Chapter 9 Chemical Bonding I lewis Theory

Chapter 9 Chemical Bonding I lewis Theory

Chemistry: A Molecular Approach, 1st Ed. Nivaldo Tro Chapter 9 Chemical Bonding I: Lewis Theory Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA 2008, Prentice Hall Bonding Theories explain how and why atoms attach together explain why some combinations of atoms are stable and others are not why is water H2O, not HO or H3O one of the simplest bonding theories was developed by G.N. Lewis and is called Lewis Theory Lewis Theory emphasizes valence electrons to explain bonding using Lewis Theory, we can draw models called Lewis structures that allow us to predict many properties of molecules aka Electron Dot Structures such as molecular shape, size, polarity

Tro, Chemistry: A Molecular Approach 2 Why Do Atoms Bond? processes are spontaneous if they result in a system with lower potential energy chemical bonds form because they lower the potential energy between the charged particles that compose atoms the potential energy between charged particles is directly proportional to the product of the charges the potential energy between charged particles is inversely proportional to the distance between the charges Tro, Chemistry: A Molecular Approach 3 Potential Energy Between Charged Particles E potential 0 is a constant

1 q1 q2 4 0 r = 8.85 x 10-12 C2/Jm for charges with the same sign, Epotential is + and the magnitude gets less positive as the particles get farther apart for charges with the opposite signs, Epotential is and the magnitude gets more negative as the particles get closer together remember: the more negative the potential energy, the more stable the system becomes Tro, Chemistry: A Molecular Approach 4 Potential Energy Between Charged Particles The attraction repulsion between

like-charged particles opposite-charged increasesincreases particles as the as particles the particles get get closer closer together. Bringing To bring them closer lowers requiresthe the addition potential energy of more of the energy. system. Tro, Chemistry: A Molecular Approach 5 Bonding a chemical bond forms when the potential

energy of the bonded atoms is less than the potential energy of the separate atoms have to consider following interactions: nucleus-to-nucleus repulsion electron-to-electron repulsion nucleus-to-electron attraction Tro, Chemistry: A Molecular Approach 6 Types of Bonds Types of Atoms metals to nonmetals nonmetals to nonmetals metal to metal Tro, Chemistry: A Molecular Approach Type of Bond Ionic Bond Characteristic electrons transferred

Covalent electrons shared Metallic electrons pooled 7 Types of Bonding 8 Ionic Bonds when metals bond to nonmetals, some electrons from the metal atoms are transferred to the nonmetal atoms metals have low ionization energy, relatively easy to remove an electron from nonmetals have high electron affinities, relatively good to add electrons to Tro, Chemistry: A Molecular Approach 9 Covalent Bonds

nonmetals have relatively high ionization energies, so it is difficult to remove electrons from them when nonmetals bond together, it is better in terms of potential energy for the atoms to share valence electrons potential energy lowest when the electrons are between the nuclei shared electrons hold the atoms together by attracting nuclei of both atoms Tro, Chemistry: A Molecular Approach 10 Determining the Number of Valence Electrons in an Atom the column number on the Periodic Table will tell you how many valence electrons a main group atom has Transition Elements all have 2 valence electrons; Why? 1A 2A 3A

4A 5A 6A 7A 8A Li Be B C N O F Ne 1 e-1 2 e-1

3 e-1 4 e-1 5 e-1 6 e-1 7 e-1 8 e-1 Tro, Chemistry: A Molecular Approach 11 Lewis Symbols of Atoms aka electron dot symbols use symbol of element to represent nucleus and inner electrons use dots around the symbol to represent valence electrons pair first two electrons for the s orbital put one electron on each open side for p electrons then pair rest of the p electrons

Li Be B Tro, Chemistry: A Molecular Approach C N

O F Ne

12 Lewis Symbols of Ions Cations have Lewis symbols without valence electrons Lost in the cation formation Anions have Lewis symbols with 8 valence electrons Electrons gained in the formation of the anion Li Li+1 F Tro, Chemistry: A Molecular Approach

F 1 13 Stable Electron Arrangements And Ion Charge Metals form cations by losing enough electrons to get the same electron configuration as the previous noble gas Nonmetals form anions by gaining enough electrons to get the same electron configuration as the next noble gas The noble gas electron configuration must be very stable

