Intermolecular ForcesIntermolecular Forces

Forces between moleculesForces between molecules

Intermolecular Forces (IMFs)Intermolecular Forces (IMFs)

Different molecules have different forces that act Different molecules have different forces that act between them.between them.

These forces attracting the separate molecules These forces attracting the separate molecules together control many physical propertiestogether control many physical properties Boiling PointBoiling Point Melting PointMelting Point SolubilitySolubility ViscosityViscosity Surface TensionSurface Tension

Three Main IMFsThree Main IMFs

Dipole-dipole forcesDipole-dipole forces

Hydrogen bondingHydrogen bonding

London dispersion forces (LDFs)London dispersion forces (LDFs)

Dipole-Dipole ForcesDipole-Dipole Forces

Electronegativity – the ability of an atom in Electronegativity – the ability of an atom in a compound to attract electrons to itself.a compound to attract electrons to itself.

Fluorine has the highest electronegativityFluorine has the highest electronegativity

The electrons in a compound spend more The electrons in a compound spend more time around the most electronegative time around the most electronegative atoms than the other atoms.atoms than the other atoms.

ElectronegativityElectronegativity If the electrons are spending more time If the electrons are spending more time

around the one particular atom, how will around the one particular atom, how will that atom be different from the other that atom be different from the other atoms?atoms?

It will have a more negative charge than It will have a more negative charge than the other atoms in the compound.the other atoms in the compound.

Does not have a full negative charge but a Does not have a full negative charge but a partial negative charge (partial negative charge (δδ - - , lower case , lower case delta)delta)

Hydrofluoric Acid (HF)Hydrofluoric Acid (HF)

The red end (fluorine) has a partial negative charge.

Polar MoleculesPolar Molecules

One end of the One end of the molecule is positive molecule is positive while the other end of while the other end of the molecule is the molecule is negative.negative.

This difference in This difference in charge is called a charge is called a “dipole”“dipole”

What effect does this have?What effect does this have?

How does this change How does this change the way two the way two molecules interact?molecules interact?

The positive end of The positive end of the molecule is the molecule is attracted to the attracted to the negative end of a negative end of a different molecule.different molecule.

How to spot dipole-dipole forces?How to spot dipole-dipole forces?

Look for molecular shapes that have Look for molecular shapes that have uneven placements of atoms.uneven placements of atoms.

BentBentTrigonal pyramidalTrigonal pyramidalAnything that has more than one type of Anything that has more than one type of

atom around the outsideatom around the outside

Is Carbon Dioxide Polar?Is Carbon Dioxide Polar?

Is carbon tetrafluoride polar?Is carbon tetrafluoride polar?

Is Water Polar?Is Water Polar?

Why is Polarity ImportantWhy is Polarity Important

Things that are polar or have charges Things that are polar or have charges dissolve in things that are polar.dissolve in things that are polar.

Things that are nonpolar dissolve in things Things that are nonpolar dissolve in things that are nonpolar.that are nonpolar.

““Like dissolves Like”Like dissolves Like”Opposing types do not dissolve in each Opposing types do not dissolve in each

other.other.

A Bio ReminderA Bio Reminder

HydrophilicHydrophilic – “water loving” – polar – “water loving” – polar

HydrophobicHydrophobic – “water fearing” - nonpolar – “water fearing” - nonpolar

Dissolution ProcessDissolution Process

How are strong ionic bonds broken in How are strong ionic bonds broken in water?water?

Dissolving_NaCl-Electrolyte.exeDissolving_NaCl-Electrolyte.exe

The polar nature of water creates The polar nature of water creates attractions between the water and ionic attractions between the water and ionic compound.compound.

Hydrogen BondingHydrogen Bonding

Special case of Special case of dipole-dipole forces.dipole-dipole forces.

The difference in The difference in electronegativity electronegativity between some atoms between some atoms and hydrogen is so and hydrogen is so strong that it creates strong that it creates a very strong dipolea very strong dipole

What elements can do this?What elements can do this?

