CHEM 115 Course Review, Second Half

Transcription

1 CEM 115 Curse Review, Secnd alf Lecture Slides May 10, 2007 Prf. Sevian Agenda Thermchemistry (ch. 5) Electrnic structure f atms (ch. 6) Peridic prperties (ch. 7) Chemical bnding basics (ch. 8) Mlecular gemetry and bnding theries (ch. 9) T surrundings Exthermic q = transfer invlves System heat flw may be Thermchemistry eat, q rm surrundings Endthermic q = + Adapted frm McMurray & ay, 2001 measured using at cnstant vlume Energy change ΔE = q V ess s law Thermchemistry Key Cncepts Calrimetry at cnstant pressure Enthalpy change Δ = q P may be calculated using Standard heats f frmatin, Δ f º Bnd dissciatin energies Mre t cme: entrpy (S), entrpy change (ΔS), free energy change (ΔG), spntaneity, etc. Map f chapter 5 Energy in chemistry Kinetic and ptential energy changes as heat energy is added t a pure substance irst law f thermdynamics Transfer f energy and the Law f Cnservatin f Energy Endthermic vs. exthermic changes Enthalpy Measuring heat energy (enthalpy) changes (Δ) in the labratry eat energy and heat capacity f a material Calrimetry technique Using labratry measurements t calculate Δ fr reactins we can t measure in the lab Sevian 1

2 Energy cntent r Internal energy, E Sum f the kinetic and ptential energies f all the particles in the system Can change in nly tw ways: When heat (q) is transferred t the system (frm the surrundings) r vice versa When wrk (w) is dne n the system (by the surrundings) r vice versa r the systems belw, describe what is happening t ΔE = q + w Energy Energy can be cnverted frm ne frm t anther Energy transfer ccurs in such a way that the ttal energy f the universe remains cnstant (irst Law f Thermdynamics) Energy transfer ccurs in such a way that matter and energy becme mre dispersed, that is, mre spread ut (Secnd Law f Thermdynamics) Let s cnsider hw energy transfer happens when we are cncerned nly with thermal energy (a.k.a., heat) eat Transfer Thermal energy Temperature A measurement made using an instrument called a thermmeter w it wrks: Transfer f thermal energy frm ne lcatin t anther When thermal energy is transferred, it always transfers frm a lcatin with mre thermal energy t a lcatin with less thermal energy. Energy cntinues t transfer until thermal equilibrium is established. (Energy gets mre spread ut.) Simply: ht t cld. T understand mre abut thermdynamic equilibrium, try all 7 thught experiments at S, what des temperature measure? rm Chemistry & Chemical Reactivity 5 th editin by Ktz / Treichel. C Reprinted with permissin f Brks/Cle, a divisin f Thmsn Learning: ax Sevian 2

3 Relative vs. Abslute Temperature Scales Relative temperature (linear) scale measures temperature f an bject relative t tw pints Cldest and warmest temperatures at which humans can typically survive ahrenheit scale is relative reezing and biling pints f water Celsius scale is relative Abslute temperature (linear) scale measures abslute mtin f particles Kelvin scale is abslute Abslute zer temperature is a pint f reference fr disrder: there is n disrder at zer (Third Law f Thermdynamics) Cnvenience: the size f 1 ºC is equal t the size f 1 K What happens t a SYSTEM when heat transfer ccurs? As cffee eventually cls, heat energy is transferred t the air arund the therms and the cunter beneath it. System: Particles in the cffee slw dwn their mtin. Surrundings: Particles in the air, and particles in the cunter, speed up their mtin. EXTERMIC CANGE Energy f system befre the change q sys < 0 Energy f system after the change System eat energy leaves the system Cnservatin f Energy Endthermic vs. Exthermic When heat energy enters r leaves matter, energy is cnserved. This means energy has t cme frm smewhere, and it has t g smewhere. It can be accunted fr. Particle level: energy can g int r cme ut f the system, thereby increasing r decreasing the energy in the particles Kinetic energy: mtin f particles (translatin, vibratin, rtatin) in slid, liquid and gas states Ptential energy: electrn states in atms r within bnds Endthermic Example: ice melting eat enters system System gains energy q sys > 0 Exthermic Example: fire burning eat exits system System lses energy q sys < 0 rm Chemistry & Chemical Reactivity 5 th editin by Ktz / Treichel. C Reprinted with permissin f Brks/Cle, a divisin f Thmsn Learning: ax Sevian 3

