Chem Hydrogen bonds. Highlights of last lecture. Only F, O, and N? Drawing H-bonds. A/Prof Sébastien Perrier

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1 Chem 1101 A/Prf Sébastien Perrier Rm: 351 Phne: Unless therwise stated, all images in this file have been reprduced frm: Blackman, Bttle, Schmid, Mcerin and Wille, Chemistry, 2007 (Jhn Wiley) ISBN: Slide 37-1 Prf Sctt Kable Rm: 311 Phne: s.kable@chem.usyd.edu.au A/Prf Adam Bridgeman Rm: 222 Phne: a.bridgeman@chem.usyd.edu.au Slide 37-2 ighlights f last lecture CNCEPTS Intermlecular Frces w t determine whether a mlecule is plar Three type f charge distributin: in, diple, plarizability Different type f IM Frces In diple Diple diple In induced diple Diple induced diple Dispersin (Lndn) Which frces are present in different mlecules Which frces are mre imprtant Slide 37-3 ydrgen bnds A -bnd arises frm an unusually strng diple-diple frce. When is bnded t a very electrnegative element (F,, N) the bnd is plar cvalent. is unusual because with nly ne electrn, it leaves a partially expsed nucleus ( has n ther cre electrns t shield the nucleus). The bnd can be thught f as frming between the hydrgen atm and the lne pairs f the F, N, r. F 2 N 3 Slide 37-4 nly F,, and N? Drawing -bnds The -bnd ccurs between -atm and a lne pair. Can ther elements with lne pairs frm -bnds? e.g. Cl? F, and N are all 2 nd rw elements, which cnveys certain prperties: They are the mst electrnegative elements They are small (nly 2s, 2p in uter shell) They have lne pairs Cnsequently, althugh, e.g. Cl ([Ne] 3s 2 3p 5 ) lne pairs D cntribute t the diple-diple frce, this frce is nt unusual in size (see plt later in lecture). S we cnsider -bnds NLY t frm between bnds invlving F, and N Slide 37-5 We indicate a -bnd using a dtted line. The -bnd is strngest when the bnd angle is 180º. Slide 37-6

2 The Ammnia Funtain N 3 (g) + 2 N 3 (aq) N 3 (aq) + 2 N 4+ (aq) + - (aq) Exam-type questin Example: Which f the fllwing substances shw -bnding? Draw the structure fr thse that d. a) C 2 6 b) C 3 c) C 3 (C=)N 2 N YES YES 3 C C 3 3 C C N N C C 3 Slide 37-7 Slide 37-8 The special case f water Water is perhaps the mst unusual liquid. Each water mlecule is -bnded t FUR ther water mlecules (dnating 2 - atms and accepting tw -atms t the lne pairs), frming a tetrahedral netwrk. 1 2 Intramlecular -bnds All the -bnds we ve seen thus far have been between tw mlecules (intermlecular -bnds). -bnds can als ccur within the same mlecule (intramlecular - bnd). 4 3 Slide hydrxybenzic acid = salicylic acid = metablite f aspirin p-hydrxybenzic acid (n intramlecular -bnding) Slide Physical prperties Biling pints We shall examine sme physical prperties f substances that are affected by the strength f the intermlecular frce. Liquid vapur prperties Biling pint Vapur pressure eat f vaprisatin Liquid prperties Surface tensin ne direct measure f the strength f the IM Frce is the biling pint f a substance. All the species in the table at right nly have dispersin interactins between the mlecules. The dispersin interactin clearly scales with mlecular weight. Viscsity Slide Slide 37-12

3 Biling pints When the mlecular weights are similar, then the strength f the diple interactin can influence the biling pint strngly. Is C 60 a gas, a liquid r a slid at RT? Is the mlecule plar? N Name Frmula MW State Radn Rn 222 gas Benzene C liquid Graphite C slid Fullerene C slid Slide Slide The effect f -bnds Biling pint data fr a hmlgus series:?? Liquid vapur prperties We have already examined the behaviur f the biling pint as a functin f the strength f the intermlecular frce. The vapur pressure is inextricably linked t the biling pint. The quantity that links the tw is the eat f Vaprisatin. Substance B.P. (ºC) Vapur 25ºC (mm g) vap (kj/ml) Butane Slide Diethyl ether Methanl Ethanl Water Decane Slide Liquid vapur prperties Think f the equilibrium between liquid and vapur: (mlecules) n (l) n (mlecules) (g) vap > 0 Exam type questin: Q: Explain the trend in the table in terms f the type and size f IM Frces present. r (mlecules) n (l) + heat n (mlecules) (g) S as temperature ges up, eq m is shifted t right, mre vapur (higher vapur pressure) As vap increases, eq m shifts t the left less vapur (lwer v.p. and higher b.p.) Substance vap (kj/ml) Butane, C 3 (C 2 ) 2 C 3 22 Diethyl ether, (C 2 5 )--(C 2 5 ) 27 Methanl, C 3 38 Ethanl, C 3 C 2 43 Water, 2 44 Decane, C 3 (C 2 ) 8 C bnds? N N Yes Yes Yes x 2 N M.W Slide Slide 37-18

4 Liquid Prperties Slubility f gases in water Nn-plar gases are generally nly very weakly sluble in water, the slubility increasing with plarisability. (What is the frce acting here?) Plar gases are generally mre sluble in water, the slubility increasing with diple mment and size. (What is the frce acting here?) Liquid Prperties Surface tensin Mlecules in the centre f a liquid experience a frce in all directins. Mlecules at the surface, hwever, will have a nett attractin pulling them int the liquid. The mst stable cnfiguratin is t minimise the surface area. C 2 is very sluble in water (523.2 x 10-4 ) because it actually reacts: C 2 (g) + 2 (l) + (aq) + C 3- (aq) Q: Why are the bubbles in Guinness much smaller than the bubbles in ther beers? Slide Slide Liquid Prperties Surface tensin Mlecules in the centre f a liquid experience a frce in all directins. Mlecules at the surface, hwever, will have a nett attractin pulling them int the liquid. Liquid Prperties Viscsity Viscsity arises when mlecules must slide past each ther. The strnger the IM Frce, the mre difficult t slide past each ther and hence higher the viscsity. The mst stable cnfiguratin is t minimise the surface area. Slide Slide Liquid Prperties Summary Viscsity Als, as the temperature rises, and mre f the IM bnds are brken, it becmes easier t slide past each ther s fr mst liquids, viscsity decreases with increasing temperature. Viscsity f water Viscsity vs Surface tensin Temperature (ºC) Viscsity (N s /m 2 ) Substance Viscsity (N s /m 2 ) Surf. tensin (J/m 2 ) x x x x 10-3 pentane 0.24 x x 10-2 water 1.00 x x 10-2 ethanl 1.20 x x 10-2 glycerl x 10-2 Slide Slide 37-24

5 Summary Summary CNCEPTS -bnding intermlecular, intramlecular the special case f water Effect f IM frces n biling pt, vapur pressure, vap viscsity, surface tensin CALCULATINS Nne Slide Slide 37-26