Tro, Chemistry: A Molecular Approach Atom Na Atoms Electron Config [Ne]3s1 Ion Na+1 Ions Electron Config [Ne] Mg [Ne]3s2 Mg+2 [Ne] Al

[Ne]3s23p1 Al+3 [Ne] O [He]2s22p4 O-2 [Ne] F [He]2s22p5 F-1 [Ne] 15 Octet Rule when atoms bond, they tend to gain, lose, or share electrons to

result in 8 valence electrons ns2np6 noble gas configuration many exceptions H, Li, Be, B attain an electron configuration like He He = 2 valence electrons Li loses its one valence electron H shares or gains one electron though it commonly loses its one electron to become H+ Be loses 2 electrons to become Be2+ though it commonly shares its two electrons in covalent bonds, resulting in 4 valence electrons B loses 3 electrons to become B3+ though it commonly shares its three electrons in covalent bonds, resulting in 6 valence electrons expanded octets for elements in Period 3 or below using empty valence d orbitals Tro, Chemistry: A Molecular Approach 16 Lewis Theory the basis of Lewis Theory is that there are

certain electron arrangements in the atom that are more stable octet rule bonding occurs so atoms attain a more stable electron configuration more stable = lower potential energy no attempt to quantify the energy as the calculation is extremely complex Tro, Chemistry: A Molecular Approach 17 Properties of Ionic Compounds Melting an Ionic Solid hard and brittle crystalline solids all are solids at room temperature melting points generally > 300C the liquid state conducts electricity the solid state does not conduct electricity many are soluble in water the solution conducts electricity well Tro, Chemistry: A Molecular Approach 18

Conductivity of NaCl in NaCl(s), the ions are stuck in position and not allowed to move to the charged rods Tro, Chemistry: A Molecular Approach in NaCl(aq), the ions are separated and allowed to move to the charged rods 19 Lewis Theory and Ionic Bonding Lewis symbols can be used to represent the transfer of electrons from metal atom to nonmetal atom, resulting in ions that are attracted to each other and therefore bond Li

+ F Tro, Chemistry: A Molecular Approach F Li + 1 20

Predicting Ionic Formulas Using Lewis Symbols electrons are transferred until the metal loses all its valence electrons and the nonmetal has an octet numbers of atoms are adjusted so the electron transfer comes out even Li Li O O

Tro, Chemistry: A Molecular Approach 2 Li + 2 Li2O 21 Energetics of Ionic Bond Formation the ionization energy of the metal is endothermic Na(s) Na+(g) + 1 e H = +603 kJ/mol the electron affinity of the nonmetal is exothermic Cl2(g) + 1 e Cl(g) H = 227 kJ/mol generally, the ionization energy of the metal is larger

than the electron affinity of the nonmetal, therefore the formation of the ionic compound should be endothermic but the heat of formation of most ionic compounds is exothermic and generally large; Why? Na(s) + Cl2(g) NaCl(s) Tro, Chemistry: A Molecular Approach Hf = -410 kJ/mol 22 Ionic Bonds electrostatic attraction is nondirectional!! no direct anion-cation pair no ionic molecule chemical formula is an empirical formula, simply giving the ratio of ions based on charge balance ions arranged in a pattern called a crystal lattice every cation surrounded by anions; and every anion surrounded by cations maximizes attractions between + and - ions Tro, Chemistry: A Molecular Approach 23 Lattice Energy

the lattice energy is the energy released when the solid crystal forms from separate ions in the gas state always exothermic hard to measure directly, but can be calculated from knowledge of other processes lattice energy depends directly on size of charges and inversely on distance between ions Tro, Chemistry: A Molecular Approach 24 Born-Haber Cycle method for determining the lattice energy of an ionic substance by using other reactions use Hesss Law to add up heats of other processes Hf(salt) = Hf(metal atoms, g) + Hf(nonmetal atoms, g) + Hf(cations, g) + Hf(anions, g) + Hf(crystal lattice) Hf(crystal lattice) = Lattice Energy metal atoms (g) cations (g), Hf = ionization energy dont forget to add together all the ionization energies to get to the desired cation M2+ = 1st IE + 2nd IE

