Chapter09_LEC

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Chapter 9Chemical Bonding I:Lewis Theory : 

Chapter 9Chemical Bonding I:Lewis Theory 2008, Prentice Hall Chemistry: A Molecular Approach, 1st Ed.Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

Bonding : 

Tro, Chemistry: A Molecular Approach 2 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

Types of Bonds : 

Tro, Chemistry: A Molecular Approach 3 Types of Bonds

Types of Bonding : 

4 Types of Bonding

Ionic Bonds : 

Tro, Chemistry: A Molecular Approach 5 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 [chapter 8] nonmetals have high electron affinities, relatively good to add electrons to [chapter 8]

Covalent Bonds : 

Tro, Chemistry: A Molecular Approach 6 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

Determining the Number of Valence Electrons in an Atom : 

Tro, Chemistry: A Molecular Approach 7 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?

Lewis Symbols of Atoms : 

Tro, Chemistry: A Molecular Approach 8 Lewis Symbols of Atoms aka electron dot symbols use dots around the symbol to represent valence electrons

Lewis Symbols of Ions : 

Tro, Chemistry: A Molecular Approach 9 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

What We Know : 

Tro, Chemistry: A Molecular Approach 10 What We Know the noble gases are the least reactive group of elements the alkali metals are the most reactive metals and their atoms almost always lose 1 electron when they react the halogens are the most reactive group of nonmetals and in a lot of reactions they gain 1 electron

Stable Electron ArrangementsAnd Ion Charge : 

Tro, Chemistry: A Molecular Approach 11 Stable Electron ArrangementsAnd 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

Octet Rule : 

Tro, Chemistry: A Molecular Approach 12 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

Lewis Theory : 

Tro, Chemistry: A Molecular Approach 13 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

Lewis Theory and Ionic Bonding : 

Tro, Chemistry: A Molecular Approach 14 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 +

Covalent Bonding:Bonding and Lone Pair Electrons : 

Tro, Chemistry: A Molecular Approach 15 Covalent Bonding:Bonding and Lone Pair Electrons Covalent bonding results when atoms share pairs of electrons to achieve an “octet” Electrons that are shared by atoms are called bonding pairs Electrons that are not shared by atoms but belong to a particular atom are called lone pairs aka nonbonding pairs

Single Covalent Bonds : 

Tro, Chemistry: A Molecular Approach 16 Single Covalent Bonds two atoms share a pair of electrons 2 electrons one atom may have more than one single bond

Double Covalent Bond : 

Tro, Chemistry: A Molecular Approach 17 Double Covalent Bond two atoms sharing two pairs of electrons 4 electrons

Triple Covalent Bond : 

Tro, Chemistry: A Molecular Approach 18 Triple Covalent Bond two atoms sharing 3 pairs of electrons 6 electrons

Bond Polarity : 

Tro, Chemistry: A Molecular Approach 19 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

HF : 

Tro, Chemistry: A Molecular Approach 20 HF d+ d- EN 2.1 EN 4.0

Electronegativity : 

Tro, Chemistry: A Molecular Approach 21 Electronegativity measure of the pull an atom has on bonding electrons increases across period (left to right) and decreases down group (top to bottom) the larger the difference in electronegativity, the more polar the bond negative end toward more electronegative atom

Electronegativity Scale : 

Tro, Chemistry: A Molecular Approach 22 Electronegativity Scale

Electronegativity and Bond Polarity : 

23 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 “100%”

Bond Polarity : 

Tro, Chemistry: A Molecular Approach 24 Bond Polarity

Bond Dipole Moments : 

Tro, Chemistry: A Molecular Approach 25 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, m, 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

Dipole Moments : 

Tro, Chemistry: A Molecular Approach 26 Dipole Moments

Water – a Polar Molecule : 

Tro, Chemistry: A Molecular Approach 27 Water – a Polar Molecule stream of water attracted to a charged glass rod stream of hexane not attracted to a charged glass rod

Lewis Structures of Molecules : 

Tro, Chemistry: A Molecular Approach 28 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

Lewis Structures : 

Tro, Chemistry: A Molecular Approach 29 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

Writing Lewis Structures of Molecules HNO3 : 

Tro, Chemistry: A Molecular Approach 30 Writing Lewis Structures of Molecules HNO3 Write skeletal structure H always terminal in oxyacid, H outside attached to O’s make least electronegative atom central N is central Count valence electrons sum the valence electrons for each atom add 1 electron for each - charge subtract 1 electron for each + charge N = 5 H = 1 O3 = 3·6 = 18 Total = 24 e-

Writing Lewis Structures of Molecules HNO3 : 

Tro, Chemistry: A Molecular Approach 31 Writing Lewis Structures of Molecules HNO3 Attach central atom to the surrounding atoms with pairs of electrons and subtract from the total Electrons Start 24 Used 8 Left 16