4 rm Chemistry & Chemical Reactivity 5 th editin by Ktz / Treichel. C Reprinted with permissin f Brks/Cle, a divisin f Thmsn Learning: ax Reactants Enthalpy Change Enthalpy axis Δ rxn < ( g ) + ( g ) 2 2 1) Stichimetry 2) What if yu duble the amunts f reactants? 3) What if yu reverse the rxn? 2 ( g) Prducts BKS_ _3446,00.html Tw ways t write the reactin s that it includes enthalpy infrmatin 1 2 ( g) + 2 ( g) 2 ( g) kj ( g) + 2 ( g) 2 ( g) Δ = kj 2 Imprtant things t knw abut state functins like Δ and ΔE 1. The delta (Δ) always means change frm initial t final, calculated as final minus initial. Δ rxn = (prducts) (reactants) Therefre, when Δ is psitive, it means the prducts were higher than the reactants 2. Reversing a reactin means changing the sign f the state functin, since prducts and reactants are switched. 3. Δ rxn can be given in tw ways: as kj r as kj/ml. If it is given in kj, then it depends n the amunt f reactant. 4. The physical states f the chemicals in the reactin matter. 2 2 (g) + 2 (g) 2 2 (l) Δ rxn = kj 2 2 (g) + 2 (g) 2 2 (g) Δ rxn = kj 5. State functins dn t depend n the path*** A ht metal blck placed in cld water Measuring heat transferred frm a system ht metal bject Aluminum metal blck f 5.00 g initially at 90.0 ºC change takes place insulatin Given infrmatin Initial temperature f Al blck = 90.00ºC Mass f Al blck = 5.00 g Mass f water = g Temperature f water befre = 23.00ºC Temperature f bth after = 23.71ºC C water = J/g K Questin: What is the heat capacity f Al? g f 2 Prblem Slving Strategy q = m C ΔT w sys w w w where ΔTw = temp change f 2 q is ppsite f q w Beaker image: cre.ecu.edu/chem/chemlab/ equipment/ebeaker.htm Sevian 4

5 Calrimetry is the same idea Measuring heat transferred frm a system system reactin takes place insulatin Prblem Slving Strategy q = m C ΔT w sys w w g f 2 w where ΔTw = temp change f 2 q is ppsite f q w Given infrmatin Mass f water = g Temperature f water befre = 23.3ºC Temperature f water after = 47.3ºC C water = J/g K The cnfusing part is that nce the change takes place, the system and the water are mixed tgether, and the heat energy gets distributed thrughut the mixture Calrimetry prblems Water is smething we knw a lt f very accurate data abut Measure heat changes that get transferred t water by a (reactin) system Calculate amunt f heat that water received frm r gave t a system If the calrimeter is insulated, then all heat that enters (r leaves) the water must have cme frm (r gne t) the system being studied igure ut things abut the system that yu didn t knw befre Beaker image: cre.ecu.edu/chem/chemlab/ equipment/ebeaker.htm eat transfer measurement and enthalpy f reactin Systems can either lse r gain heat during a change Exthermic: heat flws ut t surrundings Endthermic: heat flws in t system eat changes can be measured using calrimetry Typical calrimetry uses liquid water as the surrundings Liquid water either absrbs heat frm the system (T water increases) r gives heat t the system (T water decreases) If calrimeter is well insulated then q water = q system, and q water = m water C water ΔT water (where C water is slpe f line fr liquid regin) Under typical labratry cnditins (cnstant pressure), heat change is equal t enthalpy change Enthalpy change (Δ) is a state functin (path independent) Because f its path independence, enthalpy change can be calculated by several methds: ess s law Standard heats f frmatin Bnd dissciatin energies Reversing the directin f a reactin 1) rmatin f water 2 (g) + ½ 2 (g) 2 (l) Δ = kj _BKS_ _3446,00.html 2) Electrlysis f water 2 (l) 2 (g) + ½ 2 (g) Δ = kj Sevian 5

