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Chapter 12 IMF.docx

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McMaster University
Pippa Lock

Chapter 12: Intermolecular Forces: Liquids and Solids Intermolecular Forces  Van der Waals Forces o Several different types of intermolecular forces  Instantaneous and Induced Dipoles o A possibility is that at some particular instance, electrons are concentrated in one region of an atom or molecule o This displacement of electrons causes a normally non-polar species to become momentarily polar o An instantaneous dipole is formed (molecule has an instantaneous dipole moment) o After this, electrons in a neighbouring atom or molecule may be displaced to also produce a dipole o This is the process of induction and a newly formed dipole is called an induced dipole o Commonly used names for this intermolecular force of attraction are dispersion forces and London forces o Polarizability is the term used to describe the relative tendency for a charge distribution to distort from its normal shape in an atom or molecule o The greater the tendency, the more polarizable an atom or molecule is said to be o Polarizability increases with atomic or molecular size, which is defined by the volume of the electron cloud around a substance o Also, in large molecules, some electrons, being farther from atomic nuclei, are less firmly held o These electrons are more easily displaced and the polarizability of the molecule increases o Because dispersion forces become stronger (more attractive), as polarizability increases and because polarizability generally increases with molecular mass, the melting points and boiling points of molecular substances generally increase with increasing molecular mass o The strength if dispersion forces also depends on molecular shape o Electrons in elongated molecules are more easily displaced than are those in small, compact, symmetrical molecules (elongated molecules more polarizable)  Dipole-Dipole Interactions o In a polar substance, molecules have permanent dipole moments so the molecules tend to line up with the positive end of one dipole directed toward the negative ends of neighbouring dipoles o This additional partial ordering of molecules can cause a substance to persist as a solid or liquid at temperatures higher than otherwise expected o Considering N , 2 an2 NO, if we only looked at dispersion forces, we would expect the BP of NO to be between N a2d O 2  The BP of NO is higher than both because of its additional permanent dipole  Hydrogen Bonding o The boiling points of NH , 3 O 2nd HF are as high or higher than those of any other hydride ion their group, not lowest as might be expected o A special type of intermolecular force causes this behaviour and using HF, we see that  The alignment of HF dipoles places an H atom between two F atoms  Because of the very small size of the H atom, the dipoles come close together and produce strong dipole-dipole attractions  Although an H atom is covalently bonded to one F atom, it is also weakly bonded to the F atom of a nearby HF molecule  This occurs through a lone pair of electrons on the F atom  Each H atom acts as a bridge between two F atoms o  The bond angle between two F atoms bridged by an H atom is about 180 o The type of intermolecular force just described is called a hydrogen bond o In a hydrogen bond, an H atom is covalently bonded to a highly electronegative atom, which attracts electron density away from the H nucleus o This in turn allows the H nucleus, a proton, to be simultaneously attracted to a lone pair of electrons on a highly electronegative atom in a neighbouring molecule o Hydrogen bonds are possible only with certain hydrogen-containing compounds because all atoms other than H have inner-shell electrons to shield their nuclei from attraction by lone-pair electrons of nearby atoms o Only F, O and N meet the requirements for hydrogen-bond formation o Weak hydrogen bonding is occasionally encountered between an H atom of one molecule and a Cl or S atom in a neighbouring molecule o Compared with other intermolecular forces, hydrogen bonds are relatively strong o By contrast, single covalent bonds (intramolecular bonds) are much stronger still  Hydrogen bonding in water o The most common substance in which hydrogen bonding occurs o One water molecule is held to four neighbours in a tetrahedral arrangement by hydrogen bonds o In ice, hydrogen bonds hold the water molecules in a rigid, but open structure o As ice melts, only a fraction of the hydrogen bonds are broken o The open structure of ice gives it a low density o When ice melts, some of the H-bonds are broken o This allows water molecules to be more compactly arranged, accounting for the increase in density when ice melts o As liquid water is heated above the melting point, hydrogen bonds continue to break o The molecules become even more closely packed and the density of liquid water continues to increase o o Liquid water attains its maximum density at 3.98 C, above this temperature, the water behaves in a normal fashion, density increases as temperature increases o The unusual freezing-point behaviour of water explains why a freshwater lake freezes from the top down o o When water temperature falls below 4 C, the denser water sinks to the bottom of the lake and the colder surface water freezes o The ice over the top of the lake then tends to insulate the water below from further heat loos o This allows fish to survive in the winter in a lake that has been frozen over  Other properties affected by hydrogen bonding o In acetic acid, pairs of molecules tend to join together to form dimers (double molecules), both in liquid and in vapour states o Not all hydrogen bonds are disrupted when liquid acetic acid vaporizes and as a result, the heat of vaporization is