nonmetal atoms (g) anions (g), Hf = electron affinity Tro, Chemistry: A Molecular Approach 25 Born-Haber Cycle for NaCl Tro, Chemistry: A Molecular Approach 26 Practice - Given the Information Below, Determine the Lattice Energy of MgCl2 Mg(s) Mg(g) H1f = +147.1 kJ/mol Cl2(g) Cl(g) H2f = +121.3 kJ/mol Mg(g) Mg+1(g) H3f = +738 kJ/mol Mg+1(g) Mg+2(g) H4f = +1450 kJ/mol Cl(g) Cl-1(g) H5f = -349 kJ/mol Mg(s) + Cl2(g) MgCl2(s) H6f = -641.3 kJ/mol Tro, Chemistry: A Molecular Approach 27

Practice - Given the Information Below, Determine the Lattice Energy of MgCl2 Mg(s) Mg(g) Cl2(g) Cl(g) Mg(g) Mg+1(g) Mg+1(g) Mg+2(g) Cl(g) Cl-1(g) Mg(s) + Cl2(g) MgCl2(s) H1f = +147.1 kJ/mol H2f = +121.3 kJ/mol H3f = +738 kJ/mol H4f = +1450 kJ/mol H5f = -349 kJ/mol H6f = -641.3 kJ/mol H 6f H1f 2H 2 f H 3f H 4f 2H 5f H f lattice energy H f lattice energy H 6 f H1f 2H 2 f H 3f H 4 f 2H 5f H f lattice energy ( 641.3 kJ) - (147.1 kJ) 2(121.3 kJ) (738 kJ) (1450 kJ) 2(-349 kJ) H f lattice energy 2521 kJ Tro, Chemistry: A Molecular Approach 28 Trends in Lattice Energy Ion Size the force of attraction between charged particles is inversely proportional to the

distance between them larger ions mean the center of positive charge (nucleus of the cation) is farther away from negative charge (electrons of the anion) larger ion = weaker attraction = smaller lattice energy Tro, Chemistry: A Molecular Approach 29 Lattice Energy vs. Ion Size Lattice Energy Metal Chloride (kJ/mol) LiCl -834 NaCl -787 KCl -701 CsCl -657

Tro, Chemistry: A Molecular Approach 30 Trends in Lattice Energy Ion Charge the force of attraction between oppositely charged particles is directly proportional to the product of the charges larger charge means the ions are more strongly attracted Lattice Energy = -910 kJ/mol larger charge = stronger attraction = larger lattice energy of the two factors, ion charge generally more important Tro, Chemistry: A Molecular Approach Lattice Energy = -3414 kJ/mol 31

Example 9.2 Order the following ionic compounds in order of increasing magnitude of lattice energy. CaO, KBr, KCl, SrO First examine the ion charges and order by product of the charges Ca2+& O2-, K+ & Br, K+ & Cl, Sr2+ & O2 (KBr, KCl) < (CaO, SrO) Then examine the ion sizes of each group and order by radius; larger < smaller (KBr, KCl) same cation, Br > Cl (same Group) (CaO, SrO) same anion, Sr2+ > Ca2+ (same Group) KBr < KCl < SrO (CaO, < SrO) CaO Tro, Chemistry: A Molecular Approach 32

Ionic Bonding Model vs. Reality ionic compounds have high melting points and boiling points MP generally > 300C all ionic compounds are solids at room temperature because the attractions between ions are strong, breaking down the crystal requires a lot of energy the stronger the attraction (larger the lattice energy), the higher the melting point Tro, Chemistry: A Molecular Approach 33 Ionic Bonding Model vs. Reality ionic solids are brittle and hard the position of the ion in the crystal is critical to establishing maximum attractive forces displacing the ions from their positions results in like charges close to each other and the repulsive forces take over + +

- + + + + -+ - +- + + + Tro, Chemistry: A Molecular Approach -+ -+ +- +- +- + + + + -+ + + + - + - + - + - 34 Ionic Bonding Model vs. Reality ionic compounds conduct electricity in the liquid state or when dissolved in water, but not in the solid state to conduct electricity, a material must have charged

particles that are able to flow through the material in the ionic solid, the charged particles are locked in position and cannot move around to conduct in the liquid state, or when dissolved in water, the ions have the ability to move through the structure and therefore conduct electricity Tro, Chemistry: A Molecular Approach 35 Single Covalent Bonds two atoms share a pair of electrons 2 electrons one atom may have more than one single bond F F

