112 - Chemistry Assignment PDF

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Chemistry Assignment...

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Experiment*#2.*Lewis*Structures** *

A!Lewis!structure!shows!how!the!valence!electrons!are!arranged!and!indicates!the!bonding!between!atoms!in!a! molecule.!We!represent!the!elements!by!their!symbols.!The!shared!electron!pair!is!shown!as!a!line/bond!between!the! two!atoms.!All!the!other!valence!electrons!are!shown!as!dots!or!lines!around!the!symbol!of!the!element.! For!Example:!!The!Lewis!structure!for!Cl2!is! ! Let!us!now!see!how!to!draw!the!Lewis!structure!for!CO2! !

Steps*for*drawing**Lewis*Structures* 1. Sum!the!valence!electrons!of!all!the!atoms.! • For!anions,!add!one!electron!for!each!negative!charge.! • For!cations,!subtract!one!electron!for!each!positive!charge.!! 2.!!!!Choose!the!least!electronegative!element!(other!than!H)!as! the!central!atom.! • For!a!molecule!of!type!XYn,!X!is!always!the!central!atom.!

Example*with*CO2* The!total!number!of!valence!electrons!is!16!(4!from! carbon!and!6!from!each!oxygen).*

3.!!!!Use!single!bonds!(lines)!to!connect!the!central!atom!to!the! surrounding!atoms.!

Attach!the!two!oxygens!to!carbon!by!single!bonds.! O!–!C!–!O! We!subtract!4!electrons!(2!for!each!bond)!from!16! leaving!12!electrons!to!distribute!to!the!remaining! atoms.!

4.!!!!Subtract!2!electrons!for!each!bond!from!the!original!total! number!of!electrons.!!

Choose!carbon!as!the!central!atom!since!it!is!less! electronegative!and!also!follows!the!XYn!rule.! *

5.!!!!Complete!the!octet!for!all!outer!atoms!(other!than!H).!Each! bond!to!an!atom!will!count!as!2!electrons!for!that!atom's! octet.! ! *

Complete!the!octet!for!each!of!the!oxygen!atoms! which!uses!the!last!12!electrons.!(We!need!to!add!6! electrons!to!each!oxygen.)!!

6.!!!Complete!the!octet!for!the!central!atom.!

There!are!no!more!electrons,!so!we!cannot!add! more!to!the!central!atom!to!complete!its!octet.!

7.!!!!If!you!run!out!of!electrons!before!the!octet!for!the!central! atom!can!be!completed,!start!forming!multiple!bonds!until! the!central!atom!has!a!complete!octet.!!When!you!form!a! multiple!bond,!remember!to!remove!an!electron!pair!from! the!outer!atom.*

Since!carbon!does!not!have!an!octet,!form!multiple! bonds!by!taking!the!lone!pairs.!

*

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! becomes** ! The!central!atom!is!still!electron!deficient,!so!share! another!pair.* becomes********* *

8.!!!!Make!sure!the!correct!number!of!electrons!has!been!used! 4!bonds!+!2!electron!pairs!equal!16!electrons.!All! and!that!every!atom’s!octet!is!complete.!! atoms!have!an!octet.! !!!!!!!!The$exceptions$are$atoms$of$group$2$and$3$which$can$have$ incomplete$octets$and$atoms$in$period$3$or$greater$which$can$ ! have$expanded$octets.!

! * Molecular*Geometry*–*the*Valence*Shell*Electron*Pair*Repulsion*(VSEPR)*Theory! The!electron!groups!around!the!central!atom!repel!each!other!and!therefore!prefer!to!be!as!far!apart!from!each!other!as! possible.! This! is! the! main! idea! of! the! VSPER! theory.! ! We! can! apply! the! VSEPR! theory! to! predict! the! molecular! shape/geometry!of!a!molecule.! 1. Draw!the!Lewis!structure!for!the!molecule!in!question.! 2. Count! the! total! number! of! electron! groups! on! the! central! atom.! ! Add! the! number! of! atoms! bonded! to! the! central! atom! and! the! number! of! lone! pairs! on! the! central! atom! –!this! is! the! total! number! of! electron! groups.! Note!that!multiple!bonds!to!one!outer!atom!still!count!as!one!electron!group.! 3. The!arrangement!of!the!electron!groups!is!determined!by!minimizing!the!repulsions!between!them.! 4. Remember!that!lone!pairs!require!more!space!than!bonding!pairs.!Therefore,!choose!an!arrangement!that!gives! lone!pairs!as!much!room!as!possible.! ! ! Electron! Groups! 2!

0!Lone!Pairs!

1!Lone!Pair!

2!Lone!Pairs!

3!Lone!Pairs!

4!Lone!Pairs!

linear*(180o)! ! ! ! trigonal*planar! (120o)!

!

!

!

!

bent*

!

!

!

bent*

!

!

linear*

!

3! ! ! tetrahedral!(109o)!

trigonal*pyramid*

4! ! trigonal*bipyramid! (120o,!90o)!

!

! seesaw*

THshaped*

5! !

!

!

!

octahedral(90o)!

square*pyramid*

square*planar*

THshaped*

linear*

6! ! ! ! ! ! ! ! ! ! ! !

!

!

!

!

