LiquidsLiquidsPolar bonds and dipolesPolar bonds and dipoles

Intermolecular forcesIntermolecular forcesLiquid propertiesLiquid propertiesPhase changesPhase changes

Evaporation, vapour pressure and boiling pointEvaporation, vapour pressure and boiling pointClausius-Clapeyron equationClausius-Clapeyron equation

Intermolecular forcesIntermolecular forces

In the sequence gas In the sequence gas → liquid → solid→ liquid → solid Intermolecular attractions increaseIntermolecular attractions increase

Gases – essentially no interactionsGases – essentially no interactions Liquids – movement allowedLiquids – movement allowed Solids – completely rigidSolids – completely rigid

Polarity reduxPolarity redux

Electronegativity differences between atoms Electronegativity differences between atoms creates polar bonds – the more electronegative creates polar bonds – the more electronegative atom attracts the electronsatom attracts the electrons

Molecular dipoleMolecular dipole

Molecules are assemblies of several Molecules are assemblies of several bondsbonds

Molecular polarity depends on the Molecular polarity depends on the orientation of the individual dipolesorientation of the individual dipoles

If the dipoles cancel out, molecular is non-If the dipoles cancel out, molecular is non-polarpolar

If the dipoles don’t cancel, the molecule is If the dipoles don’t cancel, the molecule is polarpolar

Symmetry and polaritySymmetry and polarity

Studying bonds is an approximationStudying bonds is an approximation

We can calculate the centers of gravity of the negative We can calculate the centers of gravity of the negative and positive charges in a moleculeand positive charges in a molecule

If they do not coincide, the molecule is polarIf they do not coincide, the molecule is polar These calculations are involved, so studying individual These calculations are involved, so studying individual

bonds is a good approximationbonds is a good approximation

+-

Dipole momentsDipole moments

The dipole moment is the charge x length of the The dipole moment is the charge x length of the dipoledipole

An electron and proton separated by 0.1 nm (a An electron and proton separated by 0.1 nm (a typical bond length)typical bond length)

Where 1 D (Debye) = 3.336 x 10Where 1 D (Debye) = 3.336 x 10-30-30 Cm Cm

rQ

DCmxxxx 8.4106.1101.0106.1 29919

Algorithm for predicting molecular Algorithm for predicting molecular polaritypolarity

Establish molecular skeletonEstablish molecular skeleton Draw Lewis dot structureDraw Lewis dot structure Count groups of charge around central atomCount groups of charge around central atom Establish electronic geometry using VSEPREstablish electronic geometry using VSEPR Determine molecular shapeDetermine molecular shape Identify polar bonds and lone pairsIdentify polar bonds and lone pairs Inspect molecule: do polar bonds/lone pairs Inspect molecule: do polar bonds/lone pairs

cancel out?cancel out?

Percent ionic characterPercent ionic character

We have seen that we can calculate the dipole We have seen that we can calculate the dipole moment for a given charge separationmoment for a given charge separation

Comparison with experimental values permits Comparison with experimental values permits estimation of “ionic character”estimation of “ionic character” In HCl the experimental dipole moment is 1.03 D.In HCl the experimental dipole moment is 1.03 D. The theoretical dipole given the bond length of 0.127 The theoretical dipole given the bond length of 0.127

nm is 6.09 Dnm is 6.09 D Percent ionic character = 1.03/6.09 x 100 % = 16.9 %Percent ionic character = 1.03/6.09 x 100 % = 16.9 %

May the force be with youMay the force be with you

Covalent and ionic bonds are the Covalent and ionic bonds are the intraintramolecular forces that hold the atoms molecular forces that hold the atoms in molecules togetherin molecules together

InterIntermolecular forces hold the molecules molecular forces hold the molecules togethertogether

Collectively, the intermolecular forces are Collectively, the intermolecular forces are called called van der Waalsvan der Waals forces forces

All arise from electrostatic interactionsAll arise from electrostatic interactions

Name of Name of forceforce

OriginOrigin StrengthStrength

Ion-dipoleIon-dipole Between ions Between ions and and moleculesmolecules

Quite strong Quite strong (10 – 50 (10 – 50 kJ/mol)kJ/mol)

Dipole-Dipole-dipoledipole

Between Between permanent permanent dipolesdipoles

Weak (3 – Weak (3 – 4 kJ/mol)4 kJ/mol)

Hydrogen Hydrogen bondsbonds

Polar bonds Polar bonds with H and with H and (O,N)(O,N)

Quite strong Quite strong (10 – 40 (10 – 40 kJ/mol)kJ/mol)

London London dispersion dispersion forcesforces

Fluctuating Fluctuating dipoles in non-dipoles in non-polar bondspolar bonds

Weak (1 – Weak (1 – 10 kJ/mol)10 kJ/mol)