F F H O H H O H F

F Tro, Chemistry: A Molecular Approach 37 Double Covalent Bond two atoms sharing two pairs of electrons 4 electrons O O O

O O O Tro, Chemistry: A Molecular Approach 38 Triple Covalent Bond two atoms sharing 3 pairs of electrons 6 electrons N N N Tro, Chemistry: A Molecular Approach

N N N 39 Covalent Bonding Predictions from Lewis Theory Lewis theory allows us to predict the formulas of molecules Lewis theory predicts that some combinations should be stable, while others should not because the stable combinations result in octets Lewis theory predicts in covalent bonding that the attractions between atoms are directional the shared electrons are most stable between the bonding atoms resulting in molecules rather than an array

Tro, Chemistry: A Molecular Approach 40 Covalent Bonding Model vs. Reality molecular compounds have low melting points and boiling points MP generally < 300C molecular compounds are found in all 3 states at room temperature melting and boiling involve breaking the attractions between the molecules, but not the bonds between the atoms the covalent bonds are strong the attractions between the molecules are generally weak the polarity of the covalent bonds influences the strength of the intermolecular attractions Tro, Chemistry: A Molecular Approach 41 Intermolecular Attractions vs. Bonding Tro, Chemistry: A Molecular Approach

42 Ionic Bonding Model vs. Reality some molecular solids are brittle and hard, but many are soft and waxy the kind and strength of the intermolecular attractions varies based on many factors the covalent bonds are not broken, however, the polarity of the bonds has influence on these attractive forces Tro, Chemistry: A Molecular Approach 43 Ionic Bonding Model vs. Reality molecular compounds do not conduct electricity in the liquid state molecular acids conduct electricity when dissolved in water, but not in the solid state in molecular solids, there are no charged particles around to allow the material to conduct when dissolved in water, molecular acids are ionized,

and have the ability to move through the structure and therefore conduct electricity Tro, Chemistry: A Molecular Approach 44 Bond Polarity covalent bonding between unlike atoms results in unequal sharing of the electrons one atom pulls the electrons in the bond closer to its side one end of the bond has larger electron density than the other the result is a polar covalent bond bond polarity the end with the larger electron density gets a partial negative charge the end that is electron deficient gets a partial positive charge Tro, Chemistry: A Molecular Approach 45 HF EN 2.1 H

F EN 4.0 H F Tro, Chemistry: A Molecular Approach 46 Electronegativity measure of the pull an atom has on bonding electrons increases across period (left to right) and decreases down group (top to bottom) fluorine is the most electronegative element francium is the least electronegative element the larger the difference in electronegativity, the more polar the bond negative end toward more electronegative atom

Tro, Chemistry: A Molecular Approach 47 Electronegativity Scale Tro, Chemistry: A Molecular Approach 48 Electronegativity and Bond Polarity If difference in electronegativity between bonded atoms is 0, the bond is pure covalent equal sharing If difference in electronegativity between bonded atoms is 0.1 to 0.4, the bond is nonpolar covalent If difference in electronegativity between bonded atoms 0.5 to 1.9, the bond is polar covalent If difference in electronegativity between bonded atoms larger than or equal to 2.0, the bond is ionic 4% 0 0.4 Percent Ionic Character

51% 2.0 Electronegativity Difference 100% 4.0 49 Bond Polarity ENCl = 3.0 3.0 - 3.0 = 0 Pure Covalent Tro, Chemistry: A Molecular Approach ENCl = 3.0 ENH = 2.1 3.0 2.1 = 0.9 Polar Covalent ENCl = 3.0 ENNa = 1.0 3.0 0.9 = 2.1 Ionic 50 Tro, Chemistry: A Molecular Approach

