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Mr. ShieldsMr. Shields Regents Chemistry Regents Chemistry U07 L01 U07 L01

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Forces of attractionForces of attraction

There exist between Molecules of gases and liquids There exist between Molecules of gases and liquids Forces of attraction.Forces of attraction.

Some forces may be Some forces may be strongstrong

other forces may be other forces may be weakweak..

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Forces of attractionForces of attraction

It’s these forces of attraction that ultimately determineIt’s these forces of attraction that ultimately determineHow How easy or difficulteasy or difficult it is for Gases to it is for Gases to coalescecoalesce into intoLiquids and liquids toLiquids and liquids tosolidifysolidify into solids. into solids.

These forces of These forces of attraction will, forattraction will, forexample, determineexample, determinea liquids a liquids boiling point.boiling point.

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Forces of AttractionForces of Attraction

What are the What are the forces of attractionforces of attraction we’re talking about? we’re talking about?

For gases and liquids these forces are of two types:For gases and liquids these forces are of two types:

intermolecular - intermolecular - betweenbetween moleculesmolecules

intramolecular - intramolecular - within the moleculewithin the molecule

Intramolecular forcesIntramolecular forces involve the formation of involve the formation of truetruechemical bondschemical bonds

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Intermolecular Forces of Intermolecular Forces of AttractionAttraction

Intramolecular forcesIntramolecular forces of attraction are much of attraction are much strongerstrongerThan Than intermolecular forcesintermolecular forces

- on the order of 30 – 400x stronger- on the order of 30 – 400x stronger

Intermolecular forces of attraction are also known asIntermolecular forces of attraction are also known asvan der Waalsvan der Waals forces forces

So how many types of So how many types of intermolecularintermolecular or or van der Waalsvan der Waalsforces of attraction exist? forces of attraction exist?

Let’s take a look …Let’s take a look …

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Intermolecular Forces of Intermolecular Forces of AttractionAttraction

There are three types of intermolecular forces:There are three types of intermolecular forces:

1. London 1. London DispersionDispersion force force (sometimes called induced dipole force*)(sometimes called induced dipole force*)

2. 2. Dipole-DipoleDipole-Dipole force force

3. 3. Hydrogen bondHydrogen bond

NYS Regents also refers (incorrectly) to induced dipoleNYS Regents also refers (incorrectly) to induced dipoleIMF as “Van Der Waal’s” IMF as “Van Der Waal’s”

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Intermolecular Forces of Intermolecular Forces of AttractionAttraction

As we mentined before As we mentined before IntermolecularIntermolecular forces are much forces are muchweaker than intramolecular (i.e. chemical bond) forcesweaker than intramolecular (i.e. chemical bond) forces

AndAnd … Intermolecular forces… Intermolecular forces also vary in strength from also vary in strength fromone another:one another:

DispersionDispersion WeakestWeakest (<1 Kcal)(<1 Kcal)Dipole-dipoleDipole-dipole StrongerStronger (2-5 Kcal)(2-5 Kcal)Hydrogen bondHydrogen bond The StrongestThe Strongest (12-16 Kcal)(12-16 Kcal)

(for comparison, a true covalent bond is about 400Kcal)(for comparison, a true covalent bond is about 400Kcal)

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Intermolecular Forces of Intermolecular Forces of AttractionAttraction

Intermolecular forces hold moleculesIntermolecular forces hold moleculestogether in the Solid & Liquid phase & together in the Solid & Liquid phase & Allow gases to condense on cooling.Allow gases to condense on cooling.

They also prevent instantaneousThey also prevent instantaneous

evaporation from occurring.evaporation from occurring.

Strong Attractive forces between Strong Attractive forces between molecules also make Vaporization molecules also make Vaporization (L (L G) phase changes more G) phase changes more difficult. difficult.

Why?Why?

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Dispersion ForcesDispersion ForcesDispersion forcesDispersion forces, the weakest force, is sometimes called, the weakest force, is sometimes calledLONDON DispersionLONDON Dispersion Forces or Forces or induced dipoleinduced dipole forces forces

- They result when there is a momentary shift in- They result when there is a momentary shift in electron density within the molecules electron cloudelectron density within the molecules electron cloud

This intermolecular force is the predominantThis intermolecular force is the predominant intermolecular force for intermolecular force for non-polar compoundsnon-polar compounds

Formation ofFormation of

Temporary DipolesTemporary Dipoles Non-polar moleculesNon-polar molecules

??????