Writing Lewis Structures of Molecules HNO3 : 

Tro, Chemistry: A Molecular Approach 32 Writing Lewis Structures of Molecules HNO3 Complete octets, outside-in H is already complete with 2 1 bond and re-count electrons N = 5 H = 1 O3 = 3·6 = 18 Total = 24 e- Electrons Start 24 Used 8 Left 16 Electrons Start 16 Used 16 Left 0

Writing Lewis Structures of Molecules HNO3 : 

Tro, Chemistry: A Molecular Approach 33 Writing Lewis Structures of Molecules HNO3 If all octets complete, give extra electrons to central atom. elements with d orbitals can have more than 8 electrons Period 3 and below If central atom does not have octet, bring in electrons from outside atoms to share follow common bonding patterns if possible

Practice - Lewis Structures : 

Tro, Chemistry: A Molecular Approach 34 Practice - Lewis Structures CO2 SeOF2 NO2-1 H3PO4 SO3-2 P2H4

Formal Charge : 

Tro, Chemistry: A Molecular Approach 35 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- - ½ bonding e- left O FC = 6 - 4 - ½ (4) = 0 S FC = 6 - 2 - ½ (6) = +1 right O FC = 6 - 6 - ½ (2) = -1 sum of all the formal charges in a molecule = 0 in an ion, total equals the charge

Writing Lewis Formulas of Molecules (cont’d) : 

Tro, Chemistry: A Molecular Approach 36 Writing Lewis Formulas of Molecules (cont’d) Assign formal charges to the atoms formal charge = valence e- - lone pair e- - ½ bonding e- follow the common bonding patterns 0 +1 -1 all 0

Common Bonding Patterns : 

Tro, Chemistry: A Molecular Approach 37 Common Bonding Patterns -

Practice - Assign Formal Charges : 

Tro, Chemistry: A Molecular Approach 38 Practice - Assign Formal Charges CO2 SeOF2 NO2-1 H3PO4 SO3-2 P2H4 all 0 -1 P = +1 rest 0 S = +1 Se = +1 -1 -1 all 0 -1 -1 -1

Resonance : 

Tro, Chemistry: A Molecular Approach 39 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

Resonance : 

Tro, Chemistry: A Molecular Approach 40 Resonance

Rules of Resonance Structures : 

Tro, Chemistry: A Molecular Approach 41 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

Exceptions to the Octet Rule : 

Tro, Chemistry: A Molecular Approach 42 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

Bond Energies : 

Tro, Chemistry: A Molecular Approach 43 Bond Energies chemical reactions involve breaking bonds in reactant molecules and making new bond to create the products the DH°reaction 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 ½ bonding electrons

Trends in Bond Energies : 

Tro, Chemistry: A Molecular Approach 44 Trends in Bond Energies the more electrons two atoms share, the stronger the covalent bond C=C (837 kJ) > C=C (611 kJ) > C-C (347 kJ) C=N (891 kJ) > C=N (615 kJ) > C-N (305 kJ) the shorter the covalent bond, the stronger the bond Br-F (237 kJ) > Br-Cl (218 kJ) > Br-Br (193 kJ) bonds get weaker down the column

Using Bond Energies to Estimate DH°rxn : 

Tro, Chemistry: A Molecular Approach 45 Using Bond Energies to Estimate DH°rxn the actual bond energy depends on the surrounding atoms and other factors we often use average bond energies to estimate the DHrxn works best when all reactants and products in gas state bond breaking is endothermic, DH(breaking) = + bond making is exothermic, DH(making) = - DHrxn = ? (DH(bonds broken)) + ? (DH(bonds formed))

Slide 46: 

46

Estimate the Enthalpy of the Following Reaction : 

47 Estimate the Enthalpy of the Following Reaction

Estimate the Enthalpy of the Following Reaction : 

Tro, Chemistry: A Molecular Approach 48 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 DHrxn = (+934 kJ) + (-1070. kJ) = -136 kJ (Appendix DH°f = -136.3 kJ/mol)

Bond Lengths : 

Tro, Chemistry: A Molecular Approach 49 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

Trends in Bond Lengths : 

Tro, Chemistry: A Molecular Approach 50 Trends in Bond Lengths the more electrons two atoms share, the shorter the covalent bond C=C (120 pm) < C=C (134 pm) < C-C (154 pm) C=N (116 pm) < C=N (128 pm) < C-N (147 pm) decreases from left to right across period C-C (154 pm) > C-N (147 pm) > C-O (143 pm) increases down the column F-F (144 pm) > Cl-Cl (198 pm) > Br-Br (228 pm) in general, as bonds get longer, they also get weaker

Bond Lengths : 

Tro, Chemistry: A Molecular Approach 51 Bond Lengths