Electron Density Lab PDF

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Electron Density Lab: There is not a formal lab report for this lab. Complete the below pages using the Microsoft version of this file that is available for download on the lab D2L page. Once the worksheet is complete, submit the worksheet on time and to your TA in the dropbox on D2L. 1. Read sections 9.6 and 10.5 in your text book. DONE 2. What are the three types of bonding? There are three types of bonding: Ionic, nonpolar covalent, and polar covalent bonds. The type of bond seen in elements and compounds depends on the chemical properties as well as the attractive forces governing the atoms and molecules.

3. What does the description polar bond refer to? A Polar bond is a type of covalent bond between two atoms in which electrons are shared unequally, resulting in a bond in which one atom has a slightly negative charge and the othr a slightly positive charge.

4. The interactions between water molecules can be described as electrostatic or coulombic, where areas of positive charge are attracted to areas of negative charge. Draw a cartoon of how three water molecules might be arranged in space, given this electrostatic interaction. Figure 1: Diagram of the electrostatic interactions between water molecules

5. Provide a brief explanation as to why atoms may have different values of electronegativity. What causes a low electronegativity? High? El e c t r o ne g a t i v i t yi same a s u r eo ft het e n d e n c yofa na t o mt oa t t r a c tab o n d i n gp a i rofe l e c t r o n s . Th ePa u l i n g s c a l ei smo s tc ommo nl yu s e dt ome a s u r et h i s . El e c t r o n e g a t i v i t yi sr e l a t e dt oi o n i z a t i o ne n e r g y . El e c t r o nswi t h l o wi o n i z a t i o ne n e r g yh a v el o we l e c t r o n e g a t i vi t yb e c a u s et h e i rn uc l e id ono te x e r tas t r o n ga t t r a c t i v ef o r c eo n e l e c t r on s . El e me n t swi t hh i g hi o n i z a t i o ne ne r g yh a v eh i g he l e c t r o n e g a t i v i t yd u et ot hes t r o n gp u l le x e r t e do n e l e c t r on sb yt h en u c l e u s . El e c t r o n e g a t i v i t yi n c r e a s e sa c r o s st h ep e r i od i ct a b l ea st h en u mbe ro fc h a r g e so nt h e n u c l e u si nc r e a s e s . El e c t r o ne g a t i v i t yde c r e a s e sd o wnt hep e r i o d i ct a bl ea sa t o mi cn u mb e ri n c r e a s e sd o wna 1

g r o u p , a n dt h u st h e r ei sa ni n c r e a s e dd i s t a n c eb e t we e nt hev a l e n c ee l e c t r o n sa n dn u c l e u s , o rag r e a t e ra t o mi c r a d i u s .Fl u o r i n ei st h emo s t e l e c t r o n e g a t i v ee l e me nta n dCa e s i uma ndFr a n c i u ma r et h el e a s te l e c t r on e g a t i v e . 6. Briefly describe the difference between a nonpolar covalent and polar covalent bond. No n p o l a rc o v a l e ntb on d sa r eat y p eo fb on dt h a to c c u r swh e nt woa t omss h a r eapa i ro fe l e c t r on swi t he a c h o t h e r . Th e s es h a r e de l e c t r o n sg l uet woo rmor ea t omst o g e t h e rt of o r mamo l e c u l e . a t o msi n v o l v e di nan o n p ol a r c o v a l e ntb o n de q u a l l ys h a r ee l e c t r o n s . E. GHyd r o g e nb o n d s . Po l a rc o v a l e ntb o n d i n gi sat y p eo fc h e mi c a lb o n dwh e r eapa i ro fe l e c t r o nsi su n e q ua l l ys h a r e db e t we e nt wo a t o ms . I nap o l a rc o v a l e ntb o nd , t h ee l e c t r o n sa r eno te q u a l l ys h a r e db e c a u s eo n ea t o ms p e n d smor et i mewi t h t h ee l e c t r o n st h a nt h eo t he ra t o m. I np o l a rc o v a l e ntb o n d s , o n ea t o mh a sas t r o n g e rp u l lt h a nt h eo t h e ra t o ma n d a t t r a c t se l e c t r o n s .

