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Liquids. Polar bonds and dipoles Intermolecular forces Liquid properties Phase changes Evaporation, vapour pressure and boiling point Clausius-Clapeyron equation. Intermolecular forces. In the sequence gas → liquid → solid Intermolecular attractions increase

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Polar bonds and dipoles

Intermolecular forces

Liquid properties

Phase changes

Evaporation, vapour pressure and boiling point

Clausius-Clapeyron equation

intermolecular forces
Intermolecular forces
  • In the sequence gas → liquid → solid
  • Intermolecular attractions increase
    • Gases – essentially no interactions
    • Liquids – movement allowed
    • Solids – completely rigid
polarity redux
Polarity redux
  • Electronegativity differences between atoms creates polar bonds – the more electronegative atom attracts the electrons
molecular dipole
Molecular dipole
  • Molecules are assemblies of several bonds
  • Molecular polarity depends on the orientation of the individual dipoles
  • If the dipoles cancel out, molecular is non-polar
  • If the dipoles don’t cancel, the molecule is polar
studying bonds is an approximation



Studying bonds is an approximation
  • We can calculate the centers of gravity of the negative and positive charges in a molecule
  • If they do not coincide, the molecule is polar
  • These calculations are involved, so studying individual bonds is a good approximation
dipole moments
Dipole moments
  • The dipole moment is the charge x length of the dipole
  • An electron and proton separated by 0.1 nm (a typical bond length)
  • Where 1 D (Debye) = 3.336 x 10-30 Cm
algorithm for predicting molecular polarity
Algorithm for predicting molecular polarity
  • Establish molecular skeleton
  • Draw Lewis dot structure
  • Count groups of charge around central atom
  • Establish electronic geometry using VSEPR
  • Determine molecular shape
  • Identify polar bonds and lone pairs
  • Inspect molecule: do polar bonds/lone pairs cancel out?
percent ionic character
Percent ionic character
  • We have seen that we can calculate the dipole moment for a given charge separation
  • Comparison with experimental values permits estimation of “ionic character”
    • In HCl the experimental dipole moment is 1.03 D.
    • The theoretical dipole given the bond length of 0.127 nm is 6.09 D
    • Percent ionic character = 1.03/6.09 x 100 % = 16.9 %
may the force be with you
May the force be with you
  • Covalent and ionic bonds are the intramolecular forces that hold the atoms in molecules together
  • Intermolecular forces hold the molecules together
  • Collectively, the intermolecular forces are called van der Waals forces
  • All arise from electrostatic interactions
ion dipole
Ion - dipole
  • Characteristic of interactions in solutions of ionic compounds in polar solvents
    • Negative ion with the positive dipole end
    • Positive ion with the negative dipole end
dipole dipole
Dipole - dipole
  • Important attractive force in polar substances
  • Strength of the order of 3 – 4 kJ/mol (compared with 200 – 400 kJ/mol for covalent bonds)
manifested in boiling points
Manifested in boiling points:
  • Nonpolar substances have much lower boiling points
    • Acetone (polar) 56ºC butane (nonpolar) -0.5ºC
  • Boiling point increases with dipole strength
london calling
London calling
  • Even molecules with no net dipole moment attract each other.
  • Electrons are not static but mobile:
    • Fluctuation creates dipole in one molecule which induces dipole in another molecule
  • Effect increases with atomic number – as atom becomes more polarizable
    • Boiling increases with atomic weight
  • Conventionally, dispersion forces are said to be weaker than other inter-molecular forces. For large molecules this is not really true. Large molecules are solids because of dispersion forces
hydrogen bonds the most important bond
Hydrogen bonds: the most important bond?
  • Key to life
  • Between H and O, N or F
  • Dipole-dipole bonds of unusual strength (up to 40 kJ/mol)
hydrogen bonding
Hydrogen bonding
  • The ultimate expression of polarity
  • Small positive H atom exerts strong attraction on O atom
  • Other H-bonding molecules: HF, NH3
  • H2O is the supreme example: two H atoms and two lone pairs per molecule
water the miracle
Water: the miracle
  • All the properties of water that make it unique and life sustaining can be traced to hydrogen bonding
    • Density of ice lower than water
    • Anomalous high b.p.
    • High heat capacity
    • Universal solvent
understanding the force
Understanding the force
  • Predicting the forces acting between molecules means understanding the molecules
  • All molecules experience London forces, but only some will have dipole-dipole or hydrogen bonds. Where present, the latter will dominate
properties of liquids depend on intramolecular forces
Properties of liquids depend on intramolecular forces
  • Water flows but syrup is sticky
  • Viscosity measures resistance to flow
    • Small non-polar molecules flow easily
    • Large or highly polar molecules flow less easily
  • Units of viscosity are kg/m-s
surface tension take a tablet
Surface tension? Take a tablet
  • Surface tension is the tendency of a liquid to resist spreading out
  • Arises from molecules at the surface experiencing inward pull
  • Walking on water: it’s no miracle, it’s surface tension
  • Surface tension is the energy required to increase the surface area of a liquid – units are J/m2
cohesive and adhesive
Cohesive and adhesive
  • Cohesive forces are the attractive forces between like molecules
  • Adhesive forces are the attractive forces between unlike molecules
  • Adhesive forces pull H2O molecules to maximize coverage
  • Cohesive forces between H2O molecules drag liquid up
  • Gravity pushes liquid down
capillary action
Capillary action
  • Combined effects of cohesive, adhesive and gravitational forces cause liquid to rise towards edge of container
  • In very thin columns the effect of gravity is diminished and the liquid rises higher
  • Originally used as explanation (incorrect) for transport of water through plants (Osmosis is the cause)
just a phase i m going through
Just a phase I’m going through
  • A phase change occurs when matter changes from one state to another
  • Solids can exhibit more than one phase which also undergo phase changes (gray tin to white tin)
energetics of phase changes
Energetics of phase changes
  • In the series: solid → liquid → gas
    • Energy is required to break intermolecular forces
    • Distribution of molecules is more disordered (entropy) – greater disorder is more favourable
roadmap of changes
Roadmap of changes
  • More condensed to less condensed means heat absorption and entropy gain which are opposing
phase changes involve latent heats
Phase changes involve “latent” heats
  • With matter in a single phase, heating the substance gives a T increase depending upon S.H.
  • At a phase change, two phases are in equilibrium and heat is absorbed to convert one into the other without a change in T. Hence the term “latent” heat – a term no longer in popular use.
fusion versus vaporization
Fusion versus vaporization
  • For all substances, the heat of vaporization is much larger than the heat of fusion
    • More bonds are broken in creating the vapour
vapour pressure
Vapour pressure
  • Liquids do not turn into a vapour only at the boiling point
  • At any temperature, there is vapour in equilibrium with the liquid
    • A puddle of water on the sidewalk evaporates
    • A liquid develops a pressure in a manometer
  • The pressure exerted by the vapour in equilibrium with the liquid is the vapour pressure
maxwell boltzmann and vapour pressure
Maxwell, Boltzmann and vapour pressure
  • Molecules exhibit a range of energies, which moves to higher energy as T increases
    • More molecules have sufficient energy to escape liquid as T increases
  • When the vapour pressure = atmospheric pressure, the liquid boils
clausius clapeyron equation
Clausius – Clapeyron equation
  • The vapour pressure in equilibrium with a liquid obeys the following equation
    • Calculate ΔHvap from vapour pressure data
    • Calculate vapour pressure as f(T) given ΔHvapand one vapour pressure value