liquids and solids chapter 13. solids, liquids, and gases solids & liquids are condensed...

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  • Liquids and SolidsChapter 13

  • Solids, Liquids, and GasesSolids & liquids are condensed statesatoms, ions, molecules are close to one anotherhighly incompressibleLiquids & gases are fluidseasily flowIntermolecular attractions in liquids & solids are strongLook at Table 13-1 for a general description of characterisitics

  • Kinetic-Molecular Description of Liquids and SolidsWhen a gaseous sample is cooled or compressed the rapid, random motion of the molecules decrease. The attraction between molecules becomes significant. When the attractive forces overcome the reduced kinetic energies, condensation occurs (or the gas turns to a liquid).In the liquid phase the particles are close. Very little space is unoccupied (discuss).The particles, however, still have sufficient KEs to partially overcome the attractive forces. Therefore, liquids are considered fluids and can take the shape of a container.

  • Kinetic-Molecular Description of Liquids & SolidsLiquids that diffuse into one another are miscible (i.e. one liquid is soluble in the other)Water and methanol/gasoline and motor oilImmiscible liquid do not diffuse into one another (i.e. one liquid is not soluble in the other)Water and hexane/water and gasoline

  • Kinetic-Molecular Description of Liquids & SolidsCooling the liquid lowers the KE even more. Shorter-range attractive force become important and the liquid solidifies. Particles in a solid cannot move freely past one another as they can in a liquid. This is why solids have definite shapes and volumes. Solids are essentially incompressible.If diffusion occurs in solids, it is very slow.

  • Intermolecular Attractive ForcesIntermolecular attractive forces are the forces between individual particles of a substance.Generally, these forces are very weak compared to intramolecular forces (e.g. covalent and ionic bonds)Covalent bonding and attractive forces in H2O(l)Important physical properties such as boiling points, vapor pressure, heat of vaporation, melting points, and heat of fusion depend on the strength of these intermolecular attractive forces. If intermolecular attractive forces did not exist, solids and liquids would not exist (only gases)

  • Ion-Ion InteractionsThe force of attraction between two charged particles can be determined from Coulombs Law

    where q+ and q- are the charges on the particles and d is the distance between them.

  • Ion-Ion InteractionsThe energy of attraction between charged particles is given by

    The energy of attraction is large for ionic compounds due to the charged particles that are close together when substance is a solid.The melting points for ionic compounds is relatively high.For a given substance, the separation between particles in the solid is less than the separation in liquids.Energy of attraction is greater in the solid phase (generally)

  • Ion-Ion InteractionsIonic compounds that possess multiply-charged ions (e.g. Al3+ and O2-) usually have higher melting points than ionic compounds that posses singly-charged particles. Why (two reasons)?Arrange the following ionic compounds in the expected order of increasing melting and boiling points.NaF, CaO, CaF2

  • Dipole-Dipole InteractionsPermanent dipole-dipole interactions occur between polar covalent molecules because of the attraction between - and + on different molecules.Generally, these forces are not as strong as ion-ion interactionsAttraction between partial chargesDipole-dipole forces vary as 1/d4 instead of 1/d2Decrease faster

  • Dipole-Dipole InteractionsThe partial positive charge on the hydrogen atoms is attracted to the partial negative charge on the nitrogen atoms.Note: Dipole-dipole interactions are dependent on temperature. Why?

  • Hydrogen BondingHydrogen bonding is a special type of strong dipole-dipole interactionOccurs between covalent molecules containing H and of the three small, highly electronegative elements-F, O, or N.One molecule most possess a H atom attached to one of these highly electronegative atoms.The other molecule most possess one these highly electronegative atoms.

  • Hydrogen BondingThe partial negative charge of one molecule is attracted to the partial positive charge of another molecule. The small sizes of F, O, and N and their high electronegativities concentrate the electrons around these atoms.

  • Hydrogen BondingTypical hydrogen-bond energies are greater than dipole-dipole energies15-20 kJ/mol for hydrogen-bond energies4 kJ/mol for dipole-dipole energies400 kJ/mole for ion-ion interaction energiesHydrogen bonding is responsible for the high boiling points of water and methanol.

  • Dispersion ForcesThese are attractive forces that are present in all types of molecules.Dispersion forces are weak in small molecules.They are important at extremely small distances which vary as 1/d7.They are the only attractive force present in symmetrical nonpolar substances such as Cl2 and monatomic species.

