09.17 - When Molecules Dissolve… G a in in te r a c tio n...

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Unformatted text preview: When Molecules Dissolve… G a in in te r a c tio n o f s o lu te s a n d s o lv e n t. L o s e in te r a c tio n o f s o lu te s w ith e a c h o th e r . L o s e in te r a c tio n o f s o lv e n t w ith s e lf. F o r io n ic s o lu te s , fir s t tw o a r e m o s t im p o r ta n t. T h e d is s o lv e d s p e c ie s a r e ( ty p ic a lly ) th e io n s r a th e r th a n t h e io n p a ir ( in ta c t s a lt) Different Ways to Dissolve • Sugar vs. salt • E le c tr o ly te s – S tr o n g – W eak • N o n - e le c tr o ly te s Acids as Electrolytes • H 2 S O 4 , H N O 3 , H C l ( a n d o th e r s ) s h a r e c h e m ic a l p r o p e r tie s ( e .g ., r x n w ith b a k in g s o d a ) • T h e y a r e s tr o n g e le c tr o ly te s • N a 2 S O4 , N a N O3 , N a C l a ls o s tr o n g e le c tr o ly te s • Infer that acids dissociate to form H+ ( R e a c tiv ity o f a c id s a n d c o r r e s p o n d in g s o d iu m s a lts a r e , h o w e v e r , q u ite d iffe r e n t) 1 More Acids & Bases • CH3CO2H (acetic acid, vinegar), weak a c id • NaOH ---> Na+ + OH-, strong base • NH3 + H2O ---> NH4+ + OH-, weak base • H+ + OH- ---> H2O (& vice versa) a c id sour base b itte r Reactions of Ions • Io n s a r e s e p a r a te d , s o tw o c h o ic e s : – S ta y s e p a r a te d – C o m b in e a n d p r e c ip ita te • A c tu a lly , b r o a d m id d le g r o u n d th a t y o u s h o u ld ig n o r e fo r n o w • B y a n d la r g e , a fu n c tio n o f p a ir w is e in te r a c tio n s o f th e io n s Keep track of the ions! Ba( NO3 )2 + K2CrO4 ----> BaCrO4 + 2 K NO 3 Ba2+ + 2 NO3- + 2K+ + CrO42- ---> BaCrO4 (s) + 2 K+ + 2 NO3 137 62 39 116 253 39 62 C a lc u la te lim itin g r e a g e n ts , t h e o r e tic a l y ie ld s , % y ie ld s , e tc … ? O n c e d is s o lv e d , in d iv id u a l io n s a r e th e im p o r ta n t q u a n titie s 2 Rules/Guidelines to Know p . 1 0 2 in te x t. O v e r ly s im p lis tic ( s e e C h . 8 ) , b u t u s e fu l. Molecular View Application How to isolate Ag+, Ba2+, Fe3+ from a common solution? 3 N o te th e u s e o f re a g e n ts th a t a r e in itia lly s o lu b le . D id n o t s ta r t w ith h y d r o x id e . W h a t if o r ig in a l m ix tu r e w e r e Ag+, Pb2+, Fe3+? Quantification • 4 0 0 m L o f 0 .2 5 M N a C l g iv e s 1 0 .2 g o f A g C l . S h o u ld y o u a d d m o re N a C l ? If N a C l w e r e lim itin g : 0 .4 0 0 L * 0 .2 5 M = 0 .1 m o l N a C l , y ie ld s 0 .1 m o l A g C l * 1 4 3 .4 g /m o l = 1 4 .3 g A g C l A n s w e r : N o . A g + is lim itin g r e a g e n t ( a n d m u s t b e g o n e ) Not just an exercise… ++C12H 19N 2P dCl ----> C12H19N2Pd(CF3 S O3) ? s tro n g weak Answer: Use Ag (CF3SO3 )! A gCl forms and precipitates. J e n n ife r H a w k ( D u k e , P h .D . p r o g r a m ) u s in g th e r e v e r s ib ility o f th e w e a k b o n d to c r e a te s e lf- r e p a ir in g m a te r ia ls . 4 Reaction Types • P r e c ip ita tio n r e a c tio n s ( c o m b in a tio n ) ; b e tte r in c lu d e d is s o lu tio n r e a c tio n s , to o . • A c id /b a s e ( tr a n s fe r o f p r o to n ) • O x id a tio n /r e d u c tio n ( tr a n s fe r o f e le c tr o n ) 5 ...
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This note was uploaded on 05/31/2009 for the course CHEM 21L taught by Professor Roy during the Fall '08 term at Duke.

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