6 Mre n ess s Law Cncept is simple, mathematics seems mre cmplicated w is Δ A related t Δ B and Δ C? w is Δ X related t ther enthalpy changes in the diagram? Δ = Δ + Δ A B C Δ X = Δ Z + ( ΔY ) = Δ Z ΔY ess s law says that Δ rxn fr a given reactin is equal t prducts minus reactants f the heats f frmatin fr the chemicals invlved in the reactin Summary f ess s law A reactin and its reverse have equal magnitude, ppsite sign Δ values If A B has Δ = 100 kj, then B A has Δ = 100 kj If yu multiply a reactin by a factr, then yu multiply the Δ by the same factr If A B has Δ = 100 kj, then 2A 2B has Δ = 200 kj When yu add tw reactins, yu add the Δ values If A B has Δ = 100 kj, and C D has Δ = 50 kj, then A + C B + D has Δ = kj = 150 kj What is a heat f frmatin? What is a frmatin reactin? Standard heat (r enthalpy) f frmatin, Δ f, is the enthalpy f reactin assciated with a frmatin reactin A frmatin reactin fr a cmpund is a reactin that prduces ne mle f that cmpund frm the pure elements in their standard states (p=1 atm, T=25ºC) Examples f frmatin reactins and their heats f frmatin: rmatin f sdium bicarbnate, NaC 3 (s): 1 3 Na ( s) + 2 ( g) + C( s) + 2 ( s) NaC3 ( s) Δ = rmatin f ethylene, C 2 4 (g): C( s) + 2 ( g) C ( g) Δ kj f = ml f 7 kj ml Using eats f rmatin t Calculate Enthalpies f Reactin Example prblem: Use standard enthalpies f frmatin t calculate the enthalpy f reactin fr the cmbustin f ethanl, C 2 5 (l). Slutin: Start by writing the reactin C ( l) + 32 ( g) 2 C2 (g) + 3 ( l) Δ =? Methd 1: Write all the frmatin reactins fr any nn-elements at standard state. Then figure ut hw t arrange thse reactins t sum t the verall reactin and d the same t the Δ s. Methd 2: Use the equatin Δ rxn = Standard enthalpies f frmatin Δ [ l ] kj f 2( ) = ml Δ [ g ] kj f C2( ) = ml Δ prducts f Δ [ C ( l) ] kj 2 5 = ml f f reactants Δ f Sevian 6

7 Using bnd enthalpies t predict enthalpy change during a reactin Breaking bnds csts energy (hint fr remembering: think f sticks) When bnds frm, energy is released (bnded atms are mre Matter balance Energy balance Estimate f energy change stable) Sme mlecules that have bnds in them Δ Break these bnds (put energy in) rxn = bnds brken Sme different mlecules that have bnds in them rm new bnds (energy is released) bnds frmed Cmparing Δ f methd with bnd enthalpy methd Use bnd enthalpies t estimate the enthalpy change fr the reactin f hydrgen with ethene. Then calculate the standard enthalpy change using heats f frmatin. 2 (g) + C 2 4 (g) C 2 6 (g) (g) + C C (g) C C (g) Bnds t break (endthermic): + ne kj/ml fur C- 413 kj/ml ne C=C 614 kj/ml + Bnds t frm (exthermic): ne C-C 348 kj/ml six C- 413 kj/ml Energy input = kj Energy released = kj Δ rxn = bnds brken Cmparisn t Δ f methd: Δ rxn = Δ f Δ f prducts reactants Δ rxn bnds frmed = kj = 124 kj = [ Δ ( C ( g) )] [ 1 Δ ( ( g) ) + 1 Δ ( C ( ))] 1 f 2 6 f 2 f 2 4 g = [1(-84.68)] [1(0)+1(+52.30)] kj = kj Prperties f electrmagnetic radiatin (Ch. 6) Electrmagnetic spectrum divided up based n frequency Waves can be described by three interrelated measurements: wavelength, speed and frequency These are related by c = λν Energy f a wave is prprtinal t frequency Sme prperties f light are explained by wave behavir (e.g., diffractin) ther prperties f light are explained by particle behavir (e.g., phtelectric effect) Sevian 7

8 w light waves differ frm each ther wavelength Visible light wavelength Amplitude Nde Ultavilet radiatin rm Chemistry & Chemical Reactivity 5 th editin by Ktz / Treichel. C Reprinted with permissin f Brks/Cle, a divisin f Thmsn Learning: ax Cmpare visible and UV light: Bth are light, s they have the same velcity (speed f light, c = 3.0 x 10 8 m/s) Wavelength λ visible requency ν visible > < λ UV ν UV Calculatin f Light Prperties Red light has λ = 690 nm. What is its frequency? Cnvert wavelength t standard SI units: 9 10 m 690 nm = nm Calculate frequency: c c = λν ν = = λ 7 m s m / s = m r z Spectrscpy Macrscpic bservatins When energy enters atms, atms give ff light at discrete wavelengths (line emissin spectrum) Line emissin is fingerprint f an element (demnstratins in class) Entire peridic table at Particle level explanatin Electrns are s small that their quantum mechanical prperties becme imprtant (eisenberg uncertainty principle) Electrns can reside in varius different quantum mechanical ptential energy states, nly ne f which is the lwest energy grund state r a very nice, shrt summary explanatin with helpful pictures, see Symblic representatin (mathematical mdel) Bhr Thery Emissin Spectrum f ydrgen Bhr first thught t mathematically mdel electrns as in rbit arund nucleus, and when quantizatin pstulate applied, Bhr s mdel crrectly predicts hydrgen spectrum that is experimentally seen Rydberg equatin: 1 ΔE = hcr 2 n where hcr final 1 2 n initial = See equatin 6.5, p. 226 J Sevian 8