Simulation In this atomic-level simulation, you will investigate how an atoms' electronegativity value affects the types of bonds they produce.

Enter: Phet.colorado.edu into the browser of your computer select: Play with the Sims  Chemistry  Molecule Polarity

Part A Select the two atom tab in the upper left hand corner of the simulator. Turn on (check) all “View” options. Investigate how the bond behaves when the atom's electronegativity is changed. 1. What does

represent?

Th i ss y mb o lr e pr e s e n t sBo n dDi p o l ewhi c hu s e st h ei de aofe l e c t r i cd i p o l emo me n tt ome a s ur et h ep ol a r i t yo fa c h e mi c a lb o n dwi t h i namo l e c u l e . I to c c u r swhe n e v e rt h e r ei sas e p a r a t i o no fp o s i t i v ea n dn e g a t i v ec h a r g e s .

2. What does the symbol δ+ or δ- represent?

Th el o we r c a s eδu s ua l l yme a n s" as ma l la mo u n t " . I ti sof t e nu s e dt os h o wapa r t i a le l e c t r i c a lc h a r g eo na na t o m. Fo re x a mp l e , i nap o l a rHFb o n d , wemi g h ts a yt ha tt h emo r en e g a t i v eFa t o mh a saδ −p a r t i a lc ha r g e

3. Adjust the electronegativities of atoms A and B. Can an atom with a high electronegativity form a

covalent bond? Describe under what circumstances this can occur. An atom with a high electronegativity level can form a covalent bond, however this is only the case when the electronegativities are at the maximum level. This can be seen in figure 2 below:

2

Figure 2: Diagram displaying a representation of how atoms with high electronegativity levels can form covalent bonds

4. How does changing the electronegativity of the atoms affect the bond polarity?

Wh e nt woa t omsa r ec o v a l e n t l ybo n d e d , t h es h a r e de l e c t r o np a i ri ss u p po s e dt ob ee q ua l l ydi s t r i b u t e da mo n gt h e t woa t oms . Ho we v e r , du et ot h ee l e c t r o n e g a t i v i t yd i ffe r e n c e , t h emo r ee l e c t r o n e g a t i v ea t o ma t t r a c t st h ee l e c t r o n p a i ri t s e l fa ndr e s u l t si nbr i n g i n gaf r a c t i o na ln e g a t i v ec h a r g eo ni t . 5. Turn on (check) electron density in “Surface” options. Describe how a molecule’s polarity is related to it electron density? How does the electron density around an atom with a δ- compare to the electron density around an atom with a δ+?

Le s se l e c t r on e g a t i v ea t o mh a ss omeo fi t se l e c t r o nd e n s i t yt a k e na wa yδ -i smo r ed e ns et h a nt h a to fδ +. Th eδ b e c o me smo r ed e n s ea st h ee l e c t r o n e g a t i v i t yi n c r e a s e s . Wh e namol e c u l eh a sagr e a t e rd i ffe r e n c ei ne l e c t r o n d e n s i t yi twi l lb emor ep o l a rt h a nmo l e c u l e swh e r et h ee l e c t r o nd e n s i t i e sa r emo r es i mi l a r . Ana t o mwi t ha n e g a t i v ep a r t i c l ec h a r g ewi l la l s oh a v eag r e a t e re l e c t r ond e n s i t yt ha na t o mswi t hp o s i t i v ec h a r g e s . Thi si sc l e a r l y s e e ni nFi g u r e3b e l o w Fi g ur e3 :Di a g r a md i s p l a y i n gt h ee l e c t r o nd e n s i t yoft h ea t o ma st h ee l e c t r o ne g a t i v i t yv a l u ei sc h a n g e d

6. What scenarios can bring about a higher electron density around a particular atom?

Electrostatic potential correlates with electron density. Electrostatic potential is basically a measure of how a proton would react when brought to different regions of a molecule. It provides a useful way to quickly predict 3

polarity of a molecule or a region in a larger molecule. Negative electrostatic potential (colored in shades of red) corresponds to an attraction of the proton. Positive electrostatic potential (colored in shades of blue) corresponds to a repulsion of the proton.

