E o and the equilibrium constant, K

Transcription

1 lectrchemical measuremets (Ch -5 t 6). T state the relati betwee ad K. (D x -b, -). Frm galvaic cell vltage measuremet (a) K sp (D xercise -8, -) (b) K sp ad γ (D xercise -9) (c) K a (D xercise -G, -6) (d) K f (D xercise -H, -I) (e) K f ad K a (D xercise -) Ptetimetric redx titrati (Ch 6-) T perfrm calculatis fr ptetimetric redx titrati (D xercise 6-) Chem5/P.Li/electrchem & redx/p ad the equilibrium cstat, K c b.596 A C.596 AB lg lg a d AA AD (.596 A A ) lg A c d C D a b A AB.596 lg Q Whe the cell is at equilibrium, Q K ad with curret flw. xample:.596 lg K Cathdic reacti: Adic reacti: Cd aa.99 (.).V e cc dd e Ptetimetric measuremets allw us t fid K that are t large r t small..596 /.596 lg K r K K bb Ag e Ag(.99V e Cd (. V (.) / Chem5/P.Li/electrchem & redx/p

2 lectrchemical cells as chemical prbes. quilibrium betwee half-cells ad the et reactis are t redx reactis Slubility prduct determiati CO ( e e ( CO (.56V.V CO( CO.56 (.). 6V K K sp (.6 ) /.596 is just ather way f expressig the free eergy chage r equilibrium cstat f a reacti G F ad G RT l K Chem5/P.Li/electrchem & redx/p lectrchemical cells as chemical prbes. quilibrium withi each half-cell If a galvaic cell has a -zer vltage, the the et cell reacti is t at equilibrium. Hwever, we d allw the half-cells t stad lg eugh t cme t equilibrium withi each half-cell. (A) Acid-base equilibrium cstat determiati Pt( H (.atm) H ( xm ), CH COH (.5M ), CHCO Na(.5M ) Cl (.M ) AgCl( Ag( Chem5/P.Li/electrchem & redx/p.5v H ( aq, χm ) e H ( g, atm) V The et cell reacti is t at equilibrium because the measured vltage is.5v, t V. AgCl( e P. lg [ Cl lg [ H lg(.) lg [ H H.5 [ H.8 M This allws us t evaluate the equilibrium cstat fr the acetic acid dissciati K a Ag( Cl ( aq,.m ) [ CH CO [ H (.5.8 ) (.8 ) 5.8 [ CH COH (.5.8 ) The adic half-cell (the left e) may be thught f as a prbe t measure the ukw [H, leadig t the evaluati f K a..v

3 lectrchemical cells as chemical prbes (B) Cmplex frmati cstat determiati S. H.. HgY ( aq,.5m ), DTA( aq,.5m ). V Hg e.85v H e H ( g) V lg. [ Hg.6 [ Hg. 8 M This value allws us t evaluate K f [HgY -. K [ HgY [ Hg α [ DTA.5 f 8 5 (. )(. Y 6 )(.5) Re call 5 α. at ph 6 Y Chem5/P.Li/electrchem & redx/p 5 Ptetimetric redx titrati Titrati f. ml f.5m with.m. Titrati reacti : (6 ) K i M HClO This is a rapid ad quatitative reacti. Nw, we eed t perfrm sme ptetimetric calculatis i mitrig [ /[ fr the establishmet f a titrati curve. Chem5/P.Li/electrchem & redx/p 6

4 Ptetimetric redx titrati At the S..: ½Hg Cl ( e - Cl - (aq) -.V At the Pt idicatr electrde: At the Pt electrde, reactis 6- ad 6- are bth i equilibrium..596 [.6 lg (6 9) [ Nte that there is ly e value f.596 [.. lg (6 ) [ The lie tati f either cell () r (): () e e Hg Cl ( Cl.6V (6 ).V (6 ), Pt( () Hg Cl ( Cl, Pt( There are regis i the titrati curve. Chem5/P.Li/electrchem & redx/p Ptetimetric redx titrati ) Befre P (e.g. v 6. ml) Sice there is excess, ad bth [ ad [ are kw, it is cveiet t use equati 6-9 t calculate. [ S..).6.596lg [ (..5M.L.M.6L lg.M.6L lg.55V 6. Chem5/P.Li/electrchem & redx/p 8

5 Ptetimetric redx titrati ) At P (e.g. v e 5. ml) [ [ (6 ) [ [ lg lg [ [ [ [.6.596lg.6.596lg. [ [ There will be tiy amut f ad preset because f the fllwig equilibrium. [ [ (6 8) Sice [, [, [ ad [ are t kw, we must use bth equatis 6-9 ad 6- t calculate. We add the tw equatis, v e.m.ml. 5M After the cmplete reacti f ad, equal amut f ad shuld be prduced. 6. V Nte that is just the average f f equatis 6- & 6-. ( S..) (..) V. 99V Chem5/P.Li/electrchem & redx/p 9 Ptetimetric redx titrati ) After P (e.g. v 6. ml) Sice there is excess, ad bth [ ad [ are kw, it is cveiet t use equati 6- t calculate. [ S..)..596lg [ (..5M.L lg.M.6L.5M.L lg.V. See Fig. 6- fr a ptetimetric redx titrati curve fr with. Nte the cell vltage at ay pit depeds ly the rati, but t abslute values, f the ccetratis f the reactats. We shuld expect t see the same curve if bth reactats (e.g. ad ) are diluted by a factr f. Chem5/P.Li/electrchem & redx/p 5