INTERMOLECULAR FORCES

Transcription

1 INTERMLEULAR FRES Thr ar no covalnt bonds btwn molculs in molcular covalnt compounds. Thr ar, howvr, forcs of attraction btwn ths molculs, and it is ths which must b ovrcom whn th substanc is mltd and boild. Ths forcs ar known as intrmolcular forcs. Thr ar thr main typs of intrmolcular forc: 1. Van dr Waal's forcs onsidr a molcul of oxygn, 2. Th lctrons in this molcul ar not static; thy ar in a stat of constant motion. It is thrfor likly that at any givn tim th distribution of lctrons will not b xactly symmtrical - thr is likly to b a slight surplus of lctrons on on of th atoms. + - This is known as a tmporary dipol. It lasts for a vry short tim as th lctrons ar constantly moving. Tmporary dipols ar constantly apparing and disapparing. onsidr now an adjacnt molcul. Th lctrons on this molcul ar rplld by th ngativ part of th dipol and attractd to th positiv part, and mov accordingly This is known as an inducd dipol. Thr is a rsulting attraction btwn th two molculs, and this known as a Van dr Waal's forc.

2 Van dr Waal's forcs ar prsnt btwn all molculs, although thy can b vry wak. Thy ar th rason all compounds can b liqufid and solidifid. Van dr Waal's forcs tnd to hav strngths btwn 1 kjmol -1 and 50 kjmol -1. Th strngth of th Van dr Waal's forcs in btwn molculs dpnds on two factors: a) th numbr of lctrons in th molcul Th gratr th numbr of lctrons in a molcul, th gratr th liklihood of a distortion and thus th gratr th frquncy and magnitud of th tmporary dipols. Thus th Van dr Waal's forcs btwn th molculs ar strongr and th mlting and boiling points ar largr. Eg nobl gass: Substanc N Ar Kr Numbr of lctrons Mlting point/ o Boiling point/ o Eg alkans: Substanc Numbr of lctrons Mlting point/ o Boiling point/ o a) Surfac ara of th molculs Th largr th surfac ara of a molcul, th mor contact it will hav with adjacnt molculs. Thus th gratr its ability to induc a dipol in an adjacnt molcul and th gratr th Van dr Waal's forcs and mlting and boiling points. This point can b illustratd by comparing diffrnt molculs containing a similar numbr of lctrons: Substanc Kr l 2 3 ( 3 ) Numbr of lctrons

3 Mlting point/ o Boiling point/ o ( 3 ) mthylpropan butan Not that butan has a largr surfac ara than 2-mthylpropan, although thy hav th sam molcular formula ( 4 10 ). Straightchain molculs always hav highr boiling points than thir isomrs with branchd chains. 2. Dipol-dipol bonding Tmporary dipols xist in all molculs, but in som molculs thr is also a prmannt dipol. Most covalnt bonds hav a dgr of ionic charactr rsulting from a diffrnc in lctrongativity btwn th atoms. This rsults in a polar bond and a dipol. + - l - + In many cass, howvr, th prsnc of polar bonds (dipols) dos not rsult in a prmannt dipol on th molcul, as thr ar othr polar bonds (dipols) in th sam molcul which hav th ffct of canclling ach othr out. This ffct can b sn in a numbr of linar, trigonal planar and ttrahdral substancs: l F F l l 2 BF 3 l 4 l B F

4 In all th abov cass, thr ar dipols rsulting from polar bonds but th vctor sum of ths dipols is zro; i.. th dipols cancl ach othr out. Th molcul thus has no ovrall dipol and is said to b non-polar. Non-polar molculs ar thos in which thr ar no polar bonds or in which th dipols rsulting from th polar bonds all cancl ach othr out. Th only intrmolcular forcs that xist btwn non-polar molculs ar tmporary-inducd dipol attractions, or Van dr Waal s forcs. In othr molculs, howvr, thr ar dipols on th molcul which do not cancl ach othr out: l + l - l - S + - N + l 3 S 2 N 3 In all th abov cass, thr ar dipols rsulting from polar bonds whos vctor sum is not zro; i.. th dipols do not cancl ach othr out. Th molcul thus has a prmannt dipol and is said to b polar. Polar molculs ar thos in which thr ar polar bonds and in which th dipols rsulting from th polar bonds do not cancl out. In addition to th Van dr Waal's forcs causd by tmporary dipols, molculs with prmannt dipols ar also attractd to ach othr by dipol-dipol bonding. This is an attraction btwn a prmannt dipol on on molcul and a prmannt dipol on anothr. + - Br + - Br + - Br Dipol-dipol bonding usually rsults in th boiling points of th compounds bing slightly highr than xpctd from tmporary dipols alon; it slightly incrass th strngth of th intrmolcular bonding.

