Chapter 8 Concepts of Chemial Bonding John Bowne High School：8章概念的化学键约翰邦恩高中

1、chemicalbondingchapter 8concepts of chemical bondingchemistry, the central science, 10th editiontheodore l. brown; h. eugene lemay, jr.; and bruce e. burstenjohn d. bookstaverst. charles community collegest. peters, mo 2006, prentice hall, inc.chemicalbondingnovember 15chapter 8 chemical bondingsect

2、ion 1 and 2bonding typeslewis structuresionic bondinghw: 1,3,4,7 to 19 odd ,24,25 there will be a quiz any day on hw questionschemicalbondingchemical bonds three basic types of bonds:ionic electrostatic attraction between ionscovalent sharing of electronsmetallic metal atoms bonded to several other

3、atomschemicalbondingionic bondingelectron transferchemicalbondingenergetics of ionic bondingas we saw in the last chapter, it takes 495 kj/mol to remove electrons from sodium.chemicalbondingenergetics of ionic bondingwe get 349 kj/mol back by giving electrons to chlorine.chemicalbondingenergetics of

4、 ionic bonding but these numbers dont explain why the reaction of sodium metal and chlorine gas to form sodium chloride is so exothermic!chemicalbonding the reaction is violently exothermic. we infer that the nacl is more stable than its constituent elements. why? na has lost an electron to become n

5、a+ and chlorine has gained the electron to become cl-. note: na+ has an ne electron configuration and cl- has an ar configuration. that is, both na+ and cl- have an octet of electrons surrounding the central ion.chemicalbondingenergetics of ionic bonding there must be a third piece to the puzzle. wh

6、at is as yet unaccounted for is the electrostatic attraction between the newly formed sodium cation and chloride anion.chemicalbondinglattice energy this third piece of the puzzle is the lattice energy:the energy required to completely separate a mole of a solid ionic compound into its gaseous ions.

7、 the energy associated with electrostatic interactions is governed by coulombs law:eel = q1q2dchemicalbonding lattice energy depends on the charges of the ions and the sizes of the ions is a constant (8.99 x 10 9 jm/c2), q1 and q2 are the charges on the ions, and d is the distance between ions. latt

8、ice energy increases as*the charges on the ions increase*the distance between the ions decreases.chemicalbondingchemicalbonding nacl forms a very regular structure in which each na+ ion is surrounded by 6 cl- ions. similarly, each cl- ion is surrounded by six na+ ions. there is a regular arrangement

9、 of na+ and cl- in 3d. note that the ions are packed as closely as possible. note that it is not easy to find a molecular formula to describe the ionic lattice.chemicalbondingenergetics of ionic bond formation the formation of na+(g) and cl-(g) from na(g) and cl(g) is endothermic. why is the formati

10、on of nacl(s) exothermic? the reaction nacl(s) na+(g) + cl-(g) is endothermic (h = +788 kj/mol). the formation of a crystal lattice from the ions in the gas phase is exothermic:na+(g) + cl-(g) nacl(s) h = -788 kj/molchemicalbondingenergetics of ionic bondingby accounting for all three energies (ioni

11、zation energy, electron affinity, and lattice energy), we can get a good idea of the energetics involved in such a process.chemicalbonding1. vaporize the metal (enthalpy of vaporization) na (s) na (g)2. break diatomic nonmetal molecules (if applicable) (bond enthalpy) cl2 (g) cl-3. remove electron(s

12、) from metal (ionization energy) na (g) na+ (g) + e-born-haber cycleapplication of hesss lawchemicalbonding 4 add electron(s) to nonmetal (electron affinity)cl (g) + e- cl- (g)chemicalbonding5 put ions together to form compound (lattice energy)na+ + cl- nacl (s)chemicalbonding overall reaction:na (s

13、) + cl2 (g) nacl (s) this is useful because all quantities are directly measurable except lattice energy. the born-haber cycle can be used to calculate lattice energy from the other values.chemicalbondingstepenergetics12345chemicalbondingstepenergetics1endothermic2endothermic3endothermic4exothermic5

14、exothermic (highly)chemicalbondinglattice energy lattice energy, then, increases with the charge on the ions. it also increases with decreasing size of ions.chemicalbondingexamples which in each pair would have the greatest lattice energy?1. lif or licl2. nacl or mgcl23. kbr or ki4. mgcl2 or mgo5. c