Which elements have the strongest Which elements have the strongest electronegativity?electronegativity?

Anything with an N-H bond, O-H bond, or Anything with an N-H bond, O-H bond, or F-H bond will have hydrogen bonding.F-H bond will have hydrogen bonding.

Hydrogen Bonding is Very ImportantHydrogen Bonding is Very Important

It is the reason why ice floats.

Hydrogen Bonding is Very Hydrogen Bonding is Very ImportantImportant

DNA Base PairsDNA Base Pairs

London Dispersion Forces (LDFs)London Dispersion Forces (LDFs)

Often called “induced dipoles”Often called “induced dipoles” A momentary change in where the electrons are A momentary change in where the electrons are

in one molecule, “induces” a dipole in another in one molecule, “induces” a dipole in another molecule.molecule.

LDFsLDFs

The more electrons you have in a The more electrons you have in a molecule, the more likely you are to have molecule, the more likely you are to have momentary imbalances in charges.momentary imbalances in charges.

The more electrons in an atom, the The more electrons in an atom, the stronger the London Dispersion Forces.stronger the London Dispersion Forces.

Any molecule can have London Dispersion Any molecule can have London Dispersion Forces.Forces.

LDFsLDFs This explains why the This explains why the

boiling point goes up boiling point goes up as you move down a as you move down a column.column.

Hydrogen telluride Hydrogen telluride has more electrons has more electrons than hydrogen sulfidethan hydrogen sulfide

Hydrogen telluride Hydrogen telluride has stronger LDFshas stronger LDFs

Hydrogen telluride Hydrogen telluride has a higher boiling has a higher boiling pointpoint

Crude OilCrude Oil Crude oil is Crude oil is

refined based refined based on differences on differences in LDFs.in LDFs.

Longer carbon Longer carbon chains have chains have higher boiling higher boiling pointspoints Have larger Have larger

number of number of electronselectrons

IMF ComparisonIMF Comparison

LDFs are the weakestLDFs are the weakest

Dipole-dipole are in the middleDipole-dipole are in the middle

Hydrogen bonding is the strongest.Hydrogen bonding is the strongest.

Practice ProblemsPractice Problems

List all of the intermolecular forces acting List all of the intermolecular forces acting on two phosphorus trichloride moleculeson two phosphorus trichloride moleculesFigure out the formulaFigure out the formulaDraw a Lewis structureDraw a Lewis structureFigure out the molecular geometryFigure out the molecular geometryCheck to see what IMFs it has.Check to see what IMFs it has.

Practice ProblemPractice Problem

Explain why ammonia (-33.4Explain why ammonia (-33.4ººC)C)has a higher has a higher boiling point than phosphine, PHboiling point than phosphine, PH3 3 (-87.8(-87.8ººC)C). .

Justify your answer.Justify your answer.Figure out ammonia’s formulaFigure out ammonia’s formulaDraw Lewis structures for both compoundsDraw Lewis structures for both compoundsFigure out the molecular geometry for both Figure out the molecular geometry for both

compoundscompoundsFigure out what IMFs each compound has.Figure out what IMFs each compound has.Compare the two compounds.Compare the two compounds.

Practice ProblemsPractice Problems

Explain why ammonia (-33.4Explain why ammonia (-33.4ººC)C)has a higher has a higher boiling point than phosphine, PHboiling point than phosphine, PH3 3 (-87.8(-87.8ººC)C). . Justify your answer.Justify your answer.

Practice ProblemPractice Problem

Hexane (CHexane (C66HH1414) is a liquid at room ) is a liquid at room

temperature. Its Lewis structure has each temperature. Its Lewis structure has each carbon connected to another in a long carbon connected to another in a long chain. Will sodium chloride dissolve in chain. Will sodium chloride dissolve in hexane? Justify your answer.hexane? Justify your answer.