9 Emissin Spectrum f ydrgen 1 1 ΔE = hcr 2 2 n final n initial where hcr kj = 1312 ml Using the Rydberg equatin Example: Cmpare the n=3 n=2 transitin with the n=4 n=3 transitin ΔE3 2 = J = J = J per atm Exthermic Using N A t cnvert t per ml: Release f kj/ml ΔE4 3 = J = J = J per atm Exthermic Using N A t cnvert t per ml: Release f 93.3 kj/ml See Where in the electrmagnetic spectrum are these energies? Cnverting energy t wavelength Energy equatin E = hν Wavelength equatin Substituting c c = λν ν = λ c hc E = hν = h = λ λ Slving fr wavelength λ = hc E Example cntinued Where in the electrmagnetic spectrum are these energies? 34 1 hc ( J s)( m s ) λ = = 19 E J 7 1 nm = m = 657 nm 9 RED 10 m 8 hc hc λ = = 19 E J 6 1 nm = m = 1280 nm 9 10 m Infra-RED Sevian 9

10 The Schrdinger equatin and wave functins that bey it ( θ, φ, r) E ψ ( θ, φ r) ψ = where nlm n nlm, ψ nlm are a set f functins that are mathematical slutins in threedimensinal space (radial crdinates θ, φ, r instead f Cartesian crdinates x, y, z) that depend n three quantum numbers n, l, m a is the Bhr radius given by ρ, L and Y are just special functins that depend n the parameters shwn ur quantum numbers Electrns are mathematical wave functins (rbitals) specified by: 1. Principal quantum number, n Can take values 1, 2, 3, 4, Radial distance frm the nucleus (shell #) 2. Azimuthal quantum number, l Can take values up t but nt including n value Shape f rbital (when l=0 has s-shape, when l=1 has p-shape, when l=2 has d-shape, etc.) 3. Magnetic quantum number, m l Can take values ranging frm l up t +l rientatin f the rbital 4. Spin quantum number, m s Can take ne f tw values (+½ r ½), des nt depend n ther quantum numbers Summary f all 4 quantum numbers What rbitals Are l = 0 l = 0 l = 1 l = 1 l = 2 m l can be -2, -1, 0, +1, n = 3 +2 l can be: 0, 1, 2 m l can be 0 m l can be -1, 0, +1 n = 2 l can be: 0, 1 m l can be 0 m l can be -1, 0, +1 Electrn clud pictures Prbability density describing where electrn is lcated Prprtinal t the square f the wave functin with specific quantum numbers (wave functin symblized by Greek letter ψ) Think f a bird at a bird feeder, and a time-lapse pht 1s rbital n = 1 l = 0 m l = 0 l = 0 n = 1 l can be: 0 m l can be 0 within every rbital m S can be +½ r -½ Sevian 10

11 rbital Energies in ydrgen (nly) w QM mdel simplifies t the Bhr mdel Ptential energy n = 4 n = 3 4s 4p 4d 4f 3s 3p 3d n = 2 2s 2p n = 1 1s rbital Energies in Multielectrn Atms Ptential energy n = 4 n = 3 n = 2 3s 2s 3p 2p 3d 4s 4p 4d When there is mre than ne electrn, the slutin t the Schrdinger equatin fr hydrgen is a first apprximatin, but is nt cmpletely crrect. In particular, the energies f different l values are nt the same. 4f The game f QM: What yu need t knw Study pp in the text t learn the rules Knw the difference between s, p, d and f rbitals (these are l values f 0, 1, 2 and 3) Be able t tell which cmbinatins f quantum numbers (n, l, m l ) are allwed and which cmbinatins are nt allwed Given a particular electrn address, determine a set f quantum numbers (n, l, m l ) that crrespnd t it Given an atm with a specific number f electrns, determine the electrnic grund state cnfiguratin n = 1 1s Sevian 11

