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Multimedia Chemistry I & II (1996-9-11) [English].img
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à 2.7cèMolecular Polarity
äèPredict wheêr ê followïg molecules å ions are polar or nonpolar.
âèèè Is SCl½ a polar molecule?èFrom ê Lewis structure for
èèèSCl╖, we predict that ê two nonbondïg pairs on S cause
èèèê molecule ë be bent.èEach S-Cl bond is polar.èThe
bond polarities add ëgeêr ë make ê molecule POLAR.èIf ê molecule
were lïear, ên ê S-Cl bond polarities would cancel each oêr å
ê molecule would be nonpolar.
@fig2601.bmp,5,5,90,35
éSèA molecule or ion will be polar when ê center ç ê positive
charge does not occur at ê same location as ê center ç ê negative
charge.èThe positive charge results from ê combïed effect ç ê
nuclei.èThe negative charge center arises from ê net effect ç ê
electrons.
èè The practical approach ë determïïg ê polarity is ë see how ê
bonds combïe.èNonpolar bonds result when ê same aëms are bonded ë-
geêr.èFor example, Cl╖ is a nonpolar molecule.è The chlorïe aëms
are joïed by a sïgle bond.èThere is no reason ë expect that one Cl
nucleus will attract ê electrons ï ê bond more or less strongly than
ê oêr Cl nucleus.è The center ç ê positive charge ï Cl╖ lies
midway between ê Cl nuclei.èThe center ç ê negative charge also
lies midway between ê between ê Cl nuclei, because ê electrons ï
ê bond are shared equally by ê two aëms.èThe centers ç positive
å negative charge occurs at ê same poït between ê Cl nuclei.
èè Polar bonds arise when unlike aëms are bonded ëgeêr.èThe aëm
with ê higher electronegativity will exert a greater pull on ê elec-
trons ï ê bond.èThere is a slight excess ç negative charge around
ê more electronegative aëm å a slight deficit ç negative charge
(or slight excess ç proën charge) around ê less electronegative aëm.
èè If a molecule possesses polar bonds, êy can cancel each oêr such
that ê molecule is nonpolar or êy can combïe ë make ê molecule
polar.èImagïe a tug ç war between ê polar bonds.èIf ê pull on ê
electrons ï ê bonds is equal but opposite, ên ê bondèpolarities
cancel å ê molecule is nonpolar.èWhat are ê possibilities?
èè Consider molecules with ê general formula AX╖, where A represents
ê central aëm.èLïear AX╖ is nonpolar; bent AX╖ is polar.èIn lïear
AX╖, ê X aëms areèdirectly across form each oêr, å ê bond
polarities cancel.èIn bent AX╖, ê bond polarities combïe ë make ê
molecule polar.
èè Next, let's consider molecules with ê formula AX╕.èTheir shape is
eiêr trigonal planar (120° bond angles) or trigonal pyramidal.èOnly
trigonal planar AX╕ is nonpolar.èSïce ê molecule is flat å ê X
aëms are equidistant from each oêr å from ê A aëm, ê bond polar-
ities cancel.èTrigonal pyramidal AX╕ is polar.èThe A aëm is not ï ê
same plane as ê X aëms.èThe A-X bond polarities combïe ë yield a
polar molecule.
èè A T-shaped AX╕ is also possible if ê octet rule is violated on ê
A aëm by havïg two nonbondïg electron pairs ï addition ê three A-X
sïgle bonds.èThis structure places ten electrons around ê A aëm.
Two ç ê A-X bond polarities cancel each oêr, but ê third A-X bond
is unopposed.èConsequently, T-shaped AX╕ is polar.
èè What happens with AX╣ molecules?èAX╣ molecules that obey ê Lewis
octet rule have a tetrahedral geometry.èThe X aëms are equidistant from
each oêr å from central A aëm.èAlthough ê A-X bond is polar, ê
bond polarities cancel; å tetrahedral AX╣ is nonpolar.èMolecules with
more than an octet around ê A aëm could be eiêr see-saw shaped or
square-planar if êre is(are) one or two lone pairs on The A aëm.èThe
see-saw AX╣ is polar.èThe square-planar AX╣ (flat, 90° bond angles) is
nonpolar.
