Ap chem unit 8 presentation

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  • 1. Bonding: General Concepts AP Chemistry Unit 8

2. Types of Chemical Bonds 3. Ionic BondsIonic Bonds are formed when an atomthat loses electrons relatively easilyreacts with an atom that has a highattraction for electrons.Ionic Compounds results when a metalbonds with a nonmetal. 4. Bond EnergyBond energy is the energy required to break abond.The energy of interaction between a pair of ionscan be calculated using Coulombs law-19 Q1Q2 E = (2.31x10 Jinm) r r = the distance between the ions in nm. Q1 and Q2 are the numerical ion charges.E is in joules 5. Bond EnergyWhen the calculated energy betweenions is negative, that indicates anattractive force.A positive energy is a repulsive energy.The distance where the energy isminimal is called the bond length. 6. Covalent BondsCovalent bonds form betweenmolecules in which electrons are sharedby nuclei.The bonding electrons are typicallypositioned between the two positivelycharged nuclei. 7. Polar Covalent BondsPolar covalent bonds are an intermediatecase in which the electrons are notcompletely transferred but form unequalsharing.A - or + is used to show a fractional orpartial charge on a molecule with unequalsharing. This is called a dipole. 8. Electronegativity 9. ElectronegativityElectronegativity is the ability of an atom in a molecule to attract shared electrons to itself. (electron love)Relative electronegativities are determined bycomparing the measured bond energy with theexpected bond energy.Measured in Paulings. After Linus Pauling theAmerican scientist who won the Nobel Prizesfor both chemistry and peace. 10. ElectronegativityExpected H-X bond energy=H - H bond energy + X - X bond energy 2 11. ElectronegativityElectronegativity values generally increasegoing left to right across the periodic tableand decrease going top to bottom. 12. Electronegativity andBond type 13. Bond Polarity and Dipole 14. Dipoles and Dipole MomentsA molecule that has a center of positivecharge and a center of negative chargeis said to be dipolar or to have a dipolemoment.An arrow is used to show this dipolemoment by pointing to the negativecharge and the tail at the positivecharge. 15. Dipoles and DipoleMomentsElectrostatic potentialdiagram showsvariation in charge.Red is the mostelectron rich regionand blue is the mostelectron poor region. 16. Dipoles and DipoleMoments 17. Dipoles and DipoleMoments 18. Dipoles and DipoleMoments 19. Dipoles and DipoleMomentsDipole moments are when opposingbond polarities dont cancel out. 20. Dipoles and DipoleMoments 21. Example ProblemsFor each of the following molecules,show the direction of the bondpolarities and indicate which ones havea dipole moment: HCl, Cl2, SO3, CH4, H2S 22. HCl 23. Cl2 24. SO3 25. CH4 26. H2S 27. Ions: ElectronConfigurations and Sizes 28. Electron Configurations of Compounds When two nonmetals react to form a covalent bond, they share electrons in a way that completes the valence electron configurations of both atoms. That is, both nonmetals attain noble gas electron configurations. 29. Electron Configurations of Compounds When a nonmetal and a representative-group metal react to form a binary ionic compounds, the ions form so that the valence electron configuration of the nonmetal achieves the electron configuration of the next noble gas atom and the valence orbitals of the metal are emptied. In this way both ions achieve noble gas electron configurations. 30. Predicting IonicFormulasTo predict the formula of the ioniccompound, we simply recognize that thechemical compounds are always electricallyneutral. They have the same quantities ofpositive and negative charges. 31. Sizes of IonsSize of an ion generally follows the same trend as atomic radius. The big exception to this trend is where the metals become nonmetals and the ions switch charge. 32. Sizes of IonsA positive ion is formed by removing one or more electrons from a neutral atom, the resulting cation is smaller than the neutral atom.Less electrons allow for less repulsions andthe ion gets smaller. 33. Sizes of IonsAn addition of electrons to a neutral atomproduces an anion that is significantly largerthan the neutral atom.An addition of an electron causes additionalrepulsions around the atom and therefore itssize increases. 34. Energy Effects in Binary Ionic Compounds 35. Lattice EnergyLattice energy is the change in energy that takesplace when separated gaseous ions arepacked together to form an ionic solid.The lattice energy is often defined as theenergy released when an ionic solid formsfrom its ions.Lattice energy has a negative sign to showthat the energy is released. 