Groups elements. Trends in atomization energies

Transcription

1 Groups elements CHEM 102 T. Hughbanks Trends in atomization energies Explanation?

2 Group 15 Elements Range from electronegative nitrogen to somewhat electropositive Bi Atomic configurations ns 2 np 3 Bonding in the elements is strong and p orbital participation in bonding is dominant. sp 2 (N) and sp 3 hybridization is common in compounds. Bi in oxidation state III common ( inert pair effect). Elemental properties... melting pts. C boiling pts. C Density (g cm 3 ) Form, properties N (l) colorless gas Nonmetal, many forms As 613(s) sublimes 5.78 gray semimetal Sb Blue-white, shiny semimetal Bi metallic

3 Forms of phosphorus Arsenic Gas phase

4 The story of phosphorus 4 (g) + C(g) - C2 (g) Ca 3 ( 4 ) 2 (s) black phosphorus Thermodynamically stable form condense at reduced pressure b.p. 280 C 4 (l) hν red phosphorus m.p C Heat (days), no air High pressure (12 kbar), 200 C α- 4 (s) volatile, white molecular solid, insoluble in H 2, highly soluble in CS 2.? Group 15 hydrogen compounds NH 3 H 3 AsH 3 SbH 3 BiH 3 decreasing basicity decreasing stability A-H bond strength Some laboratory ways to make ammonia: H 2 6 Li + N 2 2 Li 3 N 6 Li(H) + NH 3 H 2 6 Al + N 2 2 AlN Al(H) 3 + NH 3 Industrial method: Haber process (Fe/Fe 3 4 catalyst). More moles of NH 3 produced than any other industrial chemical. NH 3 has some similarities to water, but is less polar and has a smaller self-ionization constant (~10 30 at 50 C).

5 Haber process - reality check! All the following are important in the modern process: CH 4 (g) + H 2 (g) C(g) + 3 H 2 (g) ~750 C, 40 atm Zn(g) + H 2 S(g) ZnS(s) + H 2 (g) remove H 2 S from CH 4 2 CH 4 (g) + 1 / 2 2 (g) + 2 N 2 (g) C(g) + 2 H 2 (g) + 2 N 2 (g) Removes oxygen from added air (~ 1 / 2 2 (g) + 2 N 2 (g)) C(g) + H 2 (g) C 2 (g) + H 2 (g) Converts C to C 2 (removable) and makes more H 2 N 2 (g) + 3 H 2 (g) 2 NH 3 (g) H = kj/mol ; K 298 = Reactants are finally pure. rocess conducted at C, atm Nitrogen vs. hosphorus, As, Sb N-X π bonding important 3- and 2-coordination is common in multiple bonding to X = N, C, -X pπ-pπ bonding is weak 3 single bonds + lone-pair or 4 single bonds (cation) both common. Eg., N 2 vs. 4 or other forms of phosphorus, or As N 2 5 vs. 4 10

6 Nitrogen vs. hosphorus, As, Sb Nitrogen chemistry strongly influenced by stability of N 2, so fixing of nitrogen a major chemical challenge Many compounds have positive heats of formation for phosphorus, bonds to oxygen are important, - bonds tend to be reactive no natural occurence of - bonds (not too difficult to synthesize, however) Some important N/ compounds NH 4 N 3 (s) N 2 (g) + 3 H 2 (g) ~250 C N 2 : Useful anesthetic (laughing gas) 4 NH 3 (g) (g) 4 N(g) + 6 H 2 (g) ~1000 C, t catalyst (neurotransmitter) 2 N(g) + 2 (g) 2 N 2 (g) rapid in air Significant pollutant, in eq. with N 2 4 (g) 3 N 2 + H 2 (l) N(g) + 2 HN 3 (aq) Steps shown are the stwald process for nitric acid.

7 / compounds 4 (s) (g) 4 6 (s) Limited oxidation 4 (s) (g) 4 10 (s) excess air Both are subject to Hydrolysis to give acids pening up of -- bridges is highly exothermic in reactions with water. Biologically very important: AT + H 2 AD + H H 3 + Adenosine triphosphate 4 10 ( 2 5 ) and 4 6

8 Hydrolysis of AT NH 2 N N N N H H H H H H H + H H H H Lewis basicity The electron-pair donating property of ammonia, amines, and phosphines is important in the chemistry of these compounds in reactions with metals.