abnormally low o Certain trends in viscosity can also be explained by hydrogen bonding  In alcohols, the H atom in a –OH group in one molecule can form a hydrogen bond to the O atom in a neighbouring alcohol molecule  An alcohol molecule with two –OH groups (a diol) has more possibilities for hydrogen bond formation than a comparable alcohol with a single – OH group  Having stronger intermolecular forces, the diol is more viscous than the simple alcohol  When still more –OH groups are present (polyols), we expect a further increase in viscosity  Intermolecular and Intramolecular Hydrogen Bonding o Examples of hydrogen bonding presented to this point have involved an intermolecular force between two molecules and this is called an intermolecular hydrogen bond o Another possibility occurs in molecules with an H atom covalently bonded to one highly electronegative atom and with another highly electronegative atom in the same molecule o This type of hydrogen bonding within the molecule is called intramolecular hydrogen bonding  Hydrogen bonding in living matter o Some chemical reactions in living matter involve complex structures such as proteins and DNA and in these reactions certain bonds must be easily broken and re-formed o Hydrogen bonding is the only type of bonding with energies of the right magnitude to allow this  Ionic Interactions o Favourable in solids, but are weaker in solution and can even be unfavourable in water, where salvation of individual ions by water can me more favourable than ionic interactions  Summary of VDW forces o Dispersion forces exist between all molecules  Involve displacements of all electrons in molecules and they increase in strength with increasing molecular mass  Forces also depend on molecular shape o Forces associated with permanent dipoles involve displacements of electron pairs in bonds rather than in molecules as a whole  These forces are found only in substances with resultant dipole moments (polar molecules)  Their existence adds to the effect of dispersion forces also present o When comparing substances of roughly comparable molecular masses, dipole forces can produce significant differences in properties such as MP, BP and enthalpy of vaporization o When comparing substances of widely different molecular masses, dispersion forces are usually more significant than dipole forces o Generally  Dispersion < dipole < H-bonding < Ionic/Covalent Some Properties of Liquids  Surface Tension o Interior molecules have more neighbours and experience more attractive intermolecular forces than surface molecules o The increased number of attractions by neighbouring molecules places an interior molecule in a more stable environment (lower energy state) than a surface molecule o Consequently, as many molecules as possible tend to enter the bulk of the liquid, while as few as possible remain at the surface o Thus, liquids tend to maintain a minimum surface area o To increase the surface of a liquid requires that molecules be moved from the interior to the surface of the liquid and this requires that work be done o Surface tension is the energy or work required to increase the surface area of a liquid  Often represented by gamma (γ) and has the units of energy per unit area o As the temperature increases, intermolecular forces become less effective o Less work is required to extend the surface of a liquid, meaning that surface tension decreases with increased temperature o When a drop of liquid spreads onto a film across a surface, it wets the surface o Whether a drop of liquid wets a surface or retains its spherical shape and stands on the surface depends on the strengths of two types of intermolecular forces  The forces exerted between molecules holding them together in the drop are cohesive forces  The forces between liquid molecules and the surface are adhesive forces  If cohesive forces are strong compared to adhesive forces, the drop maintains its shape  If adhesive forces are strong enough, the energy requirement for spreading the drop into a film is met through the work done by the collapsing drop o If the liquid in a glass tube is water, the water is drawn slightly up the walls of the tube by adhesive forces between water and glass  The interface between the water and the air above it, called the meniscus, is concave or curved in  With liquid mercury, the meniscus is convex  Cohesive forces in mercury, consisting of metallic bonds between Hg atoms are strong, mercury does not wet glass o The effect of meniscus formation is greatly magnified in tubes of small diameter, called capillary tubes  In capillary action, the water level inside the capillary tube is noticeably higher than outside  Viscosity o A liquid’s resistance to flow o The stronger the intermolecular forces of attraction, the greater the viscosity o When a liquid flows, one portion of the liquid moves with respect to neighbouring portions o Cohesive forces within the liquid creates an internal friction, which reduces the rate of flow o In liquids of low viscosity, the effect is weak and they flow easily o Liquids such as honey or heavy motor oil flow much more sluggishly o Because intermolecular forces of attraction can be offset by higher molecular kinetic energies, viscosity generally decreases with increased temperature for liquids  Enthalpy of Vaporization o Molecules having kinetic energies sufficiently above the average value are able to overcome intermolecular forces of attraction and escape from the surface of the liquid into the gaseous state o This passage of molecules from the surface of a liquid into the gaseous or vapour state is called vaporization or evaporation o Vaporization occurs more readily with  Increased temperature  More molecules have sufficient kinetic energy to overcome intermo
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