51 Bond Dipole Moments the dipole moment is a quantitative way of describing the polarity of a bond a dipole is a material with positively and negatively charged ends measured dipole moment, , is a measure of bond polarity it is directly proportional to the size of the partial charges and directly proportional to the distance between them = (q)(r) not Coulombs Law measured in Debyes, D the percent ionic character is the percentage of a bonds measured dipole moment to what it would be if full ions Tro, Chemistry: A Molecular Approach 52 Dipole Moments Tro, Chemistry: A Molecular Approach 53 Water a Polar Molecule

stream of water attracted to a charged glass rod Tro, Chemistry: A Molecular Approach stream of hexane not attracted to a charged glass rod 54 Example 9.3(c) - Determine whether an N-O bond is ionic, covalent, or polar covalent. Determine the electronegativity of each element N = 3.0; O = 3.5 Subtract the electronegativities, large minus small (3.5) - (3.0) = 0.5 If the difference is 2.0 or larger, then the bond is

ionic; otherwise its covalent difference (0.5) is less than 2.0, therefore covalent If the difference is 0.5 to 1.9, then the bond is polar covalent; otherwise its covalent difference (0.5) is 0.5 to 1.9, therefore polar covalent Tro, Chemistry: A Molecular Approach 55 Lewis Structures of Molecules shows pattern of valence electron distribution in the molecule useful for understanding the bonding in many compounds allows us to predict shapes of molecules allows us to predict properties of molecules and how they will interact together Tro, Chemistry: A Molecular Approach 56 Lewis Structures use common bonding patterns C = 4 bonds & 0 lone pairs, N = 3 bonds & 1 lone pair, O= 2 bonds & 2 lone pairs, H and halogen = 1 bond, Be = 2 bonds & 0 lone pairs, B = 3 bonds & 0 lone pairs

often Lewis structures with line bonds have the lone pairs left off their presence is assumed from common bonding patterns structures which result in bonding patterns different from common have formal charges B C Tro, Chemistry: A Molecular Approach N O F 57 Writing Lewis Structures of Molecules HNO3 O 1) Write skeletal structure H always terminal in oxyacid, H outside attached to Os

H O N O make least electronegative atom central N is central 2) Count valence electrons sum the valence electrons for each atom add 1 electron for each charge subtract 1 electron for each + charge Tro, Chemistry: A Molecular Approach N=5 H=1 O3 = 36 = 18 Total = 24 e58 Writing Lewis Structures of Molecules HNO3 3) Attach central atom to the surrounding atoms with

pairs of electrons and subtract from the total O H O N O Tro, Chemistry: A Molecular Approach Electrons Start 24 Used 8 Left 16 59 Writing Lewis Structures of Molecules HNO3 4) Complete octets, outside-in : O : H is already complete with 2 H O N O 1 bond

: and re-count electrons N=5 H=1 O3 = 36 = 18 Total = 24 eTro, Chemistry: A Molecular Approach Electrons Electrons Start 24 Start 16 Used 8 Used 16 Left 16 Left 0 60 Writing Lewis Structures of Molecules HNO3 5) If all octets complete, give extra electrons to central atom. elements with d orbitals can have more than 8 electrons

Period 3 and below 6) If central atom does not have octet, bring in electrons from outside atoms to share follow common bonding patterns if possible : O | H O N : O : Tro, Chemistry: A Molecular Approach 61

Practice - Lewis Structures CO2 H3PO4 SeOF2 SO3-2 NO2-1 P2H4 Tro, Chemistry: A Molecular Approach 62 Practice - Lewis Structures CO2 H3PO4 16 e- : : :O::C::O:

SeOF2 26 e- F NO2-1 18 e- O 32 e O Se F N Tro, Chemistry: A Molecular Approach

O - O H SO3-2 26 e- O P2H4 14 e- O H

P O O S H H P P O H H

O H 63 Formal Charge during bonding, atoms may wind up with more or less electrons in order to fulfill octets - this results in atoms having a formal charge FC = valence e- - nonbonding e- - bonding eleft O FC = 6 - 4 - (4) = 0 O S O S FC = 6 - 2 - (6) = +1 right O FC = 6 - 6 - (2) = -1 sum of all the formal charges in a molecule = 0 in an ion, total equals the charge Tro, Chemistry: A Molecular Approach 64 Writing Lewis Formulas of Molecules (contd) 7) Assign formal charges to the atoms

a) formal charge = valence e- - lone pair e- - bonding eb) follow the common bonding patterns O 0 S +1 O -1