  • Dispersion ForcesDispersion forces result from the attraction of a positively charged nucleus to the electron cloud of another atom in nearby molecules. As a result, temporary dipoles are induced in the neighboring atoms or molecules. The magnitude of the temporary dipole increases with increasing size of the electron cloud (or size of the molecule). The larger electron cloud is more diffuse and easily distorted. Adjacent molecules are polarized by the adjacent nuclei. Polarizibility increases with increasing sizes of molecules.Dispersion forces are, therefore, stronger for molecules that are larger or have more electrons.

  • Dispersion Forces for ArgonDispersion forces can also exist between cations/anions and polarizable atom or molecule. Examine Figure 13-6.

  • Dispersion ForcesSome trends observed in increasing boiling points can largely be attributed to dispersion forces (Figure 13-5).CH4, SiH4, GeH4, and SnH4HCl, HBr, and HIUnderstand Table 13-3.Heat of vaporization measures the energy required to overcome attractive forces in the liquid.

  • The Liquid StateViscosity - the resistance to flow of a liquid.Generally, the higher the attractive forces in a liquid, the greater the viscosity.Water versus honey or Karo syrup.Pentane versus dodecaneViscosity decreases with increasing temperature. Why?

  • The Liquid StateSurface Tension - measure of the inward forces that must be overcome to expand the surface area of a liquid.Molecules at the surface are attracted unevenly.Water bugs and floating razor blades (or needles)Demo: Razor blade or needle

  • Liquid StateCapillary action - ability of a liquid to rise or fall in a tube.Cohesive forces forces holding a liquid together.Adhesive force forces between a liquid and another surfaceCapillary actionCapillary rise implies adhesive forces > cohesive forcesCapillary fall implies cohesive forces > adhesive forcesWater is attracted to a glass capillary tube due to attractive forces between the partial negative oxygens on the surface of the glass and the partial positive charges on the hydrogen atoms.

  • Liquid StateCapillary ActionThe meniscus of water has a concave shape due to the strong adhesive forces between the water molecules and the glass graduated cylinder.The meniscus of Hg has a convex shape because the cohesive forces are much stronger than the adhesive forces (Figure 13-9).

  • Liquid StateEvaporation process by which molecules escape from the surface of a liquid.Only the molecules at the surface with sufficient KE will be able to escape the attractive forces present in the liquid. Therefore, the rate of evaporation is proportional to ________.

  • Liquid StateEvaporationAs the faster molecules leave the liquid, the flower molecules are left behind. What will this do to the temperature?This process is termed as the cooling by evaporation.DEMO: Thermometer in acetone

  • Liquid StateEvaporation and condensationIn an open container/beaker, all the water that is present will eventually escape into the gaseous phase (Figure 13-10).What if the beaker is sealed? What will happen? After molecules enter the gas phase they may be recaptured by the liquid by collisions. This process is called _______. At some point in time, the amount of gaseous molecule leaving the gaseous phase will equal the amount reentering the liquid phase. This is termed as dynamic equilibrium.

  • Liquid StateVapor pressure the partial pressure of vapor molecules above the surface of a liquid at equilibrium.Vapor pressure increases with temperature. Why? Look at Figure 13-13.Vapor pressure decreases with increasing attractive forces. Why? Look at Table 13-4.Hydrogen bonding, dipole-dipole, and dispersion forces.DEMO: H2O and ethyl ether (add H2O to ethyl ether)

  • Liquid StateAs the temperature is increased, the vapor pressure increases until the liquid boils.Boiling point temperature at which the vapor pressure equals the external pressure. The boiling point of H2O is less in Rexburg. Why?Normal boiling point temperature at which the vapor pressure of a liquid is equal to 1 atm.Boiling point at 1 atmWhat if the external pressure is lower over water?DEMO: Evacuate a flask filled with water

  • Liquid StateDistillation a method used to separate components in a solution based on differences in boiling point temperatures.Separation of liquids in a solutionPurification of water (distilled water)Separate impurities (e.g. ions) from tap waterDescribe the basics of operation (Figure 13-14)Of course, this can be related to vapor pressures.

  • The Liquid StateHeat transfer involving liquidsSpecific heat or molar heat capacity of a liquid is the amount of heat that mu