12 w t find grund state electrn cnfiguratin fr an element 1. Aufbau ( building up/assembly ) principle Add electrns sequentially frm lwest energy rbitals n upward. 2. Pauli exclusin principle Electrns are uniquely specified. If tw electrns have the same first three quantum numbers (same rbital), then their furth quantum number (m l ) must differ. 3. und s rule Electrns that are in the same subshell (same l value) tend t distribute s that they are in different rbitals (different m l values) and have parallel spin (same m s values). Perid 3 cmplete Perid 2 cmplete Perid 1 cmplete Aufbau (Building) Elements 4s 3s 2s 1s Scandium Atmic number 21 4p 3p 2p 3d spdf ntatin 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1 Paramagnetic because it has ne unpaired electrn spdf ntatin written with nble gas cre [Ar] 4s 2 3d 1 Aufbau (Building) Elements Germanium Atmic number 32 Peridic Table Structure Cl Cl = 1s 2 2s 2 2p 6 3s 2 3p 5 Perid 3 cmplete Perid 2 cmplete Perid 1 cmplete 4s 3s 2s 1s 4p 3p 2p 3d Paramagnetic because it has tw unpaired electrns spdf ntatin 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 2 spdf ntatin written with nble gas cre [Ar] 4s 2 3d 10 4p 2 Zr Zr = 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d Sevian 12

13 Inizatin Energies f Elements The energy required t remve the mst weakly bund electrn frm an atm r in. ev Inizatin ptentials Atmic number Trend seen: As atmic number increases, the first inizatin energy f the nble gases d 1st Inizatin Energy 2nd Inizatin Energy Data frm. Sevian et al, Active Chemistry r see Table 7.2, p. 271 f text Nble gases Cmparing Nble Gas Grup Elements Pd Elem Electrn Cnfiguratin 1 e 1s 2 2 Ne 1s 2 2s 2 2p 6 3 Ar 1s 2 2s 2 2p 6 3s 2 3p 6 4 Kr 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5 Xe 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 2 4d 10 5p 6 w des this explain the trend seen? Trend: As atmic number increases, the first inizatin energy f the nble gases decreases. Explanatin: This happens because the number f shells increases as yu g dwn the grup, s the mst lsely bund electrn is getting further away frm the nucleus that is hlding it n the atm. The further away the electrn is frm the nucleus, Example f cre vs. valence: Sdium Atm A neutral sdium atm has 11 prtns and 11 electrns Electrnic cnfiguratin is 1s 2 2s 2 2p 6 3s 1 (Nte: nt drawn t scale!) Bk calls this Z eff CRE Net +1 charge VALENCE SELL Net -1 charge 11 prtns and sme neutrns (charge: +11) All cmplete inner shells f electrns (charge: -10) uter electrns beynd all cmplete inner shells (charge: -1) 3s 1 1s 2 2s 2 2p 6 Image mdified frm Recall Culmb s Law rce f attractin (r repulsin): Increases when magnitudes f charges increase Decreases as distance between charges increases prprtinality cnstant rce f attractin k Q = r Charge n psitive part + 2 Q distance between parts Charge n negative part T reasn using Culmb s law, yu must talk abut the magnitudes f the charges (Q + and Q - ) and the separatin f the charges (r) Sevian 13

14 Inizatin Energies f Elements The energy required t remve the mst weakly bund electrn frm an atm r in. Cre vs. Valence Inizatin ptentials 1st Inizatin Energy 2nd Inizatin Energy +1C, -1V An abbreviated peridic table (shwing nly the s- and p-blcks) Perid 2 elements e +2C, -2V ev Perid 2 elements Atmic number Trend seen: As atmic number increases, the first inizatin energy f the perid 2 elements increases. Li +1C, -1V Be +2C, -2V B +3C, -3V C +4C, -4V N +5C, -5V +6C, -6V +7C, -7V Ne +8C, -8V w des Na this Mg explain the trend Al seen? Si P S Cl Ar +1C, -1V +2C, -2V +3C, -3V +8C, -8V Trend: As atmic number increases, the first inizatin energy f the perid 2 elements increases. Explanatin: K This Ca happens because Ga the Ge cre charge As (ZSe Br Kr eff ) increases as yu g frm left t right acrss a perid. Since we are cmparing the mst lsely bund electrn, which is sitting in the valence shell, and since the valence shell is the same fr all the perid 2 elements, then Z eff increasing means the frce f attractin hlding the electrn increases, resulting in larger inizatin Summary f Inizatin Energy Trends Summary f Atmic Radii Trends Inizatin energy generally: Increases acrss a perid (rw) Decreases dwn a grup (clumn) rm Chemistry & Chemical Reactivity 5 th editin by Ktz / Treichel. C Reprinted with permissin f Brks/Cle, a divisin f Thmsn Learning: ax Atmic radius generally: Increases dwn a grup (clumn) Decreases acrss a perid (rw) Sevian 14

