AP Review: Unit 6ABy Harrison Alch and Karen SittigMay 1, 2008

Quantum Mechanics: Constantsc = the speed of light = 2.998*108 m/s = wavelength (generally in nm)h = Plancks constant = 6.63*10-34 J*sE = energy (generally in J)m = mass mass of an electron = 9.11*10-31 kg

Quantum Mechanics: Equationsc=vWavelength of a photon (massless)E = hvEnergy of a photon = h / mvdeBroglie wavelength takes mass of a particle into account

Example problemIt requires a photon with a minimum energy of 4.41 x 10-19 J to emit electrons from sodium metal.What is the minimum frequency needed to emit electrons from sodium via the photoelectric effect?E = hv4.41 x 10-19 J = (6.626 x 10-34 J*s) * vv = 6.66 x 1014 HzCalculate the wavelength of one photon of this light. What color would it be? = c/v = (2.998 x 108 m/s)/(6.66 x 1014 Hz) = 450 nmThis light would be blue

Example problem cont.If sodium is irradiated with light of 439 nm, what is the maximum possible kinetic energy of the emitted electrons?E439 = hc/ E439 = (6.626 x 10-34 J*s)(2.998 x 108 m/s)/(439 x 10-9 m)E439 = 4.52 x 10-19 JEk = E439 EminEk = (4.52 x 10-19 J) (4.41 x 10-19 J)Ek = 1.1 x 10-20 J/e-

IMFAsIntermolecular forces of attractionGeneral types:London Dispersion ForcesDipole-Dipole ForcesIon-Dipole ForcesHydrogen Bonds

London Dispersion ForcesExist between all atoms and moleculesAre the only forces between nonpolar atoms and moleculesThe weakest type of intermolecular forces, caused by instantaneous dipolesDispersion forces tend to increase in strength with increasing molecular weight.

Dipole-Dipole and Ion-DipoleDipole-Dipole ForcesExist between polar molecules when the net positive end of one attracts the net negative end of anotherOnly exist when molecules are close together.Ion-Dipole Forcesexist between ions and polar moleculesA cation attracts the negative ends of polar molecules, an anion attracts the positive ends.

Hydrogen BondsExist only between an H bonded to an O, F, or N in a very polar bond and usually another O, F, or N, or other very electronegative atom. The strongest type of intermolecular forces.from http://kentsimmons.uwinnipeg.ca/

Comparative Strength of IMFAsWhen molecules of 2 substances are relatively the same mass, the different strengths of attractive forces are due to differences in the length of the dipole moment.When molecules of 2 substances are different masses, the different strengths of attractive forces are due to the strength of dispersion forces (the more massive one generally has stronger attractive forces).

Some Properties of LiquidsViscosityThe resistance of liquid to flowLiquids that have stronger IMFAs are more viscousViscosity also increases with the increasing ability of molecules to become tangled (some isomers are more viscous than others)Surface TensionThe amount of energy required to increase the surface area of a liquid by a given amountLiquids that have stronger IMFAs tend to have more surface tensionThe surface tension of water at 293 K is 0.0729 J/m2The surface tension of mercury at 239 K is 0.46 J/m2

Example ProblemsWhy is the surface tension of CHBr3 greater than CHCL3?CHBr3 has a higher molar mass, is more polarizable, and has stronger dispersion forces, so the surface tension is greater.Why does as temperature increases, oil flow faster through a narrow tube?The viscosity of the oil decreases because the average kinetic energy of the molecules increase.

Periodic TrendsSizeGenerally, size decreases across a period and increases down a familyAs the nuclear charge increases but no new electron orbitals are added, the valence electrons are held more strongly by the nucleus, decreasing the sizeIn transition metals, the electrons in filled d orbitals tend to repel one another strongly enough to result in a deviation from this trendElectron AffinityGenerally, electron affinity increases across a period and decreases down a familyAcross a period, the effective nuclear charge increases, more readily attracting electronsDown a family, the distance from the nucleus decreases, making it more difficult to attract electrons

Period Trends cont.Ionization EnergyIonization energy tends to increase across a period and up a familyAs more protons are added to the nucleus, they increase the effective nuclear charge and hold the valence electrons more tightly, so removing an electron requires more energyAs more orbitals are added, the valence electrons are farther away from the nucleus, reducing the pull of the protons on the electrons and allowing them to be removed more easily.Ionic RadiiCations are smaller than their parent atoms due to the remaining electrons being held more tightly than beforeAnions are larger than their parent atoms due to increased electron-electron repulsionsFor a series of isoelectronic ions and atoms, atomic radius decreases with increasing atomic number (and increasing nuclear charge)

Example ProblemsPut these atoms in order of increasing size: Na, Ca, K, Be, & SBe, S, Na, Ca, KWhich gas has a lower boiling point, Cl2 or Kr?Kr would have a lower boiling point, despite its higher molar mass.Which has stronger IMFAs, CH4 or CH3OH?CH3OH would have stronger IMFAs due to its polar OH group.

Drawing Lewis StructuresPick a central atom. This should be of the element with the lowest electronegativity (generally from families 3, 4, or 5, or a noble gas)Determine how many bonds are needed by subtracting the valence electrons (number of family) from the total electrons needed to complete octets (or duets etc) and dividing by two.Connect each atom with a single bond.Add more bonds if needed. Otherwise, complete the octets on the outer atoms.If there are extra electrons, add them to the central atom.

Example ProblemsXeF4CH4NH3H3O+

Molecular ShapesCan be classified by electron geometry or molecular geometryElectron geometry doesnt take lone pairs into account when assigning names, molecular geometry doesThe hybridization of the central atom is determined by the electron geometry

Electron GeometryLinear2 bonding domainsNo hybrid orbitalsTrigonal Planar3 bonding domainssp hybridizedTetrahedral4 bonding domainssp2 hybridizedPictures from www.wikipedia.org

Electron Geometry cont.Trigonal Bipyramidal5 bonding domainssp2d hybridizedOctahedral6 bonding domainssp2d2 hybridizedPictures from www.wikipedia.org

Molecular GeometryPictures from www.wikipedia.orgBent1 or 2 lone pairssp or sp2 hybridizedTrigonal Pyramidal1 lone pairsp2 hybridizedSee-Saw6 bonding domainssp2d hybridized

Molecular Geometry cont.Pictures from www.wikipedia.orgSquare Planar2 lone pairssp2d2 hybridizedSquare Pyramidal1 lone pairsp2d2 hybridizedLinear3 lone pairssp2d hybridizedT-Shaped2 lone pairssp2d hybridized

Sample ProblemsName the electron geometry:XeF4CH4SF6Name the molecular geometry:NH3H2OPCl5State the hybridization of the central atom:XeF2BrF5SO2

Solutions to Sample ProblemsName the electron geometry:XeF4 - OctahedralCH4 - TetrahedralClF3 Trigonal BipyramidalName the molecular geometry:NH3 Trigonal PyramidalH2O - BentPCl5 - OctahedralState the hybridization of the central atom:XeF2 - sp2d2BeCl2 - unhybridizedSO2 sp2