15、ao or nafchemicalbondingexamples which in each pair would have the greatest lattice energy?1. lif or licl2. nacl or mgcl23. kbr or ki4. mgcl2 or mgo5. cao or nafchemicalbondingenergetics of ionic bonding these phenomena also helps explain the “octet rule.” metals, for instance, tend to stop losing e

16、lectrons once they attain a noble gas configuration because energy would be expended that cannot be overcome by lattice energies.chemicalbondingnovember 17 properties of ionic compounds/ naming ionic compounds covalent bonds lewis structures resonance structures exceptions to octect rule strengths o

17、f covalent bondschemicalbondingproperties of ionic substances crystalline structure, result from electrostatic forces that keep ions in a rigid well defined three dimensional arrangement. hard and brittle. represented by empirical formulas high melting points do not conduct electricity in the solid

18、state but they conduct electricity when are dissolved in water or in the liquid statechemicalbondingnaming ionic compounds binary compounds ending ide when metal has several oxidation states indicate the o.n. with the stock system. examples lead (iv) oxide manganese (iii) nitrate iron (ii) phosphate

19、 calcium sulfatechemicalbondingcovalent bondingsharing of electrons.typical bond between non metals.can be polar or non polar.can be single, double or triple bonds.chemicalbondinggeneral properties of substances containing covalent bonds they are molecular substances. generally they have low melting

20、 points. they exists in the 3 states. in the solid state many are pliable (like plastics). they are not conductors of heat or electricity.chemicalbondingnaming covalent compounds use iupac nomenclature with roman numerals for the oxidation numberschemicalbondingbond types and nomenclature in general

21、, the least electronegative element is named first. the name of the more electronegative element ends in ide. compounds are named according to their ions (or perceived ions), including the charge on the cation if it is variable. compounds of metals with high o.n. have properties similar to covalent

22、compounds and sometimes are named using the convention for molecular compounds mn2o7 can be called dimanganese heptoxide or manganese(vii)oxide chemicalbondingbond types and nomenclatureionicmolecularmgh2h2sfef2of2mn2o3cl2o3chemicalbondingbond types and nomenclatureionicmolecularmgh2magnesium hydrid

23、eh2shydrogen sulfidefef2iron(ii) fluorideof2oxygen (ii) fluoridemn2o3manganese(iii) oxidecl2o3chlorine (iii) oxidechemicalbondingcovalent bonding in these bonds atoms share electrons. pairs of electrons become the glue that bind to atoms together. there are several electrostatic interactions in thes

24、e bonds:attractions between electrons and nucleirepulsions between electronsrepulsions between nucleichemicalbondingpolar covalent bonds although atoms often form compounds by sharing electrons, the electrons are not always shared equally. fluorine pulls harder on the electrons it shares with hydrog

25、en than hydrogen does. therefore, the fluorine end of the molecule has more electron density than the hydrogen end.chemicalbondingelectronegativity: the ability of atoms in a molecule to attract electrons to itself. on the periodic chart, electronegativity increases as you gofrom left to right acros

26、s a row.from the bottom to the top of a column.chemicalbondingelectronegativity and bond polarity difference in electronegativity is a gauge of bond polarity: electronegativity differences between 0 and 0.5 result in non-polar covalent bonds (equal or almost equal sharing of electrons); electronegat

27、ivity differences around between 0.5 and 2 result in polar covalent bonds (unequal sharing of electrons); electronegativity differences greater that 2 result in ionic bonds (transfer of electrons).chemicalbondingnon polar covalent bondsequal electron sharing happens between atoms with same electrone

28、gativity. all diatomic molecules have non polar covalent bonds!chemicalbondingpolar covalent bonds when two atoms share electrons unequally, a bond dipole results. the dipole moment, , produced by two equal but opposite charges separated by a distance, r, is calculated: = qr it is measured in debyes

29、 (d).chemicalbondingpolar covalent bondsthe greater the difference in electronegativity, the more polar is the bond.chemicalbondinglewis structures(lewis electron-dot structures)lewis structures are representations of molecules showing all electrons, bonding and nonbonding. a line between 2 atoms re