5 Th ffct of dipol-dipol bonding can b sn by comparing th mlting and boiling points of diffrnt substancs which should hav Van dr Waal's forcs of similar strngth: Substanc l 2 Br 3 ( 3 ) Numbr of lctrons Prmann t dipol Mlting point/ o Boiling point/ o 3. ydrogn bonding N YES N YES In most cass as sn abov, th prsnc of prmannt dipols only maks a slight diffrnc to th magnitud of th intrmolcular forcs. Thr is on xcptional cas, howvr, whr th prmannt dipol maks a hug diffrnc to th strngth of th bonding btwn th molculs. onsidr a molcul of hydrogn fluorid, F. This clarly has a prmannt dipol as thr is a larg diffrnc in lctrongativity btwn (2.1) and F (4.0). Th lctrons in this bond ar on avrag much closr to th F than th : + - F Th rsult of this is that th atom has on almost no lctron dnsity around its nuclus at all and is thrfor vry small. Th atom is thrfor abl to approach lctrongativ atoms on adjacnt molculs vry closly and form a vry strong intrmolcular dipoldipol bond. F F F This is known as hydrogn bonding. It is only possibl if th hydrogn atom is bondd to a vry lctrongativ lmnt; i.. N, or F. It is not fundamntally diffrnt from dipol-dipol bonding; it is just a strongr form of it.

6 A hydrogn bond can b dfind as an attraction btwn an lctropositiv hydrogn atom (i covalntly bondd to N, or F) and an lctrongativ atom on an adjacnt molcul. Exampls of substancs containing hydrogn bonds ar F, 2, N 3, alcohols, carboxylic acids, amins, acid amids and ura. N N N hydrogn bonds a) Effct on boiling point Th ffct of hydrogn bonding on mlting and boiling points of substancs is hug, unlik othr dipol-dipol bonds. Many substancs containing hydrogn bonds hav much highr boiling points than would b prdictd from Van dr Waal's forcs alon. Substanc 3 3 Structur Numbr of lctrons ydrogn bonding Mlting point/ o Boiling point/ o N YES N YES

7 BILING PINT / Anothr important sris of trnds ar th boiling points of th hydrids of lmnts in groups V, VI and VII of th priodic tabl: Group V: N 3, P 3, As 3, Sb 3 Group VI: 2, 2 S, 2 S, 2 T Group VII: F, l, Br, I Th boiling points of ths graphs ar shown graphically blow: 100 BILING PINTS F YDRIDES M r GRUP IV GRUP V GRUP VI GRUP VII -160 In ach cas th hydrid of priod 2 shows a boiling point which is abnormally high ( 2, N 3 and F). Th gnral incras in boiling point down th groups rsult from th incras in Van dr Waal's forcs which rsults from an incrasing numbr of lctrons in th molculs. Thr ar prmannt dipols but thy ar not vry strong. Th abnormally high boiling points of 2, N 3 and F ar a rsult of hydrogn bonding btwn th molculs. Thus rsults in vry strong intrmolcular forcs btwn th molculs dspit th fact that th Van dr Waal's forcs ar wakr than in th othr hydrids.

8 Explain this trnd in boiling point of priod 2 mmbrs of groups 5,6,7--- [ N 3 <F< 2 ] Thr ar 2 atoms and 2 non bonding pairs in 1 molcul. So is capabl of 2 hydrogn bonds pr molcul, xtnt of bonding is th gratst, BP highst. F has 1 atom and 3 non bonding pairs pr molcul. So xtnt of bonding lssr than watr so BP lssr than watr. N3 has 3 atoms and 1 non bonding pair pr molcul so th xtnt of hydrogn bonding lssr b) othr ffcts of hydrogn bonding Th ffcts of hydrogn bonding on th physical proprtis of a substanc ar not rstrictd to lvatd mlting and boiling points; it can influnc th proprtis of substancs in othr ways: Th low dnsity of ic. This is du to hydrogn bonding. In ic, th watr molculs arrang thmslvs in such a way as to maximis th amount of hydrogn bonding btwn th molculs. This rsults in a vry opn hxagonal structur with larg spacs within th crystal. This accounts for its low dnsity. Whn th ic mlts, th structur collapss into th opn spacs and th rsulting liquid, dspit bing lss ordrd, occupis lss spac and is thus mor dns.

9 Thus ic floats on watr. Th hlical natur of DNA. This is also du to hydrogn bonding. Molculs of DNA contain N- bonds and so hydrogn bonding is possibl. Th long chains also contain = bonds and th atoms can form a hydrogn bond with this lctrongativ atom. This rsults in th molcul spiralling, as th = bonds and th N- bonds approach ach othr. This is an xampl of an intramolcular hydrogn bond, whr th attraction is btwn a hydrogn atom and an lctrongativ atom on th sam molcul. This must b distinguishd from intrmolcular hydrogn bonding, in which th attraction is btwn a hydrogn atom and an lctrongativ atom on an adjacnt molcul. Dimr of thanoic acid Ethanoic acid has an apparnt M r of 120 in non polar solvnt such as bnzn