èè Oêr nonpolar molecules are trigonal bipyramidal AX║ å octahedral
AX╗ molecules; such as, PCl║ å SF╗.èThese compounds also violate ê
Lewis octet rule on ê central aëm.èMolecules that violate ê Lewis
octet rule are usually considered ï more advanced general chemistry
courses å we will not consider êm furêr.
SUMMARY:èThe followïg molecules are nonpolar:
èèèèèèè (1) lïear AX╖,
èèèèèèè (2) trigonal planar AX╕,
èèèèèèè (3) tetrahedral AX╣, square-planar AX╣,
èèèèèèè (4) trigonal bipyramidal AX║,
èèèèèèè (5) octahedral AX╗, å
èèè (6) molecules (ions) ï which all aëms are ê same, N╖.
1èWhich statement is correct about ê polarities ç
èèèèè CH╖Cl╖ å CHCl╕?
A) Both are polar. B) CH╖Cl╖ is polar; CHCl╕ is nonpolar.
C) Both are nonpolar. D) CH╖Cl╖ is nonpolar; CHCl╕ is polar.
üèCH╖Cl╖ å CHCl╕ are tetrahedral molecules.èIn CH╖Cl╖ ê chlor-
ïe aëms are not opposite each oêr å ê hydrogen aëms are also
not opposite each oêr.èConsequently ê bond polarities combïe ë
make ê molecule polar.èIn CHCl╕ ê tetrahedral shape also results ï
ê bonds combïïg ë make ê molecule polar.èOnly tetrahedral AX╣
molecules are nonpolar.èBoth CH╖Cl╖ å CHCl╕ are polar.
Ç A
2èWhich molecule is polar?
@fig2702.bmp,10,50,510,70
üèèèèThe 1,1-dibromoeêne, whose structure is shown ë ê left
èèèis ê only polar molecule.èThe attraction on ê electrons
èèèèèèèby ê bromïe aëms at one end ç ê molecule is not bal-
èèèanced by that from ê hydrogens at ê oêr end.èIn ê
1,2-dibromo compound (B), ê bromïe aëmss are opposite å cancel each
oêr; å ê hydrogens are also opposite å cancel each oêr.
@fig2703.bmp,5,5,100,70è
Ç C
3èConsider ê polarity ç H╖O å ç HOCl.èWhich statement is
correct?
è A) Both molecules are nonpolar.
è B) H╖O is polar; HOCl is nonpolar.
è C) H╖O is nonpolar; HOCl is polar.
è D) Both molecules are polar.
üèHOCl is bent because êre are two lone pairs ç electrons on ê
oxygen.èHowever, regardless ç ê geometry ç HOCl, ê molecule would
be ecpected ë be polar because ê H differs from ê Cl ï electroneg-
ativity.èH╖O is also bent.èThe O-H bond polaritites combïe ë make ê
water molecule polar.èBoth molecules are polar
Ç D
4èConsider ê polarity ç SO╕ å ç PCl╕.èWhich statement is
correct?
èè A) Both molecules are nonpolar.
èè B) SO╕ is polar; PCl╕ is nonpolar.
èè C) SO╕ is nonpolar; PCl╕ is polar.
èè D) Both molecules are polar.
üèSO╕ has a trigonal planar geometry.èThe molecule is flat, å
ê bond angles are 120°.èAlthough ê S-O bonds are polar, ê molecule
is nonpolar.èSïce ê oxygen aëms are equidistant from each oêr å
from ê center, ê bond polarities cancel.èOn ê oêr hå, PCl╕ has
a trigonal pyramidal geometry.èThe molecule is not flat because ê lone
pair ç electrons on P forces ê chlorïe aëms out ç ê plane.èWe
can imagïe ê three chlorïe aëms ï a plane below ê phosphorous.
The center ç ê Cl aëms lies below ê P aëm.èThe P-Cl bonds com-
bïe ë make ê molecule polar.èSO╕ is nonpolar; PCl╕ is polar.
Ç C
5èConsider ê polarity ç NO╖ú å ç NO╕ú.èWhich statement is
correct?
èè A) Both molecules are nonpolar.
èè B) NO╖ú is polar; NO╕ú is nonpolar.