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How does this shift happen?How does this shift happen?

Non-polar compounds are molecules that Non-polar compounds are molecules that do not havedo not havea permanent charge distribution within a permanent charge distribution within the Molecule.the Molecule.

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                                                                                                                                                   δ δ --δδ++ δδ- - δδ++δ δ --δδ++ δ δ --δδ++

12The larger the molecule the more easily The larger the molecule the more easily δδ charges developcharges develop

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Dispersion ForcesDispersion ForcesAs we said before Non-polar compounds are moleculesAs we said before Non-polar compounds are moleculesthat do not have a permanent charge distribution withinthat do not have a permanent charge distribution withinThe Molecule.The Molecule.

Molecules that have an even electron distribution Molecules that have an even electron distribution Are typicallyAre typically 1) 1) single single atomsatoms, 2) , 2) molecules of the same molecules of the same elementelement, 3) hydrocarbons 4) symmetrical molecules, 3) hydrocarbons 4) symmetrical molecules

For example:For example: Ne, Kr, HNe, Kr, H22, O, O22, N, N22, S, S88, C, C33HH88, CCl, CCl44, CO, CO22

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Dispersion forcesDispersion forcesInduced Dipole forces become stronger the larger the Induced Dipole forces become stronger the larger the Atom or Molecule.Atom or Molecule.

- larger electron clouds are easier to deform- larger electron clouds are easier to deform

One might expect the boiling pt. of compounds toOne might expect the boiling pt. of compounds tobe low for compounds that interact by this type of force.be low for compounds that interact by this type of force.Why?Why?

The minimal force holding these Molecules togetherThe minimal force holding these Molecules togetherAllow them to move readily from the liquidAllow them to move readily from the liquidPhase into the gas phase. Or even from the solid phasePhase into the gas phase. Or even from the solid phaseto gas phase - like Ito gas phase - like I22

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                                                                                                                                                   Smaller moleculeSmaller molecule Larger moleculeLarger molecule

CHCH44 CC88HH1818

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Dispersion forcesDispersion forcesIf Molecules have High BP’s they will haveIf Molecules have High BP’s they will have Low VPLow VP (and conversely (and conversely Low Bp’sLow Bp’s mean mean High VPHigh VP))

Since dispersion forces Since dispersion forces increaseincrease with increasing with increasingMolecular weight the Trend for non-polar compounds is…Molecular weight the Trend for non-polar compounds is…increasing BP with increasing MWincreasing BP with increasing MW . .

(this trend is also true with melting pts/freezing pts; as(this trend is also true with melting pts/freezing pts; asMW inc so does mp/fp) MW inc so does mp/fp)

Let’s look at some bp data…Let’s look at some bp data…

Low MWLow MW

Low BPLow BP

Higher MWHigher MW

Higher BPHigher BP

Low dispersion force higher dispersion forceLow dispersion force higher dispersion force

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Dispersion forcesDispersion forcesWhich member of each pair should have the higher B.P.Which member of each pair should have the higher B.P.(and thus the stronger Induced dipole)(and thus the stronger Induced dipole)

NN22, O, O22

OO2 – 2 – Higher MWHigher MW B.P: O B.P: O22 -183.0 C / -183.0 C / NN22 -195.6 C -195.6 C larger moleculelarger molecule Ne, KrNe, Kr Kr - Higher MWKr - Higher MW B.P: Kr -152.9 C B.P: Kr -152.9 C / Ne -245.9 C / Ne -245.9 C Larger AtomLarger Atom

FF22, Cl, Cl22

ClCl22 – Higher MW – Higher MW B.P: Cl B.P: Cl22 -34.6 C / -34.6 C / FF22 -187 C -187 C Larger Molecule Larger Molecule

CC33HH88 (propane), C (propane), C44HH1010 (butane) (butane)ButaneButane (larger) (larger) B.P: C B.P: C44HH1010 -0.5 C / -0.5 C / CC33HH88 -42.1 C -42.1 C

M.P: -134.4 CM.P: -134.4 C -187.7 C -187.7 C

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Dipole- Dipole ForcesDipole- Dipole Forces

Molecules that have Molecules that have permanent dipolespermanent dipoles are said to be are said to bePolar.Polar.