H H O | C || C | H Tro, Chemistry: A Molecular Approach O H all 0

65 Common Bonding Patterns B B - C N O + C + N + O - - -

C Tro, Chemistry: A Molecular Approach N O F F F + - 66 Practice - Assign Formal Charges CO2 H3PO4 O H

SeOF2 F NO2-1 O O SO3-2 O Se F

N Tro, Chemistry: A Molecular Approach O O P2H4 H P O O S H H

P P O H H O H 67 Practice - Assign Formal Charges CO2 H3PO4 P = +1 H rest 0 all 0

SeOF2 Se = +1 -1 O O P O O H F NO2-1 O

-1 O Se F N Tro, Chemistry: A Molecular Approach -1 O -1 O SO3-2 S = +1 -1 O

S P2H4 all 0 H H H P P H -1 O H 68

Resonance when there is more than one Lewis structure for a molecule that differ only in the position of the electrons, they are called resonance structures the actual molecule is a combination of the resonance forms a resonance hybrid it does not resonate between the two forms, though we often draw it that way look for multiple bonds or lone pairs O S O Tro, Chemistry: A Molecular Approach O S O 69

Resonance Tro, Chemistry: A Molecular Approach 70 Ozone Layer Tro, Chemistry: A Molecular Approach 71 Rules of Resonance Structures Resonance structures must have the same connectivity only electron positions can change Resonance structures must have the same number of electrons Second row elements have a maximum of 8 electrons bonding and nonbonding third row can have expanded octet

Formal charges must total same Better structures have fewer formal charges Better structures have smaller formal charges Better structures have formal charge on more electronegative atom Tro, Chemistry: A Molecular Approach 72 Drawing Resonance Structures 1. draw first Lewis structure that maximizes octets 2. assign formal charges 3. move electron pairs from atoms with (-) formal charge toward atoms with (+) formal charge 4. if (+) fc atom 2nd row, only move in electrons if you can move out electron pairs from multiple bond 5. if (+) fc atom 3rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet. Tro, Chemistry: A Molecular Approach -1

O O O -1 N +1 -1 O +1 N O -1

O 73 Exceptions to the Octet Rule expanded octets elements with empty d orbitals can have more than 8 electrons odd number electron species e.g., NO will have 1 unpaired electron free-radical very reactive incomplete octets B, Al Tro, Chemistry: A Molecular Approach 74 Drawing Resonance Structures 1. draw first Lewis structure that maximizes octets 2. assign formal charges 3. move electron pairs from atoms with (-) formal charge toward atoms with (+) formal charge 4. if (+) fc atom 2nd row, only move

in electrons if you can move out electron pairs from multiple bond 5. if (+) fc atom 3rd row or below, keep bringing in electron pairs to reduce the formal charge, even if get expanded octet. Tro, Chemistry: A Molecular Approach H H O O -1 O +2 S O

-1 0 O S 0 O 0 O O H H 75 Practice - Identify Structures with Better or Equal Resonance Forms and Draw Them

CO2 H3PO4 P = +1 H all 0 SeOF2 Se = +1 O -1 O P O O H F

NO2-1 O -1 O Se F N Tro, Chemistry: A Molecular Approach -1 O -1 O

SO3-2 S = +1 -1 O S P2H4 all 0 H H H P P H

-1 O H 76 Practice - Identify Structures with Better or Equal Resonance Forms and Draw Them CO2 H3PO4 -1 O none O P +1 O