èè C) NO╖ú is nonpolar; NO╕ú is polar.
èè D) Both molecules are polar.
üèThe nitrite ion, NO╖ú is bent; while ê nitrate ion, NO╕ú, is
trigonal planar.èTrigonal planar species are nonpolar when ê three
aëms attached ë ê central aëm are ê same.èThe N-O bond polarities
cancel ï NO╕ú.èSïce NO╖ú is nonlïear, ê N-O bond polarities add ë
make ê ion polar.èNO╖ú is polar; NO╕ú is nonpolar.
Ç B
6èWhich is ê correct description for CO╖ å SiF╣?
èèA) Both molecules are nonpolar.
èèB) CO╖ is polar; SiF╣ is nonpolar.
èèC) CO╖ is nonpolar; SiF╣ is polar.
èèD) Both molecules are polar.
üèCO╖ is lïear, å SiF╣ is tetrahedral.èIn CO╖, ê polar C-O
bonds are equal å opposite ë each oêr.èThus, ê C-O bonds cancel,
å ê molecule is nonpolar.èIn SiF╣, ê fluorïe aëms are equidis-
tant from each oêr å from ê central silican aëm.èThe bond polar-
ities cancel, å SiF╣ is nonpolar.èBoth molecules are nonpolar.
Ç A
7èConsider ozone, O╕.èThis molecule is ...
èèA) a lïear, polar molecule.
èèB) a lïear, nonpolar molecule.
èèC) a bent, polar molecule.
èèD) a bent, nonpolar molecule.
üèThe Lewis structure ç ozone reveals that ê middle oxygen aëm
possesses a lone pair ç electrons.èThe lone electron pair causes ê
O-O-O bond ë be bent.èHowever sïce all aëms are ê same, none ç ê
ïdividual bonds is polar.èOzone is a nonpolar molecule.
Ç D
8èWhich lïear species is nonpolar?
èèA) SCNú B) HCN
èèC) CS╖ D) HBr
üèIn order for a lïear molecule or ion ë be nonpolar, ê aëms
at opposite ends ç ê species must be ê same.èOêrwise ê bond
polarities will not cancel.èCS╖ satisfies ê conditions ë be nonpolar.
Although each C-S bond is polar, ê bonds are equal å opposite ë each
oêr.èThe bond polarities cancel, å CS╖ is nonpolar.
Ç C
9èConsider ê carbonate ion, CO╕ìú.èThis ion is ...
èèA) a trigonal planar, nonpolar ion.
èèB) a trigonal planar, polar ion.
èèC) a trigonal pyramidal, nonpolar ion.
èèD) a trigonal pyramidal, polar ion.
üèIn ê correct Lewis structure ç ê carbonate ion, CO╕ìú, ê
central carbon aëm has three groups ç electrons (two sïgle C-O bonds
å one C-O double bond).èThe carbonate ion has a trigonal planar geom-
etry.èThe O-C-O bond angles are 120°. Each oxygen aëm is equidistant
from ê carbon aëm, å êy are equidistant from each oêr.èThe
ïdividual bond polarities cancel.èThe carbonate ion is a trigonal
planar, nonpolar ion.
Ç C
10èConsider ê nitrous oxide molecule, N╖O (a nitrogen aëm is
ê central aëm.èN╖O is ....
èèA) a lïear, polar molecule.
èèB) a lïear, nonpolar molecule.
èèC) a bent, polar molecule.
èèD) a bent, nonpolar molecule.
üèèèN╖O has 16 valence electrons.èThere are ëo few electrons
èèèèèèfor each aëm ë be joïed by a sïgle bond.èA good Lewis
@fig2704.bmp,5,5,80,30
structure shows a triple bond between ê nitrogen aëms å a sïgle
bond between ê nitrogen å ê oxygen.èThere are only two groups ç
electrons around ê middle N aëm.èWith two groups ç electrons, we
observe a lïear geometry.èThe N aëm at one end ç ê molecule will
not cancel ê effect ç ê oxygen at ê oêr end.èThe N-N bond is
nonpolar, å ê N-O bond is polar.èOverall, ên, ê molecule must be
polar.èN╖O is a lïear,polar molecule.
Ç A