7.

Describe how electron density is related to the electrostatic potential? In the red shaded regions, what is responsible for a potential attraction to a proton? In the Blue shaded regions, what would cause the repulsion?

Th emo l e c u l a re l e c t r o s t a t i cp o t e nt i a li st h ep o t e n t i a le n e r gyofap r o t o na tap a r t i c ul a rl o c a t i o nn e a ramol e c u l e . I nt h er e ds h a d e dr e g i o n ,n e g a t i v ee l e c t r o s t a t i cp o t e n t i a lc o r r e s p o n dst oa na t t r a c t i o noft h ep r o t o nb yt h e c on c e n t r a t e de l e c t r o nd e n s i t yi nt h emo l e c u l e s ;a sap r o t o na p p r o a c h e san e g a t i v er e g i o na na t t r a c t i v ei nt e r a c t i o n r e s u l t si nn e g a t i v ep o t e n t i a le ne r g y .I nt heb l u er e gi o n ,wh e nap r ot o na p p r o a c h e sapo s i t i v er e g i o no ft h e mo l e c u l e ,t h er e pul s i v ei n t e r a c t i o nr e s u l t si na ni n c r e a s i n gpo s i t i v ep o t e nt i a le n e r g y ;p o s i t i v ee l e c t r o s t a t i c p o t e n t i a lc o r r e s p o n d st or e p u l s i ono ft hep r o t onb yt h ea t o mi cn u c l e ii nr e gi o n swh e r el o we l e c t r o nd e n s i t ye x i s t s a ndt hen u c l e a rc h a r g ei si nc o mp l e t e l ys h i e l d e d . 8. Explain how the direction of the arrow in the bond dipole symbol (

) relates to the electron

density, the partial charges and the electrostatic potential on a molecule.

Th eb o ndd i pl o ea l wa y sp oi n t st os i d eo ft h ed e l t ami n u swhe nt h ee l e c t r o ni sd e n s e r ,mor en e g a t i v e l yc h a r g e d a ndmo r en e g a t i v e l ye l e c t r o s t a t i c .Th ea r r o wsr e p r e s e n twhi c ha t o msh a v eah i g h e re l e c t r o nd e n s i t y ,p o i n t i n g a wa yf r o mt hea t o mswi t hal e s s e re l e c t r o nd e n s i t y ,wh i c ha l s oh a samo r ep o s i t i v ec ha r g e ,a ndap o s i t i v e e l e c t r os t a t i cp o t e n t i a l ,a n dv i c ev e r s a

9. What happens to a polar molecule when the electric field is turned on? Make sure to spin the molecule

several times while making observations. What if the molecule is nonpolar? Wh e nt h ep o l a rmo l e c ul ei na ne l e c t r i cfie l di st u r ne do ni tc a u s e st h ene g a t i v es i d eo fi tt oa t t r a c tt h ep o s i t i v e p o l a rmo l e c u l e , wh i c hi nt u r nr e p e l st h en e g a t i v emo l e c u l e . Th ep o s i t i v es i d eo ft h ee l e c t r i cfie l dwi l lt h e na t t r a c t t h en e g a t i v es i deo ft h ep o l a rmol e c ul ewh i l er e p e l l i n gt hep o s i t i v ep a r t .I ft h emo l e c u l ewe r et ob eno n p o l a ri t wo u l dh a v enoe ffe c to nt hi smo l e c u l eb e c a u s ea l lc ha r g e swou l db ewh o l e ,a n dwo u l db en op a r t i a lc ha r g e st o a t t r a c torr e p e l

Part B Select the three atoms tab in the upper left hand corner of the simulator. Turn on (check) all “view options”. Investigate how the bond behaves when the electronegativity of the individual atoms is changed. In addition to changing the electonegativities, orient the radial atoms relative to one another by dragging them with the mouse. 1. Provide a summary as to how the bond dipoles affect the molecular dipole. Also, describe how the geometric orientation of the bonded atoms affect the molecular dipole? 4