9 Halogen compounds of Group 15 NF 3 is stable, N 3 is reactive (bleach for flour), neither is very basic. Halides of, As, and Sb are important laboratory chemicals: AX 3 and AX 5 forms are both important. 3 and Br 3 are reactive and useful as starting materials to other -containing compounds. F 3 more stable. F 5, 5, AsF 5, SbF 5, Sb 5 are strong Lewis acids. Group 16 - general characteristics Range from highly electronegative oxygen to metalloid Te Atomic configurations ns 2 np 4 pπ-pπ bonding is again most important for the first element, oxygen. With six valence electrons, in most molecules these atoms bear from 1 to 3 lone pairs and hence have fewer bonds.

10 hysical properties melting pts. C boiling pts. C Density (g cm 3 ) Form, properties (l) diatomic gas S (monoclin.) (rhom.) Yellow, S 8 rings, S chains Se (gray) Gray, Se 8 rings, Se chains Te Te chains o radioactive semimetallic, cubic Trends in atomization energies Explanation?

11 Basic xides ( Ionic ) Alkali, alkaline-earth, and other electropositive metal oxides, e.g., Ca + 2 H 2 Ca 2+ (aq) + 2 H (aq) [ 2 + H 2 2 H (aq), K > ] Acidic xides ( Covalent ) - Acid anhydrides Element-oxygen bond not broken on dissolution in water. Either an E--E bridge is hydrolyzed or water is added across a double bond. e.g., N N + H 2 N C + H 2 C H H 2 H Acidic xides - more xide Acid S 3 H 2 S 4 = () 2 S(H) 2 S 2 H 2 S 3 = ()S(H) 2 (not isolated) 4 10 H 3 4 = ()(H) H 3 3 = H()(H) 2 N HN 3 = () 2 N(H)

12 p = 0 p = 1 p = 2 p = 3 H H B C N H H H H H H Si S H H H H H H H H H , 7.4, , H H H As H Te H H H I H H H H H H H H H H H 7.8, , , H H Sb H As Se H H H H H H , 6.9, , 8.0 p E(H) q acids: Structures and pk a s Group 17 elements CHEM 102 T. Hughbanks

13 roperties m. p. C b. p. C Density (g cm 3 ) Description F (l, -188 C) pale yellow diatomic gas (l, -70 C) greenish-yellow diatomic gas Br blood-red diatomic liquid I deep-purple diatomic solid At radioactive Halide Chemistry We have already discussed several compounds with halides in earlier lectures The elements are unremarkable: all have diatomic molecular structures, X 2, but intermolecular forces increase moving down the series. Halide compounds are important intermediates to other compounds.

14 Methods of preparing halides Direct reaction, useful for highest ox. state, e.g., (M = Si, Ge, Sn, Ti, Zr) M(s) + X 2 MX 4 or (M = Al, Ga, In, Sc, Y) M(s) + 3 / 2 I 2 MI 3 Higher ox. states favored by more electronegative X: Mo(s) + excess 2 Mo 5 Mo(s) + excess Br 2 MoBr 3 Fe(s) + excess X 2 FeX 3 (X =, Br) Fe(s) + excess I 2 FeI 2 (FeI 3 unknown) More Methods of preparing halides Reaction with HX, useful for lower ox. state (why?), e.g., Sn(s) + conc. aq. HI SnI 2 Cr(s) + 2H(g) Cr 2 - at red heat compare: Cr(s) + 3 / 2 2 Cr C Dehydration, e.g., Mn 2 4H 2 + excess S 2 (l) Mn 2 (s) + S 2 (g) + H(g) [note: S 2 (l) + H 2 (l) S 2 (g) + H(g)]

15 Halides often the route to other compounds Example: 3 R Mg R Li Br R R R R = CH 3, -CH 2 CH 3, etc. R H R R R (in base) S 8 S 2 H 2 H 3 3 R Mg Br R H R R R R R R Halides often used as volatile intermediates Example: Silicon production Mg or Zn (M II ) Si 4 fractional Si 4 "Sponge" Si + M II 2 liquid, b.p. 57 C distillation high purity 2 2 zone refining Scrap Si single crystal Si

16 Fluorine vs. others; bond energy comparison Typical average bond energies to halogens, kj/mol X XX HX BX 3 AlX 3 CX 4 F Br I