What does it mean to be a What does it mean to be a polar moleculepolar molecule??

One area is always One area is always positivepositive and another part of the and another part of theMolecule is always Molecule is always negativenegative

An example of a An example of a Polar molecule is HClPolar molecule is HCl

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Dipole-Dipole Forces

Dipole- Dipole forces are stronger than London Dipole- Dipole forces are stronger than London dispersion forces (<1 Kcal vs. 2-5 Kcal)dispersion forces (<1 Kcal vs. 2-5 Kcal)

- In this intermolecular force- In this intermolecular force DipolesDipoles are are always presentalways present as opposed to constantlyas opposed to constantly forming and reforming forming and reforming dipoles in weaker Dispersiondipoles in weaker Dispersion force moleculesforce molecules

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Hydrogen Bond ForcesHydrogen Bond ForcesThe The hydrogen bondhydrogen bond is a special form of the is a special form of theDipole-Dipole force, its Dipole-Dipole force, its notnot really a true really a true chemicalchemical bond. bond.

- It is the strongest intermolecular force- It is the strongest intermolecular force

- - much strongermuch stronger than either Dipole-Dipole than either Dipole-Dipole Or Dispersion forces.Or Dispersion forces.

However It can However It can ONLYONLY exist when a exist when a Hydrogen atomHydrogen atomis present in the molecule AND it’s bonded to …is present in the molecule AND it’s bonded to …

- - Either aEither a N, O,N, O, oror FF atom also present in theatom also present in the same moleculesame molecule..

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Hydrogen bondsHydrogen bonds

In all compounds that form H-bonds the In all compounds that form H-bonds the Hydrogen atom is Hydrogen atom is ALWAYSALWAYS the Positive Dipole. the Positive Dipole.

O, F, or N is O, F, or N is ALWAYSALWAYS the negative the negativeDipoleDipole

As one molecule approaches As one molecule approaches another the positive Hydrogenanother the positive Hydrogenaligns with the negative O, F, or N.aligns with the negative O, F, or N.

++

--

HH FF+ -+ -

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Hydrogen bondsHydrogen bonds

All Hydrogen bonds are stronger than eitherAll Hydrogen bonds are stronger than eitherDispersion or dipole-dipole interactions.Dispersion or dipole-dipole interactions.

Strength of H-bond: F>O>>NStrength of H-bond: F>O>>N

However, among molecules withHowever, among molecules withHydrogen bonds there are some thatHydrogen bonds there are some thatAre stronger than others.Are stronger than others.

How do we determine what H-bondsHow do we determine what H-bondsAre the strongest? Are the strongest?

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The Hydrogen BondThe Hydrogen BondExamples of compounds that can form H-bonds are:Examples of compounds that can form H-bonds are:

Water, Methyl or Ethyl Alcohol, HF, NHWater, Methyl or Ethyl Alcohol, HF, NH33

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Water Alignment during Water Alignment during formation of the H-Bondformation of the H-Bond

Water molecules in ice align to form a repeating hexagonWater molecules in ice align to form a repeating hexagon

Positive aligns to negativePositive aligns to negative

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Hydrogen BondsHydrogen BondsCompounds that form Hydrogen bonds take a Compounds that form Hydrogen bonds take a lotlotmore energymore energy to move molecules from the liquid phase to move molecules from the liquid phaseInto the gas phase.Into the gas phase.

Molecules of similar molecular weight without theMolecules of similar molecular weight without theAbility to form H-bonds haveAbility to form H-bonds have higher vapor pressureshigher vapor pressuresAnd thusAnd thus lower B.P.lower B.P.

CompoundCompound Molar MassMolar Mass B.P. deg CB.P. deg C

HH2200 1818 + 100+ 100

NHNH33 1717 - 33- 33

CHCH44 1616 - 164- 164

H-bondH-bond

H-bondH-bond

Non-polarNon-polar

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dec. MWdec. MW

B.PB.P

These compounds form H-Bonds

If If FF forms forms stronger H bondsstronger H bonds than than OO why is the BP of why is the BP of Water Water higher higher than HF? (Hint: Consider the Mol. Structure)than HF? (Hint: Consider the Mol. Structure)

Theoretical B.P Theoretical B.P Of WaterOf Water

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