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Chapter 8.8 Bond Enthalpies

Chemistry, The Central Science , 11th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten. Chapter 8.8 Bond Enthalpies. John D. Bookstaver St. Charles Community College Cottleville, MO. Covalent Bond Strength.

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Chapter 8.8 Bond Enthalpies

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  1. Chemistry, The Central Science, 11th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten Chapter 8.8Bond Enthalpies John D. Bookstaver St. Charles Community College Cottleville, MO © 2009, Prentice-Hall, Inc.

  2. 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. © 2009, Prentice-Hall, Inc.

  3. Average Bond Enthalpies • This table (on pg 326 in your text) lists the average bond enthalpies for many different types of bonds. • Average bond enthalpies are positive, because bond breaking is an endothermic process. © 2009, Prentice-Hall, Inc.

  4. Average Bond Enthalpies NOTE: These are average bond enthalpies, not absolute bond enthalpies; the C-H bonds in methane, CH4, will be a bit different than the C-H bond in chloroform, CHCl3. © 2009, Prentice-Hall, Inc.

  5. 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, • Hrxn = (bond enthalpies of bonds broken) - • (bond enthalpies of bonds formed) © 2009, Prentice-Hall, Inc.

  6. Enthalpies of Reaction CH4(g) + Cl2(g) CH3Cl (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. © 2009, Prentice-Hall, Inc.

  7. Enthalpies of Reaction So, H = [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 © 2009, Prentice-Hall, Inc.

  8. Bond Enthalpy and Bond Length • We can also measure an average bond length for different bond types. • As the number of bonds between two atoms increases, the bond length decreases. © 2009, Prentice-Hall, Inc.

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