Presentation on theme: "Ch. 9: Chemical Bonding I: Lewis Theory (cont’d)"— Presentation transcript:
1Ch. 9: Chemical Bonding I: Lewis Theory (cont’d) Dr. Namphol SinkasetChem 200: General Chemistry I
2I. Chapter Outline Introduction Energy of Ionic Bond Formation Energy of Covalent Bond Formation
3I. Energies of Bond Formation Previously in this chapter, we talked about how ionic and covalent bonds form.Now we look at the energies involved.For ionics, we apply Hess’s Law.For covalents, we use bond energies (enthalpies).
4II. Ionic Bond Formation Recall that we envision the creation of ionic bond as an e- transfer from the metal to the nonmetal.However, if we look at the energies involved in these two steps, we see something puzzling.
5II. Ion Formation for NaCl We break up the e- transfer process into two steps and add up the energies.Na(g) Na+(g) + e IE1 = 496 kJ/moleCl(g) + e- Cl-(g) EA = -349 kJ/moleNa(g) + Cl(g) Na+(g) + Cl-(g) IE1 + EA = 147 kJ/mole
6II. Other EnergiesFor most ionics, this is typical; ion formation is an endothermic process.So why do ionics form at all?There must be a huge exothermic process to offset these endothermic processes.The strong +/- attractions that are formed are the source of this exo step.
7II. Lattice Energylattice energy: the energy associated with forming a crystalline lattice of alternating cations and anions from gaseous ions.
8II. Born-Haber CycleAlthough lattice energy is a critical component of ionic bond formation, it cannot be measured directly.We use Hess’s Law in the Born-Haber Cycle.
10II. Calculating Lattice Energy Since enthalpy is independent of path, the following equation applies.ΔH˚f = ΔH˚atom + ΔH˚BE + ΔH˚IE + ΔH˚EA + ΔH˚lattice
11II. Sample ProblemCalculate the lattice energy for CaCl2 given the following enthalpy values: heat of sublimation for Ca = kJ/mole, Cl2 bond energy = 243 kJ/mole, 1st ionization energy Ca = kJ/mole, 2nd ionization energy Ca = kJ/mole, 3rd ionization energy Ca = kJ/mole, electron affinity Cl = -349 kJ/mole, heat of formation CaCl2 = kJ/mole.
12III. Covalent Bond Strength The strength of a covalent bond depends on how strongly the e- are held by both nuclei.The bond energy or bond enthalpy (BE) is the energy needed to overcome this attraction. It is defined as the energy needed to break 1 mole bonds in the gas phaseAnother way to envision it…
14III. Bond Energies Breaking bonds require energy… A-B(g) A(g) + B(g) ΔH˚break = BEA-BForming bonds releases energy...A(g) + B(g) A-B(g) ΔH˚form = -BEA-BDifferent bonds have different levels of attraction (have different depths to their wells) and thus different BE’s.
15III. BE’s and Chemical Change In a reaction, bonds are broken and new bonds are formed.Relative strengths of bonds determine whether rxn is exo or endo.The energy released or absorbed in a reaction is due to the difference between reactant and product bond energies.