesive IMFs

How does IMF strength affect surface tension

Stronger IMF = higher surface tension

Rank IMF from strongest to weakest

Ion-ion, ion dipole, hydrogen bonds, dipole dipole, London dispersion