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 Gases – essentially no interactions Gases – essentially no interactions Liquids – movement allowed Liquids – movement allowed Solids – completely rigid Solids – completely rigid

Polarity redux  Electronegativity differences between atoms creates polar bonds – the more electronegative atom attracts the electrons

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

Symmetry and polarity

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  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) = x Cm

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  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. In HCl the experimental dipole moment is 1.03 D. The theoretical dipole given the bond length of nm is 6.09 D The theoretical dipole given the bond length of nm 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 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

Name of force OriginStrength Ion-dipole Between ions and molecules Quite strong (10 – 50 kJ/mol) Dipole- dipole Between permanent dipoles Weak (3 – 4 kJ/mol) Hydrogen bonds Polar bonds with H and (O,N) Quite strong (10 – 40 kJ/mol) London dispersion forces Fluctuating dipoles in non- polar bonds Weak (1 – 10 kJ/mol)

Ion - dipole  Characteristic of interactions in solutions of ionic compounds in polar solvents Negative ion with the positive dipole end Negative ion with the positive dipole end Positive ion with the negative dipole end Positive ion with the negative dipole end

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:  Nonpolar substances have much lower boiling points Acetone (polar) 56ºC butane (nonpolar) - 0.5ºC Acetone (polar) 56ºC butane (nonpolar) - 0.5ºC  Boiling point increases with dipole strength

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 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 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?  Key to life  Between H and O, N or F  Dipole-dipole bonds of unusual strength (up to 40 kJ/mol)

Hydrogen bonding  The ultimate expression of polarity  Small positive H atom exerts strong attraction on O atom  Other H-bonding molecules: HF, NH 3  H 2 O is the supreme example: two H atoms and two lone pairs per molecule

H 2 O has optimum combination of lone pairs and H atoms Compound Number of lone pairs Number of H atoms HF31 H2OH2OH2OH2O22 NH 3 13

H bonding generates three- dimensional network

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 Density of ice lower than water Anomalous high b.p. Anomalous high b.p. High heat capacity High heat capacity Universal solvent Universal solvent

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  Water flows but syrup is sticky  Viscosity measures resistance to flow Small non-polar molecules flow easily Small non-polar molecules flow easily Large or highly polar molecules flow less easily Large or highly polar molecules flow less easily  Units of viscosity are kg/m-s

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/m 2

Cohesive and adhesive  Cohesive forces are the attractive forces between like molecules  Adhesive forces are the attractive forces between unlike molecules

Meniscus  Adhesive forces pull H 2 O molecules to maximize coverage  Cohesive forces between H 2 O molecules drag liquid up  Gravity pushes liquid down

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  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  In the series: solid → liquid → gas Energy is required to break intermolecular forces Energy is required to break intermolecular forces Distribution of molecules is more disordered (entropy) – greater disorder is more favourable Distribution of molecules is more disordered (entropy) – greater disorder is more favourable

Roadmap of changes  More condensed to less condensed means heat absorption and entropy gain which are opposing

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

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 puddle of water on the sidewalk evaporates A liquid develops a pressure in a manometer 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  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 More molecules have sufficient energy to escape liquid as T increases  When the vapour pressure = atmospheric pressure, the liquid boils

Property Volatile liquid Non-volatile liquid Cohesive forces LowHigh ViscosityLowHigh Surface tension LowHigh Specific heat LowHigh Vapour pressure HighLow Rate of evaporation HighLow Boiling point LowHigh Heat of vaporization LowHigh Contrasting volatile and non- volatile liquids

Clausius – Clapeyron equation  The vapour pressure in equilibrium with a liquid obeys the following equation Calculate ΔH vap from vapour pressure data Calculate ΔH vap from vapour pressure data Calculate vapour pressure as f(T) given ΔH vap and one vapour pressure value Calculate vapour pressure as f(T) given ΔH vap and one vapour pressure value