Ion - dipoleIon - dipole

Characteristic of interactions in solutions Characteristic of interactions in solutions of ionic compounds in polar solventsof ionic compounds in polar solvents Negative ion with the positive dipole endNegative ion with the positive dipole end Positive ion with the negative dipole endPositive ion with the negative dipole end

Dipole - dipoleDipole - dipole

Important attractive force in polar Important attractive force in polar substancessubstances

Strength of the order of 3 – 4 kJ/mol Strength of the order of 3 – 4 kJ/mol (compared with 200 – 400 kJ/mol for (compared with 200 – 400 kJ/mol for covalent bonds)covalent bonds)

Manifested in boiling points:Manifested in boiling points:

Nonpolar substances have much lower Nonpolar substances have much lower boiling pointsboiling points Acetone (polar) 56Acetone (polar) 56ºC butane (nonpolar) -ºC butane (nonpolar) -

0.5ºC0.5ºC Boiling point increases with dipole strengthBoiling point increases with dipole strength

London callingLondon calling

Even molecules with no net dipole moment attract each Even molecules with no net dipole moment attract each other.other.

Electrons are not static but mobile:Electrons are not static but mobile: Fluctuation creates dipole in one molecule which induces dipole Fluctuation creates dipole in one molecule which induces dipole

in another moleculein another molecule Effect increases with atomic number – as atom becomes Effect increases with atomic number – as atom becomes

more polarizablemore polarizable Boiling increases with atomic weightBoiling increases with atomic weight

Conventionally, dispersion forces are said to be weaker Conventionally, dispersion forces are said to be weaker than other inter-molecular forces. For large molecules than other inter-molecular forces. For large molecules this is not really true. Large molecules are solids this is not really true. Large molecules are solids because of dispersion forcesbecause of dispersion forces

Hydrogen bonds: the most Hydrogen bonds: the most important bond?important bond?

Key to lifeKey to life Between H and O, N or FBetween H and O, N or F Dipole-dipole bonds of unusual strength (up to Dipole-dipole bonds of unusual strength (up to

40 kJ/mol)40 kJ/mol)

Hydrogen bondingHydrogen bonding

The ultimate expression of polarityThe ultimate expression of polarity Small positive H atom exerts strong attraction on Small positive H atom exerts strong attraction on

O atomO atom Other H-bonding molecules: HF, NHOther H-bonding molecules: HF, NH33

HH22O is the supreme example: two H atoms and O is the supreme example: two H atoms and

two lone pairs per moleculetwo lone pairs per molecule

HH22O has optimum combination of O has optimum combination of

lone pairs and H atomslone pairs and H atomsCompoundCompound Number of lone Number of lone

pairspairsNumber of H Number of H atomsatoms

HFHF 33 11

HH22OO 22 22

NHNH33 11 33

H bonding generates three-H bonding generates three-dimensional networkdimensional network

Water: the miracleWater: the miracle

All the properties of water that make it All the properties of water that make it unique and life sustaining can be traced to unique and life sustaining can be traced to hydrogen bondinghydrogen bonding Density of ice lower than waterDensity of ice lower than water Anomalous high b.p.Anomalous high b.p. High heat capacityHigh heat capacity Universal solventUniversal solvent

Understanding the forceUnderstanding the force

Predicting the forces acting between Predicting the forces acting between molecules means understanding the molecules means understanding the molecules molecules

All molecules experience London forces, All molecules experience London forces, but only some will have dipole-dipole or but only some will have dipole-dipole or hydrogen bonds. Where present, the hydrogen bonds. Where present, the latter will dominatelatter will dominate

Properties of liquids depend on Properties of liquids depend on intramolecular forcesintramolecular forces

Water flows but syrup is stickyWater flows but syrup is sticky Viscosity Viscosity measures resistance to flowmeasures resistance to flow

Small non-polar molecules flow easilySmall non-polar molecules flow easily Large or highly polar molecules flow less easilyLarge or highly polar molecules flow less easily

Units of viscosity are kg/m-sUnits of viscosity are kg/m-s

Surface tension? Take a tabletSurface tension? Take a tablet

Surface tensionSurface tension is the tendency of a liquid to is the tendency of a liquid to resist spreading outresist spreading out

Arises from molecules at the surface Arises from molecules at the surface experiencing inward pullexperiencing inward pull

Walking on water: it’s no miracle, it’s surface Walking on water: it’s no miracle, it’s surface tensiontension

Surface tensionSurface tension is the energy required to is the energy required to increase the surface area of a liquid – units are increase the surface area of a liquid – units are J/mJ/m22

Cohesive and adhesiveCohesive and adhesive

Cohesive Cohesive forces are the attractive forces forces are the attractive forces between like moleculesbetween like molecules