! ! Polarity*of*Molecules* A!covalent!bond!is!polar!if!there!is!a!difference!in!electronegativity!between!the!bonded!atoms.!!A!molecule!like!HCl!has! a!polar! covalent!bond! since!there! is!a! difference!in! electronegativity!between! the!two! atoms! (the! difference!is! greater! than! 0.4! and! less! than! 1.8).! ! Thus! HCl! possesses! a! permanent! dipole! moment! because! the! molecule! has! a! distinct! negative!end! and!a!distinct!positive!end.!! !However,!just!because!there!is! a!polar!bond! present!in!a! molecule!does! not! necessarily!mean!that!the!molecule!is!polar.!!If!all!the!dipoles!in!the!molecule!cancel!each!other!out,!then!the!molecule! will!be! nonepolar.! !For! example,! CO2! has!two! polar!bonds,! but! they! point! in!opposite! directions! and! cancel! each! other! out.! ! For! simple! molecules,! polarity! can! usually! be! determined! by! looking! for! lone! pairs! on! a! central! atom! or! noneidentical! outer!atoms.!!Both!of!these!things!cause!uneven!electron!distribution!and!generally!lead!to!polar!molecules.!!! ! ! Polar!molecules!–!bond!dipoles!do!not!cancel.!!! Have!a!lone!pair!on!the!central!atom!AND/OR!different!outer!atoms.! ! Nonepolar!molecules!–!bond!dipoles!cancel.!!!! Have!no!lone!pairs!on!the!central!atom!AND!have!all!outer!atoms!the!same.!! ! ! Condensed*Structural*Formulas* Organic!(carbonebased)!molecules!can! contain! chains!of! several! central!atoms.! !Condensed!formulas! are!often!used! to! indicate!how!the!atoms!are!connected.!!To!draw!a!Lewis!structure!from!a!condensed! structure,!connect! any! elements! that! are! not! hydrogens! or! halogens! in! a! chain! in! the! order! written.! Connect! the! hydrogens! and! halogens! with! single! bonds! to! the! atom! they! are! written! next! to.! ! Count! valence! electrons! and! add! lone! pairs! and! double/triple! bonds! to! complete!octets!as!needed.! * * Molecular!Formula!!!! Condensed!Formula!!! ! Lewis!structure! ! H

H H

C3H8O! ! !

C3H8O! ! !

C3H2NF3! !

!

!

!

!

CH3CH2OCH3!!

!

CH3CH2CH2OH! !

CF3CH2CN!

!

!

H C C O C H H H H !

!

H H H H C C C O H H H H !

!

F

H

F C

C

F

H

C N

!

! ! * *

Name_________________________! *

*

CHM112*Lab*–*Lewis*Structures*–*Grading*Rubric! * Criteria* *

* Points*possible*

! Question!1!(1.33!points!each!question)! * ! Question!2!(1!point!each!question)! ! ! Question!3!(1!point!each!question)! ! * Question!4!(1!point!each!question)! ! ! Question!5!(1!points!each!question)! ! * Total* *

! ! *! * Subject$to$additional$penalties$at$the$discretion$of$the$instructor.$ $

!

* Points*earned*

! 8!

*

! 4!

*

! 2!

*

! 4!

*

! 2!

*

! 20!

*

CHM*112:*Lewis*Structures*Lab***********************************************************************************************Name*________________________! ! 1.!!!Fill!in!the!table!below.! ! Lewis*Structure!(redraw!to!reflect!geometry,!show!net! Polar?!Y/N! Geometry* dipole!direction!if!polar)! CH2Cl2! ! ! ! ! ! ! ! ! ! CBr4! ! ! ! ! ! ! ! ! ! CBr2O! ! ! ! ! ! ! ! ! ! NBr3! ! ! ! ! ! ! ! ! ! CH3CH2OH! ! ! at!C?! ! ! ! ! ! at!O?! ! ! ! C5H10!O! ! ! at!C?! (connect!C’s! ! ! &!O!in!a!6e ! ! membered! ! at!O?! ring.!!H’s!are! ! attached!to! ! C’s!)! ! ! ! ! !

! 2.!!Draw!the!Lewis!Structures!of!each!of!the!following!acids:!!(These!structures,!like!most!acids,!have!each!H!attached!to! one!of!the!oxygens.! (A)!!!H2SO4! ! ! ! ! ! (B)!HIO2! ! ! ! ! ! ! (C)!!H3PO3! ! ! ! ! ! (D)!CF3CO2H!(Both!O!atoms!are!attached!to!second!C)! ! ! ! ! ! ! 3.!Draw!the!Lewis!Structure!for!these!anions.!!Each!anion!is!a!conjugate!base!of!an!acid.! (H!attached!to!one!of!the!O’s)! ! (F)!!SO3 2─! (E)!!HCO3─! ! ! ! ! ! 4.!!Weak!bases!frequently!contain!N:!(nitrogen!with!a!lone!pair!of!electrons).!!Draw!the!Lewis!Structure!for!each!of!the! following!weak!bases.! (A)!NH3!!! ! ! ! ! ! (C)!CH3NH2!!

!

!

!

!

!

(B)!!CH3CONH2!!(Connectivity!is!!!C

O C

N !!)!

!

(D)!!C5H5N!(six!membered!ring!made!of!C!and!N,!Hs!attached!to! Cs)! ! ! !!

! ! ! ! ! 5.!!!!Draw!the!Lewis!structures!of!the!following!cations.!Each!cation!is!the!conjugate!acid!of!a!weak!base:!!! (B)!CH3NH2CH3+! (A)!!NH4 +! !

!

!

!!...