Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chemistry FIFTH EDITION by Steven S. Zumdahl University of Illinois

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 2 Chemistry FIFTH EDITION Chapter 8 Chemical Foundations Molecular Bonding and Structure play the central role in determining the course of all chemical reactions.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Section 8.8 Covalent Bond Energies Single Covalent Bonds : one pair of shared electrons

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Figure 8.14 The Molecular Structure of Methane

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Bond Energies Bond breaking requires energy (endothermic). Bond formation releases energy (exothermic).

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Stepwise Decomposition of Methane CH 4 (g)  CH 3 (g) + H (g) 435 kJ/mole CH 3 (g)  CH 2 (g) + H (g) 453 kJ/mole CH 2 (g)  CH (g) + H (g) 425 kJ/mole CH (g)  C (g) + H (g) 339 kJ/mole

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 One C—H bond broken in each case. Energy required varies depending on the environment. Usually we assign an Average value Total = = 1652 kJ/mole Divide by 4 = 413 kJ/mole for C—H bond.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 Bond energy does depend on Environment. C—H Bond Energy H—CBr kJ/mole H—CCl kJ/mole H—CF kJ/mole H—C(CH 2 )– CH kJ/mole

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Double Bond: two pairs of electrons shared. Triple Bond: three pairs of electrons shared.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 As the number of shared electrons increases, (1)Bond length shortens. (2) Bond energy increases.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Reactants  Atoms  Products Break Bonds Form Bonds

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Energy of Chemical Reactions Calculated using Bond Energies

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 Bond Energies Bond breaking requires energy (endothermic). Bond formation releases energy (exothermic).  H =  D( bonds broken )   D( bonds formed ) energy requiredenergy released D = Bond Energy per mole of bonds.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Section 8.9 Localized Electron Model A molecule is composed of atoms that are bound together by sharing pairs of electrons using the atomic orbitals of the bound atoms.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 Electron Pairs are: 1) Lone Pairs -- localized on one particular atom. particular atom. 2) Bonding Pairs -- In the space between two atoms. between two atoms.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 Localized Electron Model 1.Description of valence electron arrangement (Lewis structure). 2.Prediction of geometry (VSEPR model). 3.Description of atomic orbital types used to share electrons or hold lone pairs.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 Lewis Structure 4 Shows how valence electrons are shared among atoms in a molecule. 4 Reflects central idea that stability of a compound relates to noble gas electron configuration. 4 Uses “dots” to represent valence electrons.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Do H C N N 3- I Ba Ba 2+

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Duet Rule: Hydrogen forms stable molecules when it shares 2 electrons. H∙ + H∙  H:H Octet Rule: Stable molecules form when 8 electrons are shared (fills the valence shell).

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 Rules for Lewis Structures Add total # of valence electrons for all atoms in the molecule. Use a pair of electrons to form a bond between each pair of atoms. Arrange remaining electrons to satisfy duet rule for H & octet rule for 2 nd row elements. Create double & triple bonds as necessary.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Only C, O, N, S, & F can have Double Bonds. Only C & N can form stable Triple Bonds.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Do Examples Cl 2 NH 4 + H 2 CO SO 4 2- PO 3 3-

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Section 8.11 Comments About the Octet Rule 4 2nd row elements C, N, O, F observe the octet rule. 4 2nd row elements B and Be often have fewer than 8 electrons around themselves - they are very reactive. 4 3rd row and heavier elements CAN exceed the octet rule using empty valence d orbitals. 4 When writing Lewis structures, satisfy octets first, then place electrons around elements having available d orbitals.

Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Do Examples BeCl 2 BH 4 - Br 3 - BrF 3 SF 6