15 The big picture: The pattern f a Culmb s law argument Perids vs. Grups Cmparing tw elements in the same perid: Same number f cmplete shells, s size (radius) f cres is the same Different charges in nucleus, but same number f cre electrns, leads t different cre charge Different numbers f electrns in valence Arguments are usually based n Q + (cre charge) and Q - (valence charge) being different while distance between cre and valence (r) is nearly the same Cmparing tw elements in the same grup: Different number f cmplete shells, s size (radius) f cres is different Cre charges are the same because valence electrns same Arguments are usually based n distance between cre and valence (r) being different while Q + and Q - are the same 1. Usually cmparing ne set f circumstances t a secnd set, t explain why ne measure is larger r smaller than anther Nen atm vs. sdium atm with atmic radius MgCl 2 vs. CaCl 2 with energy required t break the inic bnds 2. r each set f circumstances, determine what the relevant attractin is between a Q - and a Q + Attractin between utermst electrn (-) and effective cre charge (+) will affect atmic radius Attractin between negatively charged in (Cl - ) and psitively charged in (Mg 2+ r Ca 2+ ) will determine strength f inic bnd 3. r each set f circumstances, determine what the distance f separatin is between the + and charges Number f shells (perids) Number f shells n + in plus number f shells n in 4. Usually ne variable, distance r (Q + and Q - ), can be cnsidered cnstant while the ther ne varies. The ne that varies is respnsible fr the difference in the measure Nen has Q+=+8 while sdium has Q+=+1. Nen has 2 shells while sdium has 3 shells. Bth differences lead t sdium s utermst electrn being further away and less tightly bund. Bth attractins are a +2 in with a -1 in. Cl - in has same radius, but Mg 2+ in is smaller than Ca 2+ in, s separatin distance between Q+ and Q- is smaller in MgCl 2, therefre harder t break the inic bnd. Qualitative cmparisns in strength f inic bnding Recall that Culmb s law predicts that frce f attractin between tw ppsitely charged bjects depends n magnitudes f charges (direct) and n distance that separates them (indirect) Cmparing the lattice energies f tw inic cmpunds depends n tw factrs: Cmpare magnitudes f inic charges ver the same separatin distance Cmpare separatin distance if the same inic charges (Bth factrs can wrk in the same directin) (If factrs wrk in ppsing directins, yu need t knw mre quantitative infrmatin t make a predictin) Map f Chapter 8 What hlds ins tgether Predicting qualitative trends What hlds mlecules tgether Predicting enthalpy f reactin frm bnd energies Inic vs. cvalent character f bnds: plarity and electrnegativity mdel Lewis structure mdel Simple structures (ctet rule), with single and multiple bnds Resnance structures Mre cmplicated structures (breaking the ctet rule) rmal charges Bnd strength and length Using Lewis structures t predict Using ess s law and bnd enthalpies Sevian 15

16 Lewis Dt Structure Mdel Thery behind Lewis dt structures is that valence electrns are distributed as either Pairs f electrns that are shared by tw atms (shared pairs) Pairs f electrns that belng t a single atm (unshared, r lne pairs) Lne pair f electrns Shared r bnding pair f electrns Lne pair f electrns (belngs exclusively t this xygen) Duble bnd (shared electrns get cunted as belnging t bth xygen atms) Building Lewis Structures 1. Determine central atm (atm with lwest electrn affinity because electrn density will spread as far as pssible, given the pprtunity) 2. Cunt ttal number f valence electrns in mlecule 3. Arrange atms arund central atm 4. Start with single bnds 5. Place remaining valence electrns 6. Mve electrns t frm ctets, making duble r triple bnds where necessary Check: Make sure yu have cnservatin f electrns Predictins that can cme frm Lewis structures Resnance Bnd rder (useful fr cmparing strengths f bnds) Bnd length cmparisns (cmparing ne mlecule t anther) rmal charge (useful fr predicting which resnance structures are mst stable) Resnance Structures N N What d these structures have in cmmn? w are they different? Which f these is the actual structure f N 3-? Average bnd rder = 4 bnds split ver 3 lcatins = 4/3 N N Nte: This is nt a crrect Lewis structure. It is drawn this way nly t emphasize the bnd rder Sevian 16

