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Chapter 12

Textbook Chapter 12 - Chem 1AA3

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McMaster University
Pippa Lock

Chem 1AA3 Chapter 12: Liquids, Solids & Intermolecular Forces 12.1 Intermolecular Forces  In pure substances: o van der Waals forces  Eg/ iodine o Dipole-dipole forces  Eg/ acetone o Hydrogen-bonding forces  Eg/ water  In solutions we also see: o ion-dipole forces  Eg/ hydrated ions o Ionic interactions  Eg/ hydrated ions London (Dispersion) Forces – Instantaneous dipole-induced dipole forces  Electrons are concentrated in one region of an atom or a molecule by chance o Causes a normally non-polar species to become momentarily polar – instantaneous dipole is formed  Process of Induction – neighboring electrons become displaced and also produce a dipole – induced dipole  Non polar molecule  Experiences an instantaneous dipole  Which induces a dipole in its neighbor  Instantaneous dipoles propagate throughout the sample  Force strength depends on polarizability (the tendency for charge separation to occur) o Polarizability – the relative tendency for a charge distribution to distort from its normal shape in an atom or a molecule  Forces strength increases with: o Atomic number and size of the molecule (because of polarizability) o Linear versus branches structures (because of increased intermolecular contact area)  Electrons in elongated molecules are more easily displaced than are those in small, compact, symmetrical molecules; elongated molecules are more polarizable  Isomers may have different properties  Melting points and boiling points of molecular substances generally increase with increasing molecular mass o Molecular Mass Increases  Polarizability Increases  Dispersion Forces Become Stronger Chem 1AA3 Dipole-Dipole Forces  Occur in polar molecules (molecules with permanent dipoles) o Polar molecules have bond dipoles and asymmetric shape (eg/ acetone is polar, while carbon dioxide is not polar)  Being polar increases b.p. over dispersion forces alone o Eg/ N (27K), NO (121 K), O (92 K) Hydrogen Bonding  Between an H atom in a polar bond (ie/ H-N, H-O, or H-F) and another electronegative atom (ie/ N, O, F) o H atom covalently bonded to highly electronegative atom – attracts elctron density away from the H nucleus o Allows the H nucleus, a proton, to be simultaneously attracted to a lone pair of electrons on a highly electronegative atom in a neighboring molecular  One H-bond is small, but many are mighty  Most atoms other than H have inner shell electrons to shield their nuclei from attraction by lone-pair electrons of nearby atoms – only F, O and N easily meet the requirements for H bond formation  Graph: Group 15, 16 and 17 are exceptions to trend (H 2, HF and NH )3 o Strong dipole-dipole attractions o H atom covalently bonded to atom in own molecule, but weakly bonded to nearby molecule  H-bonds are cumulatively very strong o Magazine example  -OH groups allow for hydrogen bonding Chem 1AA3  more –OH  more H-bonding (more viscous etc) Ionic Interactions  Ionic interactions are very favorable in solids, but are weaker in solution, and can be unfavorable in water, where solvation of the individual ions by water can be more favorable than the ionic interactions  Applications: Relative Strengths of IMFs: A Summary  All molecules have dispersion (London) forces  The range of strength for each force type overlaps, but approximately: o Dispersion < Dipole < H-Bonding < Ionic/Covalent  Dispersion (London) forces exist between all molecules o Involve displacement of all electrons in molecules o Increase in strength with increasing molecular mass o Forces depend on molecular shape  Forces associated with permanent dipoles involve displacement of electron pairs in bonds rather than in molecules as a whole Chem 1AA3 o Forces are found only in substance with resultant dipole moments (polar molecules) o Their existence adds to the effect of dispersion forces also present  When comparing substances of roughly comparable molecular masses, dipole forces can produce significant differences in properties such as melting point, boiling point and enthalpy of vaporization  When comparing substances of widely different molecular masses, dispersion forces are usually more significant than dipole forces 12.2 Impact of IMF Strength in Liquids Surface Tension  The energy or work required to increase the surface area of a liquid  Decrease when temperature increases  Cohesive Forces – forces exerted between molecules of the same substance; forces holding a drop of water together  Adhesive forces – forces between different molecules; forces holding water and glass together Viscosity  A liquids resistance to flow  Stronger intermolecular forces – greater viscosity  Decreases when temperature increases Enthalpy of Vaporization  Vaporization/Evaporation – the passage of molecules from the surface of a liquid into the gaseous or vapor state  Vaporization occurs more readily when o Temperature is increased  more molecules have sufficient kinetic energy to overcome intermolecular forces of attraction in the liquid o Surface area is increased  a greater proportion of the liquid molecules are at the surface o Strength of intermolecular forces is decreased  the kinetic energy needed to overcome intermolecular forces of attraction is less, and more molecules have enough energy to escape  Enthalpy of Vaporization – the quantity of heat that must be absorbed if a certain quantity of liquid is vaporized at a constant temperature o ΔH vaporizationvapor- liquid Chem 1AA3 o Vaporization is endothermic; ΔH vaporizationds to be positive o Molecules lost through evaporation are more energetic than the average – the average kinetic energt of the remaining molecules decreases; temperature of the liquid falls  Condensatio
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