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LewisA,cid-Base'Conc,ept

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Learniing Outcomes:S tudents w ill be able to :1.,sta'te and explain Lewiis concepts ofacids and l bases.

2,.,'cllassilfy substances as Lewis acids orbases.3.,explain this self ionization ofwater.

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I' IIn 1923 I G I .N. Lewis, Almlerlcan Ch'elml~5t:j 'put fo~a,rd a more,g;,en,erall.~ed a,cid~bas,econcept.Aocording to this concept,

An acid=.~II~Sa species (charged l e n unchargadl)1 whiich canaccept a pai r ,of electron a, M

- !I

'. A base:'11 IISspee les (charged lor uncharged) ,wh ilch can.~donate & 1 pair , o f electron ."

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' ' ' '

II ~/ ' l t i . ~ ~ ~ : : ~ . . , . . . . . . .~ l - ~ '/~;'I

~ I ~1 ' 1 ~ I " " ' . ! ii'li

I Ii!' _!.II - - -, .' -. -_

, , ' 'iii II

/ - 1 1f l~;: : - \ -~L\ ,= ~[4!O!~ a~~~'1.J-~rec . -(f'~I,i's !f;:J{j~$' -

" - J , N ttu l.-e .~ '"t o " " ty~ vtt : J . . rr:~ L e l N , ' r r / 5 , t : t , ' i ' s .

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I fil',.,

ThulS, a Lewis add is a n e~ectrophille, and ,8 L!ewis baseis ,8 nucil,eop hiile,.Fo,r'example:r'~ H3N: + --------..,. ,( H3N':~ B ' F 3 ' ) ~

P r o l d U I C ' tI(am m oln ia-b ere n ,t rifluer ide)a O t i l e(blof'lon ' t l r i l ' f r 1 I U : l ! l t d c h ~ i )

'-..J In theabo 'Ve reac tion, INH3 is ,a,donor a ,f e lectron' " pai~t rand 1 B F 3 Gu:=oept~8 pair 1 0 0 f edectron. Sa"__ --.z_ __ _ _ _ _accordinig to the L !ewJs concept"N IH I:3, is a ILew~sbase, and BF3, i s , a l L e w i l : s , a c i i d . ,

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T he boron atom in boron trffluride, B F 3 . , has on~ lys ixele ctrons in ilts va lence shell, Consequently ; B,F31~S,8very glo'o,d Le:w:i.sacid and reacts wilth many Lew'isbases ; fl~uoride , lo n ~:S~he,I~ewilsbase ln the reacno n:- [eF41.A'c~dl-baseachjiuct

. I~ !I!' : . F . ,+ B I F aL e 'W :E S Lewis

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In the fOlllowingl reac tion, each of two ammon iam olecu les i.e , a Lewis base, donates ,8 pair ofe,~ec~r,ono a, silverlo n L ,8. Lewis acid ,. .. 2 IH 3 N ID + Ag+L,ewils Llewlsbase acid

'l+[ I H 3 N - A g - N H : 3 : JAciid!..base adduct

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-In the following example, inwh ilch four amm oniamol~ecul~es,serve as Lew is bases, E ach ammoniam oillecu l~ed onates ,e pa ir of , e i l , s e t r , o n to a copper ion~hatserves asthe lew~s , B !c i id l .

._ . ,. . . . . . . . .

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Non-metal oxides act, as, Le'wis, acids react wiith, o , x i d l e i l'O ,n s . ; th e Lew is base, to fo r m oxya n l i o n s , .

I---"_

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C:las,siific,.,ti'on of' s,ubstan,'c,es as, LewiisAci'dls anldB,.,s,e's::ILewlis ,Aciids,:,~"1.Compounds having ~essthan ,8 full octet of,e~lec~ronsbehave as Lew lj:s acids.

B 1 F 3, 8 '0 31 ' AI 'Cb etc,, ,-,........_, _ ', 2 . Positive ~onsare often consid ered as Lew is acid s,~ , H I ' T _ A..g. .. . + - . Co..+ " . C i ' : I U 2 : + etc,_, , ~ _ , '.. I",' . , .. ,. _' " " 3 ,. A 1 1 1 1oiecuTe:s inw h ~ c h t h e ! central a t o m lean

a cq IU ire m a re tha n eig h t va Ie n ce e lectro ns ..' ~ C I I 1 4 : I ' , S ' i F 4 " " S I Q Q J 4 etc,"4 Mo~ecUieswith mult~p~ebonds between atoms ofd i ii f fe : r : e n t elsctroneq at iv ities...'C02 !1 802 etc",

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L,ew Basle.,::Th e fo!III,owii'n'giy lpes of compounds orspeciesare L,ewis, bases,./1" A llIIanions a:re Lawi': s bases .OH-~CN-,~'CH3C , O ' O - etc,

~ 2 :. ,A J ll lmolecules, havinlg lone-pairs olf electrons.:NH3, H 2 0 ' : : . ~ 'CQ ,: :: "and am lnes e t c . " ,

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i!i I,S,.lf Iioniz,a'tion of 'Wate'r::W,ater, aswe know, is a unique solvent, O ne

of its spscia I ~'P rope r t : i ! 9 S is i t s , ab i ~ l ~ t y to act both as ,9nacid and asa base.Water functions as,a,base inreactions with aeds such as HICI~andl'CHI3I C ' O O H I ,and it functions as an acid in reactions wi~h basessuch as Nl+, Ws'ter is a very weak electrolyte,therefore a poorconductor of ,elec,~ri'ci~YJbut it doesunderg o ionization toa s i m a l l l i l l exte'ntHi-(a~~O +OH-(,a,q)

,=.. -I

T his reaction is som etsnes caned the eutoonizstionof water. T o d escribe the acid -base

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prope rties o fwa te r inthe Bronsted fram ework, we,express, its autoionization as,

I~

acid2acid 1 base~1

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Multip eChoiceQuestions

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A . , H,20a N I H l sC. H +D . , C I I -

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.2.., Whilch 0 ' " t h e , _ ' l I Io w i in g i i s NO IT a I L . e w i s ,Base?

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3~Whli,ch o f the, f o l l l l o : w i i n g reactionsrepresent Lewis-acid-base reaction?A~CH4,+202 ~e. , 4 , I F e +3,02 - + ,C'~PbC'03 - + .ID . , A I I 'C ,: 1 3 , + , N H '3 +

C O 2 , + , 2 _ ,202Fe2 'OaPbO'+C02AICI:sNIH3,