17 Bnd rder and Bnd Length/Strength Bnd rder Single bnd is bnd rder 1 Duble bnd is bnd rder 2 Triple bnd is bnd rder 3 Bnd strength Average bnd rder = 4 bnds split ver 3 lcatins = 4/3 The greater the bnd rder, the strnger the bnd (the mre energy required t break the bnd) Bnd length The greater the bnd rder, the shrter the bnd length N rmal Charges and Alternative Structures rmal charge is a cmparisn between the valence electrns riginally cntributed by an atm and the electrns that it lks like the atm wuld have if all bnds were brken and electrns reassigned demcratically. If mre than ne Lewis structure exists, the mst stable structure is the ne in which the frmal charges make mst sense Negative frmal charges n atms with large electrn affinity Psitive frmal charges n atms with small inizatin energies (small electrn affinity) rmal charge = +1 because has 6 valence electrns but the demcratically assigned -1-1 electrns in this structure are 0 5, s is missing ne C N electrn C N C N Inic bnding in inic cmpunds, and hw Lewis structures begin t explain cvalent bnding in mlecular cmpunds Inic vs. cvalent bnding Inic bnd is attractin between + and ins, held tgether by Culmb frce f attractin acting acrss in separatin Mlecular bnd is valence electrns shared between tw atms and attracted (but nt necessarily equally) t bth atms in the bnd A Range f Bnd Types Bnd types are nt separable int a true dichtmy between cvalent and inic. Instead there is a range f cvalent character vs. inic character f bnds. Increasing inic character f bnd Increasing bnd plarity nnplar cvalent plar cvalent inic Many aspects f mlecular bnding can be mdeled by Lewis structures Bnd rder/strength and length Resnance structures f mlecules (r ins) and frmal charges n individual atms in the mlecules (r ins) Bnd plarity Mlecular gemetry Perfectly cvalently shared electrns 0 0 Bnding electrns exhibit greater electrn density (δ-) n ne atm in the bnd than n the ther (δ+) δ+ δ- Ins with full charges (unshared electrns) Sevian 17

18 Pauling s Electrnegativity Scale Electrnegativity measures the ability f an atm t attract electrns t itself Electrnegativities Allw Yu t Cmpare Bnd Plarity Which bnd is mst plar? Which bnd is least plar? Which end f the bnd is the negative ple (greater electrn density)? Which end is the psitive ple (less electrn density)? The bnd between C and N in CN - δ+ δ- C N Difference f 0.5 Is less plar than The bnd between C and in 2 C δ+ C Difference f 1.0 δ- Is less plar than δ- The bnd between C and in C δ+ C 2.5 Difference f 1.5 The electrnegativity mdel, and amendments t the Lewis structure mdel Mlecular Gemetries bserved The electrnegativity mdel explains tw things: There is a gray area between cvalent bnding and inic bnding: ranging frm nt plar at all, t s cmpletely plar that the electrn transfers cmpletely Yu can cmpare the plarities f bnds within mlecules, and ultimately yu can predict the verall plarity f a mlecule (by a vectr sum f all the bnd plarities in a mlecule) Tetrahedral See-saw There are exceptins t the ctet rule in Lewis structures Sme atms (ntably Be, B and Al) can have less than an ctet f electrns and be stable dd electrn mlecules are free radicals Atms that can have mre than ctet must have d-rbitals that can be used t create the beynd-ctet ptins (s the atms must be at atmic #13 and higher, i.e., having accessible 3d rbitals and beynd) Square planar Square pyramid Sevian 18