30、presents a pair of electrons (2 electrons = 1 line )chemicalbondingmultiple bonds it is possible for more than one pair of electrons to be shared between two atoms (multiple bonds): one shared pair of electrons = single bond (e.g. h2); two shared pairs of electrons = double bond (e.g. o2); three sha

31、red pairs of electrons = triple bond (e.g. n2). generally, bond distances decrease as we move from single through double to triple bonds.h ho on nchemicalbonding1. add the valence electrons.2. write symbols for the atoms and show which atoms are connected to which. 3. complete the octet for the cent

32、ral atom, then complete the octets of the other atoms.4. place leftover electrons on the central atom.5. if there are not enough electrons to give the central atom an octet, try multiple bonds.chemicalbondingwriting lewis structures1. find the sum of valence electrons of all atoms in the polyatomic

33、ion or molecule. if it is an anion, add one electron for each negative charge. if it is a cation, subtract one electron for each positive charge.pcl35 + 3(7) = 26chemicalbondingwriting lewis structures2. the central atom is the least electronegative element that isnt hydrogen. connect the outer atom

34、s to it by single bonds.keep track of the electrons:26 - 6 = 20chemicalbondingwriting lewis structures3. fill the octets of the outer atoms.keep track of the electrons:26 - 6 = 20 - 18 = 2chemicalbondingwriting lewis structures4. fill the octet of the central atom.keep track of the electrons:26 - 6

35、= 20 - 18 = 2 - 2 = 0chemicalbondingwriting lewis structures5. if you run out of electrons before the central atom has an octetform multiple bonds until it does.chemicalbondingexamples draw lewis structures for each:1. h2o2. co23. ncl34. so25. so3chemicalbonding practice lewis structures formal char

36、ge exceptions to octect rule a) atoms with fewer than 8 valence e- b) atoms with more than 8 valence e- c) molecules with odd number of e- resonance chemicalbondingformal chargeit is possible to draw more than one lewis structure with the octet rule obeyed for all the atoms.to determine which struct

37、ure is most reasonable, we use formal charge.formal charge is the charge on an atom that it would have if all the atoms had the same electronegativity.chemicalbondingto calculate formal charge: all nonbonding electrons are assigned to the atom on which they are found.half the bonding electrons are a

38、ssigned to each atom in a bond.formal charge is: valence electrons - number of bonds - lone pair electronschemicalbondingwriting lewis structures to assign formal charges.for each atom, count the electrons in lone pairs and half the electrons it shares with other atoms.subtract that from the number

39、of valence electrons for that atom: the difference is its formal charge.chemicalbondingwriting lewis structures the best lewis structureis the one with the fewest charges.puts a negative charge on the most electronegative atom.chemicalbondingformal chargeconsider:for c: there are 4 valence electrons

40、 (from periodic table).in the lewis structure there are 2 nonbonding electrons and 3 from the triple bond. there are 5 electrons from the lewis structure.formal charge: 4 - 5 = -1.cnchemicalbondingformal chargeconsider:for n:there are 5 valence electrons.in the lewis structure there are 2 nonbonding

41、 electrons and 3 from the triple bond. there are 5 electrons from the lewis structure.formal charge = 5 - 5 = 0.we write:cncnchemicalbondingexamples determine the formal charge on each atom for each:1.h2o2.co23.ncl34.so25.so3chemicalbondingformal chargethe most stable structure has:the lowest formal

42、 charge on each atom,the most negative formal charge on the most electronegative atoms.resonance structuressome molecules are not well described by lewis structures.typically, structures with multiple bonds can have similar structures with the multiple bonds between different pairs of atomschemicalb

43、ondingresonancethis is the lewis structure we would draw for ozone, o3.-+chemicalbondingresonance but this is at odds with the true, observed structure of ozone, in whichboth oo bonds are the same length.both outer oxygens have a charge of -1/2.chemicalbondingresonance one lewis structure cannot acc

44、urately depict a molecule such as ozone. we use multiple structures, resonance structures, to describe the molecule.chemicalbondingresonancejust as green is a synthesis of blue and yellowozone is a synthesis of these two resonance structures.chemicalbondingresonance in truth, the electrons that form

45、 the second co bond in the double bonds below do not always sit between that c and that o, but rather can move among the two oxygens and the carbon. they are not localized, but rather are delocalized.chemicalbondingresonance the organic compound benzene, c6h6, has two resonance structures. it is com