Th emo l e c u l a rd i p o s eor i e n t si t s e l fi nt h es a med i r e c t i o nt h eb o n ddi p o l e sg oe s .I ft h ea t o mse l e c t r o n e g a t i vi t yi s u n e q u a lt h emol e c u l a rd i p o l es h o wsup ,b e c a u s et h ec h a r g e sa r e n ’ tc a n c e l l e dou t .Th e r e f o r e ,de p e n d i n go nt he g e ome t r i co r i e n t a t i o nt h eb on d s ,t hep u r e l yc o v a l e nta n di o n i cbo n d swo n ’ tr e s u l ti nmo l e c u l a rd i p os e( b e c a u s e c h a r g e sa r ec a n c e l l e do u t ) . 2. Provide a scenerio in which a molecule with two strong bond dipoles can have no molecular dipole at all? Explain your answer with a drawing showing individual bond dipoles and the overall molecular dipole. Wh e na l lt h r e ea t o msa r ea l lo na ne x t r e mep o l eo fb o n dt h emo l e c u l a rd i p o l ed i s a pp e a r s ,b e c a u s et he yo n l y s h o wupwh e nt h ea t o mse l e c t r o n e g a t i v i t yi su n e q u a l . Th i sc o nc e p ti sc l e a r l ys e e ni nfig u r e4b e l o w Fi g ur e4 :Di a g r a md i s p l a y i n ga molecule with two strong bond dipoles withlo no molecular dipole

3. Provide a scenerio in which a molecule may have a very large molecular dipole. Explain your answer with a drawing showing individual bond dipoles and the overall molecular dipole. Th ea t omsr e s u l ti nav e r yl a r g emo l e c u l a rd i p o l ei f2o ft hea t o msa r eo no n es i deo ft h eb on ds c a l ea n dt h e o t h e ri so nt h eo pp o s i t es i d e . Ass h o wnbe l o w. Fi g ur e5 :Di a g r a md i s p l a y i n gt h emo l e c u l ewi t hav e r yl a r g emo l e c ul a rdi p o l e

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Part C One property between molecules which is explored more in CH222 is the solubility of one substance in another. There is an adage that describes this ability, "Like dissolves like"; molecules with similar molecular dipoles will tend to interact favorably and mix. For instance, a polar molecule will mix well (dissolve) other polar molecules (ethanol readily dissolves in water, both are polar molecules and possess strong molecular dipoles). Octane (a major component of gasoline) will not dissolve in water because it does not have a molecular dipole and is thus a nonpolar molecule. Being able to predict the polarity of a molecule is extremely important since many properties of molecules depend on whether they are polar or non-polar. Predicting a molecule’s polarity is a multi-step process that starts with drawing the Lewis structure. Using VSEPR, predict the molecule’s molecular geometry. Individual bond polarities are finally used to predict the molecular polarity 1. For the following molecules complete this step-by-step process. Table 1: Table displaying Lewis Structure, Molecular Geometry, 3D with Bond Polarities, and Polarity of the molecules listed Molecu le

Lewis Structure

Molecular Geometry

3-d Geometry with Bond Polarities

Polar or nonpol ar?

Tetrahedral CH3F POLAR

N2

Linear

NON POLAR

BF3

Triganol Planar

NON POLAR

CH2F2

Tetrehedral

POLAR

C is the central atom

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HCN

Linear

POLAR

C is the central atom

1. Using the molecular dipoles/polarity of BF3, explain why BF3 does not mix with H2O? BF3, h a sf o u ra t o msa n dt hr e ep o l a rbo n d s , a n dat r i g ona lp y r a mi d a ls h a p ewh i c hme a nst h a tt h ebo n dp o l a r i t i e s r e i n f o r c ee a c ho t h e r . Th u s , BF3i sh i g hl yp o l a ra n ds oi swa t e rma ki n gi th a r dt omi x

Submi tyourr e por tont i meandt oyourTA i nt hedr opboxonD2 L.

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