19 Stretching Lewis Structure Thery Prcedure fr drawing a Lewis structure (abbreviated) 1. Determine hw many ttal valence electrns 2. Decide n central atm and arrange ther atms arund it 3. Start with single bnds, make ctets n all atms (except ), making duble r triple bnds where necessary Amendment t prcedure 4. If it s nt pssible t draw a simple structure, determine whether central atm can accmmdate mre than an ctet Which elements can accmmdate mre than an ctet? Any element that has access t un-used d-rbitals All elements in perid 3 have access t 3d rbitals All elements in perid 4 have access t either 3d r 4d rbitals, etc. Summary: all elements at and beynd atmic #13 Examples f mre than an ctet n the central atm nly elements in perids 3 and higher (e.g., S, Cl) can d this. S 6 48 Cl 3 S Mlecular shape is ctahedral Electrn dmain gemetry is: ctahedral Cl 28 Mlecular shape is T-shaped Electrn dmain gemetry is: trignal bipyramid VSEPR w pairs f electrns arund a central atm interact with each ther 1. A lne pair f electrns repels anther lne pairs f electrns 1. mre than A lne pair f electrns repels a pair f bnding electrns and tw ther interactins mre than A pair f bnding electrns repels anther pair f bnding electrns VSEPR results Sme bnd angles are smaller than Lewis structure predicts Sme bnd angles are larger than Lewis structure predicts Lcatins where electrns are (whether bnding r nn-bnding) Electrn dmain gemetry Lewis structure predictin: All angles equal at 109.5º VSEPR Bnd angle less than 109.5º Bnd angle less than 109.5º Mre than 109.5º Mlecular gemetry Mre than 109.5º Sevian 19

20 w d bnd plarities sum t determine mlecular plarity? Sme mlecules cntaining plar diples A mlecule is a diple: 1. If it has at least ne bnd in it that is plar cvalent and 2. If the bnd diples d nt cancel each ther ut (cancellatin happens when bnd diples are symmetrically lcated) Remember hw t determine whether a bnd is a diple? Difference in electrnegativities f the tw atms in the bnd N difference: perfectly cvalent Sme difference (as between nn-metals): plar cvalent Very different (as between a metal and a nn-metal): inic Valence Bnd Thery Central ideas: 1. Atmic rbitals initially frm hybrids t get ready fr bnding t frm mlecules/ins (csts a little bit f energy less stable) 2. Bnds in mlecules/ins are frmed by the verlap f atmic rbitals (win back a lt f energy much mre stable) Three Different ybridizatins f Carbn s Atmic rbitals 1s 1s 2sp 3 2sp 3 2sp 3 2sp 3 1s 2p z π 2p z 1s sp 3 sp 2 C C π p x p x sp C C 1s p z π p z 1s p x π p x Valence bnd thery leads t predictins f bnd angles that cncur with experimentally bserved bnd angles Sevian 20

21 ther Atmic rbital ybridizatins sp 3 d fur 3d rbitals remain Lewis structure mdel + Valence bnd thery 3s 3s 3p 3p 3p These can nly ccur fr elements that have level 3 and higher atmic rbitals as valence shell 3p 3p 3p 3d 3d 3d 3d 3d 3d 3d 3d 3d five equal 3sp 3 d hybrids sp 3 d 2 3d 3d 3d 3d 3d 3d 3d 3d six equal 3sp 3 d 2 hybrids three 3d rbitals remain Lewis structures mdel Predict mlecular gemetry Need t determine hw many bnding and lne pair electrns surrund a central atm Valence bnd thery Explains hw bnd angles arise Different cmbinatins f atmic rbitals are pssible n different atms Making duble and triple bnds requires reserving sme p-rbitals Taken tgether, the tw mdels explain mst mlecular gemetries Valence Bnd Thery vs. Mlecular rbital Thery fr 2 Lewis diagram predicts sp 2 hybridizatin n atms sp 2 sp 2 p sp 2 z spπ 2 bnd B σ bnd p z B sp 2 sp 2 1. Mlecular shape predicted t be flat 2. Crrect bnd rder 2 predicted 3. All rbitals are ccupied by pairs f electrns nt paramagnetic 1. N predictin abut mlecular shape 2. Crrect bnd rder predicted (net pairs f electrns in bnding rbitals) 3. Sme unpaired electrns paramagnetic Tw cmpeting theries that predict varius prperties f mlecules Valence bnd thery Thery f quantum mechanical wave functins that wuld satisfy Schrdinger equatin fr the mlecule (if it culd be slved) Lewis structure s electrn pairs translated int quantum mechanics Electrns in a particular bnd are lcalized t specific valence bnd rbitals Mlecular rbital thery Thery f quantum mechanical wave functins that wuld satisfy Schrdinger equatin fr the mlecule (if it culd be slved) Wave functins (mlecular rbitals) are frmed frm all bnding electrns in mlecule Electrns in all bnds are spread ut (delcalized) ver all mlecular bnding rbitals in mlecule Mathematically, the appraches are different. Results (predictins) are ften the same Sevian 21