H SeOF2 O -1 F Se F +1 all 0 F N -1 O

H O O Se F O S O O

O P2H4 -1 O Tro, Chemistry: A Molecular Approach H SO3-2 NO2-1 O O H N

O H O S O S H H P P O

O O O P O O all 0 O H

O S H O S=0 in all res. forms H none 77 Bond Energies chemical reactions involve breaking bonds in reactant molecules and making new bond to create the products the Hreaction can be calculated by comparing the cost of breaking old bonds to the profit from making new bonds the amount of energy it takes to break one mole of a bond in a compound is called the bond energy

in the gas state homolytically each atom gets bonding electrons Tro, Chemistry: A Molecular Approach 78 Trends in Bond Energies the more electrons two atoms share, the stronger the covalent bond CC (837 kJ) > C=C (611 kJ) > CC (347 kJ) CN (891 kJ) > C=N (615 kJ) > CN (305 kJ) the shorter the covalent bond, the stronger the bond BrF (237 kJ) > BrCl (218 kJ) > BrBr (193 kJ) bonds get weaker down the column Tro, Chemistry: A Molecular Approach 79 Using Bond Energies to Estimate Hrxn the actual bond energy depends on the surrounding atoms and other factors we often use average bond energies to estimate the Hrxn works best when all reactants and products in gas state

bond breaking is endothermic, H(breaking) = + bond making is exothermic, H(making) = Hrxn = (H(bonds broken)) + (H(bonds formed)) Tro, Chemistry: A Molecular Approach 80 81 Estimate the Enthalpy of the Following Reaction H H + O O H O O H

82 Estimate the Enthalpy of the Following Reaction H2(g) + O2(g) H2O2(g) reaction involves breaking 1mol H-H and 1 mol O=O and making 2 mol H-O and 1 mol O-O bonds broken (energy cost) (+436 kJ) + (+498 kJ) = +934 kJ bonds made (energy release) 2(464 kJ) + (142 kJ) = -1070 Hrxn = (+934 kJ) + (-1070. kJ) = -136 kJ (Appendix Hf = -136.3 kJ/mol) Tro, Chemistry: A Molecular Approach 83 Bond Lengths the distance between the nuclei of bonded atoms is called the bond length because the actual bond length depends on the other atoms around the bond we often use the average bond length averaged for similar bonds from many compounds Tro, Chemistry: A Molecular Approach

84 Trends in Bond Lengths the more electrons two atoms share, the shorter the covalent bond CC (120 pm) < C=C (134 pm) < CC (154 pm) CN (116 pm) < C=N (128 pm) < CN (147 pm) decreases from left to right across period CC (154 pm) > CN (147 pm) > CO (143 pm) increases down the column FF (144 pm) > ClCl (198 pm) > BrBr (228 pm) in general, as bonds get longer, they also get weaker Tro, Chemistry: A Molecular Approach 85 Bond Lengths Tro, Chemistry: A Molecular Approach 86 Metallic Bonds low ionization energy of metals allows them to

lose electrons easily the simplest theory of metallic bonding involves the metals atoms releasing their valence electrons to be shared by all to atoms/ions in the metal an organization of metal cation islands in a sea of electrons electrons delocalized throughout the metal structure bonding results from attraction of cation for the delocalized electrons Tro, Chemistry: A Molecular Approach 87 Metallic Bonding Tro, Chemistry: A Molecular Approach 88 Metallic Bonding Model vs. Reality metallic solids conduct electricity because the free electrons are mobile, it allows the electrons to move through the metallic crystal and conduct electricity

as temperature increases, electrical conductivity decreases heating causes the metal ions to vibrate faster, making it harder for electrons to make their way through the crystal Tro, Chemistry: A Molecular Approach 89 Metallic Bonding Model vs. Reality metallic solids conduct heat the movement of the small, light electrons through the solid can transfer kinetic energy quicker than larger particles metallic solids reflect light the mobile electrons on the surface absorb the outside light and then emit it at the same frequency Tro, Chemistry: A Molecular Approach 90 Metallic Bonding Model vs. Reality metallic solids are malleable and ductile because the free electrons are mobile, the direction of the attractive force between the

metal cation and free electrons is adjustable this allows the position of the metal cation islands to move around in the sea of electrons without breaking the attractions and the crystal structure Tro, Chemistry: A Molecular Approach 91 Metallic Bonding Model vs. Reality metals generally have high melting points and boiling points all but Hg are solids at room temperature the attractions of the metal cations for the free electrons is strong and hard to overcome melting points generally increase to right across period the charge on the metal cation increases across the period, causing stronger attractions melting points generally decrease down column the cations get larger down the column, resulting in a larger distance from the nucleus to the free electrons

Tro, Chemistry: A Molecular Approach 92

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