Liquids Polar bonds and dipoles Intermolecular forces Liquid properties Phase changes Evaporation,...
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Transcript of Liquids Polar bonds and dipoles Intermolecular forces Liquid properties Phase changes Evaporation,...
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