Chapter 8 -Chapter 9

Chapter 8 -Chapter 9 - General Chemistry I Fall 2007 Joann...

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Unformatted text preview: General Chemistry I Fall 2007 Joann S. Monko Chemistry 9th ed. Raymond Chang Ionization Energy Energy required to remove an e- from an atom in the gas phase. 2 Mg (g) + 738 kJ Mg+ (g) + eMg+ (g) + 1451 kJ Mg2+ (g) + eMg has 12 p+ & only 11 e-'s. + IE + 7733 Mg. Mg (g) for Mg > kJ Mg (g) + eKotz &Treichel Chemistry & Chemical Reactivity 5th ed. Energy cost + too high NO Mg 1st Ionization energy (kJ/mol) 2500 He 3 Ne Trends in Ionization Energy Ar Kr 2000 1500 1000 500 0 H 1 Li 3 5 7 9 Na 11 13 15 17 19 K 21 23 25 27 29 31 33 35 Atomic Number IE increases across a period cuz' Z* increases. IE decreases down a group cuz' size increases. Metals lose electrons more easily than nonmetals. Nonmetals lose electrons with difficulty. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 4 Lithium Sodium Periodic Trends Reactivity of Alkali Metals with Water reactivity - a group Potassium Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 5 Clicker Question Arrange the following in order of increasing first ionization energy Na, Cl, Al, S, Cs 1. Cs < Cl < Al < S < Na 2. Cs < Na < Al < S < Cl 3. Al < Na < Cs < S < Cl 4. Na < Cs < Al < Cl < S Electron Affinity 6 the energy involved when an atom gains an eto form an anion. X (g) + e- ---> X (g) E.A. = - H O atom [He] - + electron O - ion [He] E is EXOthermic cuz O has an affinity for an e-. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. EA = + 141 kJ Electron Affinity of Nitrogen N atom [He] 7 E is zero for N due + electron N- ion [He] EA = 0 kJ Atom EA F +328 kJ to electron-electron Cl +349 kJ repulsions. Br +325 kJ I +295 kJ Trends: EA increases across a period ( - becomes more positive). EA decreases down a group ( - becomes less positive). Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 8 Kotz &Treichel Chemistry & Chemical Reactivity Chang 5th ed. Chemistry, 8th ed. 9 2 Forms of Chemical Bonds Ionic -- complete transfer of 1 or more e-'s from one atom to another Covalent -- some valence e-'s shared between atoms Most bonds are somewhere in between. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Forms of Chemical Bonds Ionic Bonding: complete e- transfer from an element of low IE (metal) to an element of high affinity for e-'s (nonmetal) 2 Na (s) + Cl2 (g) 2 Na+ + 2 Cl metals (Grps 1A & 2A & transition metals) + nonmetals (O & halogens) = ionic bonds 10 Covalent Bonding 2 nuclei are attracted to the same e-'s -a bond forms from Electron sharing 11 HA + HB HA HB Bond is a balance of attractive and repulsive forces. Chemical Bonding Objectives are to understand: 1. valence e- distribution in molecules and ions. 2. molecular structures 3. bond properties & effect on molecular properties. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Valence Electrons Outer shell e-'s - used in bonding. Group 1 2 3 4 5 6 7 e- Configuration ns1 ns2 ns2np1 ns2np2 ns2np3 ns2np4 ns2np5 Valence e1 2 3 4 5 6 7 12 e- Distribution in Molecules 13 Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS. G. N. Lewis 1875-1946 H Cl shared or bond pair lone pair (LP) LEWIS ELECTRON DOT structure Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 14 Chang Chemistry, 8th ed. Bond Formation 15 A bond can result from a "head-to-head" overlap of atomic orbitals on neighboring atoms. H + Cl H Cl Overlap of H (1s) and Cl (3p) Note: each atom has a single, unpaired electron. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Valence Electrons # of valence e-'s of a main group atom = Group # 16 Electrons are divided between core & valence e-'s B 1s2 2s2 2p1 Core = [He] valence = 2s2 2p1 Br [Ar] 3d10 4s2 4p5 Core = [Ar] 3d valence = 4s2 4p5 10 Octet Rule the tendency of molecules/atoms to have structures with 8 e-'s surrounding each atom. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 17 Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Central atom = lowest affinity for e-'s. 1. Count valence electrons: N = 1x5=5 H = 3x1=3 H N H Total = 8 e-'s / 4 pairs H 3. Form a single bond between the central atom & each surrounding atom. 4. Remaining e-'s form LONE PAIRS to complete octet. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. H N H H 3 bond pairs 1 lone pair Sulfite ion, SO32- 18 Step 1. Central atom = S Step 2. Count valence electrons S = 1 x 6 = 6 O O = 3 x 6 = 18 Negative charge = 2 O S O TOTAL = 26 e- or 13 pairs Step 3. Form bonds 10 e-'s pairs are left. Step 4. Remaining pairs become lone pairs, first on outside atoms and then on central atom. Each atom is surrounded by an octet of e-'s. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. O S O O C a rb o n Dio xid e , C O 2 1. 2. 3. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 19 Central atom = _______ Valence electrons = __ or __ pairs Form bonds. O C O This leaves 6 pairs. 4. Place lone pairs on outer atoms. 1. C needs an octet, form DOUBLE BONDS between C & O. O C O O C O O C O Second bonding pair forms a pi ( ) bond. 20 H2CO Double & Triple bonds ~ C, N, P, O, & S SO3 C2F4 O C O Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. S ulfur Dio xid e , S O 2 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 21 1. Central atom = S O S 2. Valence electrons = 18 or 9 pairs 1. Form double bond -S needs an octet -- but... two ways of doing this. O bring in left pair O S O O OR bring in right pair S O O S O RESONANCE STRUCTURES - true structure = HYBRID of the two 22 1. Number of valence e-'s = 24 e2. Draw sigma bonds. 