Adhesive Adhesive forces are the attractive forces forces are the attractive forces between unlike moleculesbetween unlike molecules

MeniscusMeniscus

Adhesive forces pull HAdhesive forces pull H22O molecules to O molecules to

maximize coveragemaximize coverage Cohesive forces between HCohesive forces between H22O molecules O molecules

drag liquid updrag liquid up Gravity pushes liquid downGravity pushes liquid down

Capillary actionCapillary action Combined effects of cohesive, adhesive and Combined effects of cohesive, adhesive and

gravitational forces cause liquid to rise towards gravitational forces cause liquid to rise towards edge of containeredge of container

In very thin columns the effect of gravity is In very thin columns the effect of gravity is diminished and the liquid rises higherdiminished and the liquid rises higher

Originally used as explanation (incorrect) for Originally used as explanation (incorrect) for transport of water through plants (Osmosis is the transport of water through plants (Osmosis is the cause)cause)

Just a phase I’m going throughJust a phase I’m going through

A A phasephase change occurs when matter change occurs when matter changes from one state to anotherchanges from one state to another

Solids can exhibit more than one phase Solids can exhibit more than one phase which also undergo phase changes (gray which also undergo phase changes (gray tin to white tin) tin to white tin)

Energetics of phase changesEnergetics of phase changes

In the series: solid → liquid → gas In the series: solid → liquid → gas Energy is required to break intermolecular Energy is required to break intermolecular

forcesforces Distribution of molecules is more disordered Distribution of molecules is more disordered

(entropy) – greater disorder is more (entropy) – greater disorder is more favourablefavourable

Roadmap of changesRoadmap of changes

More condensed to less condensed More condensed to less condensed means heat absorption and entropy gain means heat absorption and entropy gain which are opposing which are opposing

Phase changes involve “latent” heatsPhase changes involve “latent” heats

With matter in a single phase, heating the With matter in a single phase, heating the substance gives a T increase depending upon substance gives a T increase depending upon S.H.S.H.

At a phase change, two phases are in At a phase change, two phases are in equilibrium and heat is absorbed to convert one equilibrium and heat is absorbed to convert one into the other without a change in T. Hence the into the other without a change in T. Hence the term “latent” heat – a term no longer in popular term “latent” heat – a term no longer in popular use.use.

Fusion versus vaporizationFusion versus vaporization

For all substances, the heat of For all substances, the heat of vaporization is much larger than the heat vaporization is much larger than the heat of fusionof fusion More bonds are broken in creating the vapour More bonds are broken in creating the vapour

Vapour pressureVapour pressure Liquids do not turn into a vapour only at the Liquids do not turn into a vapour only at the

boiling pointboiling point At any temperature, there is vapour in At any temperature, there is vapour in

equilibrium with the liquidequilibrium with the liquid A puddle of water on the sidewalk evaporatesA puddle of water on the sidewalk evaporates A liquid develops a pressure in a manometerA liquid develops a pressure in a manometer

The pressure exerted by the vapour in The pressure exerted by the vapour in equilibrium with the liquid is the vapour pressureequilibrium with the liquid is the vapour pressure

Maxwell, Boltzmann and vapour Maxwell, Boltzmann and vapour pressurepressure

Molecules exhibit a range of energies, which Molecules exhibit a range of energies, which moves to higher energy as T increasesmoves to higher energy as T increases More molecules have sufficient energy to escape More molecules have sufficient energy to escape

liquid as T increasesliquid as T increases When the vapour pressure = atmospheric When the vapour pressure = atmospheric

pressure, the liquid boilspressure, the liquid boils

PropertyProperty Volatile liquidVolatile liquid Non-volatile liquidNon-volatile liquid

Cohesive forcesCohesive forces LowLow HighHigh

ViscosityViscosity LowLow HighHigh

Surface tensionSurface tension LowLow HighHigh

Specific heatSpecific heat LowLow HighHigh

Vapour pressureVapour pressure HighHigh LowLow

Rate of evaporationRate of evaporation HighHigh LowLow

Boiling pointBoiling point Low Low HighHigh

Heat of Heat of vaporizationvaporization

LowLow HighHigh

Contrasting volatile and non-volatile Contrasting volatile and non-volatile liquidsliquids

Clausius – Clapeyron equationClausius – Clapeyron equation

The vapour pressure in equilibrium with a The vapour pressure in equilibrium with a liquid obeys the following equationliquid obeys the following equation

Calculate Calculate ΔΔHHvap vap from vapour pressure datafrom vapour pressure data Calculate vapour pressure as f(T) given Calculate vapour pressure as f(T) given ΔΔHHvapvap and one and one

vapour pressure valuevapour pressure value

CTR

HP vapvap

1ln