Bond Enthalpies & Length Chapter 8

Covalent Bond Strength Most simply, the strength of a bond is measured by determining how much energy is required to break the bond. This is the bond enthalpy. The bond enthalpy for a Cl—Cl bond, D(Cl—Cl), is measured to be 242 kJ/mol.

Average Bond Enthalpies Average bond enthalpies are positive, because bond breaking is an endothermic process. Making bonds is exothermic

Average Bond Enthalpies NOTE: These are average bond enthalpies, not absolute bond enthalpies; the C—H bonds in methane, CH 4, will be a bit different than the C—H bond in chloroform, CHCl 3.

Enthalpies of Reaction Yet another way to estimate  H for a reaction is to compare the bond enthalpies of bonds broken to the bond enthalpies of the new bonds formed. In other words,  H rxn =  (bond enthalpies of bonds broken)   (bond enthalpies of bonds formed)

Enthalpies of Reaction CH 4 (g) + Cl 2 (g)  CH 3 Cl (g) + HCl (g) In this example, one C—H bond and one Cl—Cl bond are broken; one C—Cl and one H—Cl bond are formed.

Enthalpies of Reaction So,  H rxn = [D(C—H) + D(Cl—Cl)  [D(C—Cl) + D(H—Cl) = [(413 kJ) + (242 kJ)]  [(328 kJ) + (431 kJ)] = (655 kJ)  (759 kJ) =  104 kJ

Practice Problem How much energy is absorbed or released (identify which) when the following reaction takes place? CH 4 (g) + 2 O 2 (g)  CO 2 (g) + 2 H 2 O (g) The relevant bond energies are: C—H 414 kJ/mol O==O 502 kJ/mol C==O 730 kJ/mol O—H 464 kJ/mol H—H 435 kJ/mol Cl—Cl 243 kJ/mol H—Cl 431 kJ/mol C—Cl 331 kJ/mol

Bond Enthalpy and Bond Length Bond length is the distance at which the energy of interaction between attraction and repulsion is minimized –Atoms vibrate about this minimum energy As the number of bonds between two atoms increases, the bond length decreases.

Distance versus Potential Energy graph The bond length is the distance between bonded atoms’ nuclei, and is the distance of minimum potential energy

Bond Length Coulomb’s law describes why bonds length decreases with number of bonds –Since two atoms share more electrons, causes the two atoms to be closer together Decreasing distance, increases Coulombic force –Strength of bond: triple > double > single –Distance between bonded atoms: single > double > triple