3. Place remaining epairs. O 2 2 H N H C N H H O C H N H N H H 4. Complete C's octet double bond. O H N H C N H H Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 23 Clicker Question What is the correct structure for chloric acid, HClO3? Violations of the Octet Rule Usually occurs with B & elements of higher periods. 24 BF3 SF4 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Boron Trifluoride Central atom = _____________ Valence electrons = __________ or electron pairs = __________ Assemble dot structure 25 F B F The B atom shares in only 6 e-'s (or 3 pairs). B atom is electron deficient. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. F Sulfur Tetrafluoride, SF Central atom = Valence electrons = ___ or ___ pairs. Form sigma bonds and distribute electron pairs. 26 4 F S F F F 5 pairs around S atom. Common - outside the 2nd period. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 27 Clicker Question Write the Lewis structure for XeF4. There are ___ around Xe. a. 4 single bonds b. 4 single bonds & 1 lone e- pair c. 4 single bonds & 2 lone e- pairs d. 2 single bonds, 2 double bonds & 1 lone e- pair Formal Atom Charges 28 Atoms in molecules often bear a charge (+ or -). The predominant resonance structure: charges are as close to 0 as possible. Formal charge = Group # (# of Bonding e-'s) - (# of LP e-'s) Carbon Dioxide CO2 6 (1 / 2)(4) 4 = 0 O C O Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 4 (1 / 2 ) (8 ) 0 = 0 Thiocyanate Ion, SCN- 29 6 - (1/2)(2) - 6 = -1 5 - (1/2)(6) - 2 = 0 S C N 4 - (1/2)(8) - 0 = 0 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 30 Thiocyanate Ion, SCN S C N S S N C N C Which is the most important resonance form? Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 31 Boron Trifluoride, BF3 F B F B +1 -1 F F F F What if we form a B--F double bond to satisfy the B atom octet? Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Properties What is the effect of bonding & structure on molecular properties? 32 *Bond Order *Length *Strength *Polarity Buckyball in HIV-protease Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Order # of bonds between a pair of atoms 33 Double bond Single bond Acrylonitrile Triple bond Bond Order Fractional bond orders - molecules with resonance structures. Consider NO2 34 N N O O O O The N--O bond order = 1.5 Total # of e - pairs used for a type of bond Bond order = Total # of bonds of that type 3 e - pairs in N -- O bonds Bond order = 2 N -- O bonds Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 35 Clicker Question What is the bond order for the carbonate ion, CO3-2? a. 1.25 b. 1.33 c. 1.5 d. 1.75 Bo nd O rd e r -related to two important bond properties: (a) bond strength (b) bond length Bond length the distance between the nuclei of two bonded atoms. 110 pm 36 414 kJ 123 pm 745 kJ C-O single bond 143 pm & 358 kJ Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Length 37 Depends on bond order H--F H--Cl H--I Depends on size of atoms Bond distances Angstrom units 1 = 10-2 pm. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Strength 38 --measured by the energy req'd to break a bond. BOND H--H C--C C=C CC NN STRENGTH (kJ/mol) 436 346 602 835 945 The GREATER the number of bonds (bond order) the HIGHER the bond strength & the SHORTER the bond. Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Using Bond Energies 39 H--H + Cl--Cl ----> 2 H--Cl Net energy = Hrxn = = energy required to break bonds -- energy evolved when bonds are made Hrxn = D (bonds broken) - D (bonds formed) H--H = 436 kJ/mol Cl--Cl = 242 kJ/mol H--Cl = 432 kJ/mol Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. 40 Using Bond Energies 2 H--O--O--H O=O + 2 H--O--H Is the reaction exo- or endothermic? Which is larger: A) energy req'd to break bonds B) or energy evolved on making bonds? H-O = 463 kJ/mol O-O = 146 kJ/mol O=O = 498 kJ/mol 2144 kJ/mol 2350 kJ/mol Hrxn = -206 kJ/mol exothermic Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Polarity + - 41 HCl is POLAR -it has a positive end & a negative end. H Cl Cl has a greater share in bonding e-'s than does H. Cl - slight negative charge (- ) H - slight positive charge (+ ) H is + (red) and Cl is - (yellow) Calc'd charge is + or - 0.20 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Electronegativity, - a measure of the ability of an atom in a molecule to attract electrons to itself. 42 TWO unshared Nobel prizes: Peace & Chemistry Chemistry areas: bonding, electronegativity, protein structure Concept proposed by Linus Pauling 1901-1994 Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. + - H Cl Bond Polarity 43 The H--Cl bond energy is GREATER than expected for a "pure" covalent bond. ENERGY 339 kJ/mol calc'd 432 kJ/mol measured BOND "pure" bond real bond Difference = 92 kJ. proportional to the difference in ELEC TRONEGATI VI TY, . Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Electronegativity 44 Chemistry 8th ed. Raymond Chang 45 4 3.5 3 O N C H F Cl S Si P Electronegativity 2.5 2 1.5 1 0.5 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 F = maximum . Lowest Atom = center atom Relative values of BOND POLARITY (& point of attack on a molecule) Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. Bond Polarity Which bond is more polar (or DIPOLAR)? O--H 3.5 - 2.1 1.4 0.5 O--F 3.5 - 4.0 46 OH is more polar than OF - O + H + O - F polarity is "reversed." Kotz &Treichel Chemistry & Chemical Reactivity 5th ed. ...
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