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

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Presentation on theme: "Copyright©2000 by Houghton Mifflin Company. All rights reserved. 1 Chemistry FIFTH EDITION by Steven S. Zumdahl University of Illinois."— Presentation transcript:

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

2 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.

3 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 3 Section 8.1 Types of Chemical Bonds Forces that hold groups of atoms together and make them function as a unit.

4 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 4 Bond Energy 4 It is the energy required to break a bond. 4 It gives us information about the strength of a bonding interaction. 4 Various types of Bonds

5 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 5 Ionic Bonds 4 Formed from electrostatic attractions of closely packed, oppositely charged ions. 4 Formed when an atom that easily loses electrons reacts with one that has a high electron affinity. 4 Usually when a metal loses electrons & the electrons are gained by a non-metal.

6 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 6 Ionic Bonds Q 1 and Q 2 = numerical ion charges r = distance between ion centers (in nm) To calculate the Energy of Interaction between a pair of ions -- Coulombs Law.

7 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 7 Problem Calculate the energy of interaction between Ag + and Br - if the internuclear distance of AgBr is nm. E = 2.31 x J nm (Q 1 Q 2 / r) E = 2.31 x J nm ((+1) (-1)/ nm) E = x J per unit = x J per unit

8 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 8 Problem E = x J per unit x x units per mole x 1 kJ / 1000 J E = kJ/mole = kJ/mole E = Negative means Attractive force Ion Pair has lower energy than the separated ions.

9 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 9 Covalent Bonding Bonding between atoms where electrons are shared by nuclei. A bond will form if the energy of the system is lower than that of the separated atoms.

10 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 10 Figure 8.1 Interaction of Two H Atoms and the Energy Profile

11 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 11 Bond Length The distance where the system energy is a minimum. Corresponds to distance apart where the combination of repulsive forces (proton-proton & electron-electron) and attractive forces (electron-proton) allows the system to reach a minimum energy.

12 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 12 Non-polar Covalent Bond: Equal Sharing of Electrons Ex: Br---Br Polar Covalent Bond: Unequal Sharing of Electrons Ex: H---F

13 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 13 The Effect of an Electric Field on Hydrogen Fluoride Molecules Molecules tend to line up With - ends toward + pole & + ends toward – pole. Fluorine is slightly neg (δ - ) since it is more electronegative.

14 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 14 Section 8.2 Electronegativity The ability of an atom in a molecule to attract shared electrons to itself. Electronegativity Ranges from 4.0 (Fluorine) to 0.7 (Cesium).

15 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 15 Figure 8.3 The Pauling Electronegativity Values Electronegativity values range from 4.0 (F) to 0.7 (Cs). Trend??

16 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 16 EN Electronegativity Difference EN = 0 Non-polar Covalent identical atoms EN = 0.5 to 1.9Polar Covalent EN > 1.9Ionic Approximate Values

17 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 17 Problem Using the Periodic Table, order the following from lowest to highest electronegativity. Fr, Mg, Rb Fr < Rb < Mg P, As, Ga, O Ga < As < P < O

18 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 18 Section 8.3 Bond Polarity & Dipole Moments A molecule, such as HF, that has a center of positive charge and a center of negative charge is said to be polar, or to have a dipole moment. Molecule said to be dipolar.

19 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 19 Diatomic Molecules Polarity clear cut: Cl Cl non-polar, no dipole moment electrons equally shared

20 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 20 Polyatomic Molecules More difficult Determine the polarity of each bond. Consider the 3-D shape. Determine direction (if any) of the molecules dipole moment. Sometimes individual dipoles cancel each other out.

21 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 21 Figure 8.4 Dipole Moment for H 2 O

22 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 22 Figure 8.5 Dipole Moment for NH 3

23 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 23 Figure 8.6 (a) Carbon Dioxide b) Opposed Bond Polarities CO 2 is Nonpolar

24 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 24 Atoms in Stable compounds have a Noble Gas Electronic Configuration. Section 8.4 Ions: Electronic Configurations and Sizes

25 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 25 Achieving Noble Gas Electron Configurations (NGEC) Two nonmetals react: They share electrons to achieve NGEC. A nonmetal and a representative group metal react (form ionic compound): The valence orbitals of the metal are emptied to achieve NGEC. The valence electron configuration of the nonmetal achieves NGEC by gaining these electrons..

26 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 26 Atoms lose or gain electrons to achieve a NGEC. Example: Mg 1s 2 2s 2 2p 6 3s 2 lose 2 electrons Mg 2+ 1s 2 2s 2 2p 6 isoelectric with Ne

27 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 27 Example: P: 1s 2 2s 2 2p 6 3s 2 3p 3 gains 3 e - P 3- : 1s 2 2s 2 2p 6 3s 2 3p 6 isoelectric with argon

28 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 28 Isoelectronic Ions Ions containing the the same number of electrons (O 2, F, Na +, Mg 2+, Al 3+ ) O 2 > F > Na + > Mg 2+ > Al 3+ largest smallest

29 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 29 Know Exceptions Sn 2+ Sn 4+ Bi 3+ Bi 5+ Pb 2+ Pb 4+ Tl 1+ Tl 3+ (Thallium)

30 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 30 Formulas for Binary Ionic Cpds. Requires compounds to be Electrically Neutral. Sum of the cation charges MUST EQUAL Sum of the anion charges

31 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 31 Size of Ions Absolute ion sizes Impossible to define. Mostly Interested in Trends A A + + e - smaller A + e - A - larger

32 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 32 Figure 8.7 Sizes of Ions Related to Positions of the Elements in the Periodic Table

33 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 33 Consider Isoelectric Ions O 2- F - Na + Mg 2+ Al 3+ All have [Ne] EC. All have 10 electrons. But they have different #s of protons.