36. Lattice Energy ExampleEstimate the enthalpy of lithium fluoride and the changes of energy and lattice energy during formation: Li+(g) + F-(g) LiF(s)1. Break down LiF into its standard state elements (use formation reaction):Li(s) + F2(g) LiF(s) 37. Lattice Energy Example Li(s) + F2(g) LiF(s) Li+(g) + F-(g) LiF(s)2. Use sublimation and evaporation reactions to get reactants into gas form (since lattice energy depends on gaseous state). Find the enthalpies to these reactions: Li(s) Li(g) 161 kJ/mol Li(g) + F2(g) LiF(s) 38. Lattice Energy Example Li(g) + F2(g) LiF(s)Li+(g) + F-(g) LiF(s)3. Ionize cation to form ions for bonding. Use Ionization energy for the enthalpy of the reaction. Li(g) Li+(g) + e- Ionization energy: 520 kJ/mol Li+(g) + F2(g) LiF(s) 39. Lattice Energy Example Li+(g) + F2(g) LiF(s)Li+(g) + F-(g) LiF(s)4. Dissociate diatomic gas to individual atoms: F2(g) F(g) Bond dissociation energy of F-F= 154 kJ/ 2 = 77 kJ/mol Li+(g) + F(g) LiF(s) 40. Lattice Energy Example Li+(g) + F(g) LiF(s) Li+(g) + F-(g) LiF(s)5. Electron addition to fluorine is the electron affinity of fluorine: F(g) + e- F-(g) -328 kJ/mol Li+(g) + F-(g) LiF(s) 41. Lattice Energy Example Li+(g) + F-(g) LiF(s)Li+(g) + F-(g) LiF(s)6. Formation of solid lithium fluoride from the gaseous ions corresponds to its lattice energy: Li+(g) + F-(g) LiF(s) -1047 kJ/mol 42. Lattice Energy ExampleThe sum of these five processes yields the overall reaction and the sum of the individual energy changes gives theoverall energy change and the enthalpy of formation: Li(s) Li(g)161 kJ Li(g) Li+(g) + e-520 kJ F2(g) F(g)77 kJ F(g) + e- F-(g)-328 kJ Li+(g) + F-(g) LiF(s)-1047 kJTotal = -617 kJ/mol 43. Lattice Energy 44. Lattice EnergyLattice energy can be calculated with at form ofCoulombs law: Q1Q2 LatticeEnergy = k r Q is the charges on the ions and r is theshortest distance between the centers of thecations and anions. k is a constant thatdepends on the structure of the solid and theelectron configurations of the ions. 45. Partial Ionic Characterof Covalent Bonds 46. Bond CharacterCalculations of ionic character: dipole moment of x - y Percent ionic character of a bond = + y x100% dipole moment of x y Even compounds with the maximum possibleelectronegativity differences are not 100%ionic in the gas phase. Therefore theoperational definition of ionic is anycompound that conducts an electric currentwhen melted will be classified as ionic. 47. Bond Character 48. The Covalent ChemicalBond 49. Chemical Bond ModelA chemical bond can be viewed as forces thatcause a group of atoms to behave as a unit.Bonds result from the tendency of a systemto seek its lowest possible energy.Individual bonds act relatively independent. 50. ExampleIt takes 1652 kJ of energy required to break thebonds in 1 mole of methane.1652 kJ of energy is released when 1 mole ofmethane is formed from gaseous atoms.Therefore, 1 mole of methane in gas phase has1652 kJ lower energy than the total of theindividual atoms.One mole of methane is held together with 1652kJ of energy.Each of the four C-H bonds contains 413 kJ ofenergy. 51. ExampleEach of the four C-H bonds contains 413 kJof energy.CH3Cl contains 1578 kJ of energy: 1 mol of C-Cl bonds + 3 mol (C-H bonds)=1578 kJ C-Cl bond energy + 3 (413 kJ/mol) = 1578 kJ C-Cl bond energy = 339 kJ/mol 52. Properties of ModelsA model doesnt equal reality; they are usedto explain incomplete understanding of hownature works.Models are often oversimplified and aresometimes wrong.Models over time tend to get overcomplicated due to repairs. 53. Properties of ModelsRemember that simple models often requirerestrictive assumptions. Best way to usemodels is to understand their strengths andweaknesses.We often learn more when models areincorrect than when they are right.Cu and Cr. 54. Covalent Bond Energiesand Chemical Reactions 55. Bond EnergiesBond energy averages are used for individualbond dissociation energies to giveapproximate energies in a particular bond.Bond energies vary due to several reasons:multiple bonds, 4 C-H bonds in methanedifferent elements in the molecule, C-H bond inC2H6 or C-H bond in HCCl3 56. Bond Energy ExampleCH4(g)CH3(g) + H(g) 435 kJCH3(g)CH2(g) + H(g) 453 kJCH2(g)CH(g) + H(g)425 kJCH(g)C(g) + H(g)339 kJ Total 1652 kJ Average 413 kJ 57. Bond Energy ExampleHCBr3380 kJHCCl3380 kJHCF3 430 kJC2H6 410 kJ 58. Average Bond Energies 59. Bond EnergyA relationship also exists between thenumber of shared electron pairs.single bond 2 electronsdouble bond 4 electronstriple bond 6 electrons 60. Bond EnergyBond energy values can be used to calculateapproximate energies for reactions.Energy associated with bond breaking havepositive signs Endothermic processEnergy associated with forming bonds releasesenergy and is negative. Exothermic process 61. Bond EnergyA relationship exists between the number of shared electron pairs and the bond length. As the number of electrons shared goes up thebond length shortens. 62. Bond Energy 63. Bond EnergyH = sum of the energies required to break oldbonds (positive signs) plus the sum of theenergies released in the formation of new bonds(negative signs).DH = Sn x D(bonds broken) - n x D(bonds formed) D represents bond