46、monly depicted as a hexagon with a circle inside to signify the delocalized electrons in the ring.chemicalbonding exception to the octect rule bond strengthchemicalbondingexceptions to the octet rule there are three types of ions or molecules that do not follow the octet rule:ions or molecules with

47、an odd number of electrons.ions or molecules with less than an octet.ions or molecules with more than eight valence electrons (an expanded octet).chemicalbondingodd number of electronsthough relatively rare and usually quite unstable and reactive, there are ions and molecules with an odd number of e

48、lectrons. clo2 no no2 o2-superoxide ionchemicalbondingmolecules with odd # of e- it is impossible to draw a structure obeying the octet rule. they are called free radicalschemicalbondingfewer than eight electrons consider bf3:giving boron a filled octet places a negative charge on the boron and a po

49、sitive charge on fluorine.this would not be an accurate picture of the distribution of electrons in bf3.chemicalbondingfewer than eight electronstherefore, structures that put a double bond between boron and fluorine are much less important than the one that leaves boron with only 6 valence electron

50、s.chemicalbondingfewer than eight electronsthe lesson is: if filling the octet of the central atom results in a negative charge on the central atom and a positive charge on the more electronegative outer atom, dont fill the octet of the central atom.chemicalbonding bf3 reacts very energetically with

51、 molecules having an unshared pair of electrons that can be used to form a bond with b example formation of nh3bf3chemicalbondingmore than eight electrons the only way pcl5 can exist is if phosphorus has 10 electrons around it. it is allowed to expand the octet of atoms on the 3rd row or below.presu

52、mably d orbitals in these atoms participate in bonding.chemicalbondingmore than eight electronseven though we can draw a lewis structure for the phosphate ion that has only 8 electrons around the central phosphorus, the better structure puts a double bond between the phosphorus and one of the oxygen

53、s.chemicalbondingmore than eight electrons this eliminates the charge on the phosphorus and the charge on one of the oxygens. the lesson is: when the central atom is on the 3rd row or below and expanding its octet eliminates some formal charges, do so.chemicalbondingions with expanded shell icl4- xe

54、f2 clf4-chemicalbondingthe energy required to break a covalent bond is called the bond dissociation enthalpy, d. that is, for the cl2 molecule, d(cl-cl) is given by h for the reaction:cl2(g) 2cl(g).when more than one bond is broken:ch4(g) c(g) + 4h(g) h = 1660 kjthe bond enthalpy is a fraction of h

55、for the atomization reaction:d(c-h) = h = (1660 kj) = 415 kj.bond enthalpies can either be positive or negative.chemicalbondingchemicalbondingbond enthalpies and the enthalpies of reactionswe can use bond enthalpies to calculate the enthalpy for a chemical reaction.we recognize that in any chemical

56、reaction bonds need to be broken and then new bonds get formed.the enthalpy of the reaction is given by the sum of bond enthalpies for bonds broken less the sum of bond enthalpies for bonds formed. chemicalbondingthe energy required to break a covalent bond is called the bond dissociation enthalpy,

57、d. that is, for the cl2 molecule, d(cl-cl) is given by h for the reaction:cl2(g) 2cl(g).when more than one bond is broken:ch4(g) c(g) + 4h(g) h = 1660 kjthe bond enthalpy is a fraction of h for the atomization reaction (since 4 bonds are broken each is )d(c-h) = h = (1660 kj) = 415 kj.bond enthalpie

58、s can either be positive or negative.chemicalbondingchemicalbondingaverage bond enthalpies this table lists the average bond enthalpies for many different types of bonds. average bond enthalpies are positive, because bond breaking is an endothermic process.chemicalbondingaverage bond enthalpiesnote:

59、 these are average bond enthalpies, not absolute bond enthalpies; the ch bonds in methane, ch4, will be a bit different than thech bond in chloroform, chcl3.chemicalbondingbond enthalpies and the enthalpies of reactionswe can use bond enthalpies to calculate the enthalpy for a chemical reaction.we r

60、ecognize that in any chemical reaction bonds need to be broken and then new bonds get formed.the enthalpy of the reaction is given by the sum of bond enthalpies for bonds broken less the sum of bond enthalpies for bonds formed. chemicalbondingmathematically, if hrxn is the enthalpy for a reaction, t