34 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 34 Isoelectric Ions O 2- F - Na + Mg 2+ Al 3+ # p + : electrons feel greater attraction as the positive charge of nucleus increases. THEREFORE, ions become smaller.

35 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 35 Sizes of Ions For isoelectric ions, Size as Z Ion size increases as you go down a group. Trend Complicated L R

36 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 36 Figure 8.7 Sizes of Ions Related to Positions of the Elements in the Periodic Table.

37 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 37 Many separate processes go into the formation of an ionic solid. Section 8.5: Formation of Binary Ionic Compounds

38 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 38 Lattice Energy The change in energy when separated gaseous ions are packed together to form an ionic solid. M + (g) + X (g) MX(s) Lattice energy is negative (exothermic) from the point of view of the system. Dominate Energy Term

39 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 39 Reaction is broken into steps in order to calculate the energy associated with the process. Use steps where the energy of the process is known. Energy is a state function.

40 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 40 Formation of an Ionic Solid 1.Sublimation of the solid metal M(s) M(g) [endothermic] 2.Ionization of the metal atoms M(g) M + (g) + e [endothermic] 3.Dissociation of the nonmetal 1 / 2 X 2 (g) X(g) [endothermic]

41 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 41 Formation of an Ionic Solid (continued) 4.Formation of X ions in the gas phase: X(g) + e X (g) [exothermic] 5.Formation of the solid MX M + (g) + X (g) MX(s) [quite exothermic]

42 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 42 K (s) + ½ Cl 2 (g) KCl (s) Lets break this into several steps. K (s) K (g) sublimation E = + 64 kJ/mole K (g) K + (g) ionization E 1 = +419 kJ/mole Consider:

43 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 43 K (s) + ½ Cl 2 (g) KCl (s) ½ Cl 2 (g) Cl (g) Dissociation of chlorine molecules Energy to break bond = 240 kJ/mole Energy to break ½ mole = 120 kJ Cl (g) Cl - (g) Electron affinity EA = kJ/mole

44 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 44 K (s) + ½ Cl 2 (g) KCl (s) K + (g) + Cl - (g) KCl (s) Lattice energy = -690 kJ/mole Net Energy Change = sum of all the energy changes

45 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 45 H f (Energy of formation) = +64 kJ kJ kJ kJ kJ = kJ Lattice energy is the dominate energy term.

46 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 46 Consider Li (s) + ½ F 2 (g) LiF (s)

47 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 47 Figure 8.8 The Energy Changes Involved in the Formation of Solid Lithium Fluoride from its Elements

48 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 48 Figure 8.9 The Structure of solid Lithium Fluoride Ions pack together such that 1) maximize the attraction of oppositely charged ions & 2) minimize the repulsion of identically charged ions.

49 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 49 Comparison of the Energy Changes Involved in the Formation of Solid Sodium Fluoride (NaF) and Solid Magnesium Oxide (MgO). All ions involved are isoelectric with Neon. See next slide. Compare all energies.

50 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 50 Figure 8.10 Comparison of the Energy Changes Involved in the Formation of Solid Sodium Fluoride (NaF) and Solid Magnesium Oxide (MgO)

51 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 51 Q 1, Q 2 = charges on the ions Lattice energy has negative sign when Q 1 and Q 2 have opposite signs. LE increases as ionic charges increase. Higher charges More energetically stable crystal r = shortest distance between centers of the cations and anions LE increases as the distance between the ions decrease.

52 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 52 Section 8.6 Partial Ionic Character of Covalent Bonds

53 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 53 Figure 8.11 Three Possible Types of Bonds (a) Covalent bond between Identical Atoms. (b) Polar Covalent Bond. (c) Ionic Bond (no sharing of electrons)

54 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 54 How do we tell the difference between ionic bond & a polar covalent bond? % ionic character of a bond = measured dipole moment of XY calculated dipole moment of X + Y - X 100 Based on calculations done in the gas phase

55 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 55 Figure 8.12 The Relationship Between the Ionic Character of a Covalent Bond and the Electronegativity Difference of the Bonded Atoms

56 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 56 None reach 100% ionic character. Ones with more than 50% are normally considered ionic solids. These calculations are for discrete pairs of atoms, i.e., gas phase where individual XY molecules exist. Existence of ions in solid is favored by the multiple ion interactions.

57 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 57 Operational Definition of Ionic Compound Any compound that conducts an electric current when melted will be classified as ionic.

58 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 58 Section 8.7 The Covalent Chemical Bond: A Model Carefully Read. Complete Handout.

59 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 59 Models Models are attempts to explain how nature operates on the microscopic level based on experiences in the macroscopic world. Bonding is a model proposed to explain molecular stability.

60 Copyright©2000 by Houghton Mifflin Company. All rights reserved. 60 Fundamental Properties of Models 4 A model does not equal reality. 4 Models are oversimplifications, and are therefore often wrong. 4 Models become more complicated as they age. 4 We must understand the underlying assumptions in a model so that we dont misuse it.


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