Redox Reaction - the basics. Ch Electrochemistry. Redox Reaction - the basics. Important Redox Titrants and the Reactions

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1 Redox Reactio the basics Ch.46 lectrochemistry ox red <> red ox Oxidizig Aget Reducig Aget Redox reactios: ivolve trasfer of electros from oe species to aother. Oxidizig aget (oxidat): takes electros Reducig aget (reductat): gives electros Redox Reactio the basics Balace Redox Reactios (Half Reactios) Reduced Oxidizig Aget Reducig Aget Oxidized ox red <> red ox. Write dow the (two half) reactios.. Balace the (half) reactios (ass ad Charge): Redox reactios: ivolve trasfer of electros from oe species to aother. Oxidizig aget (oxidat): takes electros Reducig aget (reductat): gives electros a. Start with elemets other tha H ad O. b. Balace O by addig water. c. balace H by addig H. d. Balacig charge by addig electros. (3. ultiply each half reactio to make the umber of electros equal. 4. Add the reactios ad simplify.) 3 V V 3 xample: Balace the two half reactios ad redox reactio equatio of the titratio of a acidic solutio of Na C O 4 (sodium oxalate, colorless) with KO 4 (deep purple). O 4 (aq) C O 4 (aq) (aq) CO (g) 6H (aq) O 4 (aq) 5C O 4 (aq) (aq) 8H O(l) CO (g) xample: Balace S 3 <> S 4 O 4 <> 3 Importat Redox Titrats ad the Reactios Oxidizig Reagets (Oxidats) () Potassium Permagaate O 4 8 H 5e 4H O O4 4 H 3e O ( s) H O O e O 4 4

2 Importat Redox Titrats ad the Reactios Oxidizig Reagets (Oxidats) () Potassium Dichromate Importat Redox Titrats ad the Reactios Oxidizig Reagets (Oxidats) (3) Potassium Iodate 3 CrO7 4 H 6e Cr 7H O 4 3 Cr O7 3U H Cr 3UO HO IO3 6 H 5e I 3H O Importat Redox Titrats ad the Reactios Reducig Reaget ( Reductats ) () Potassium Iodide Importat Redox Titrats ad the Reactios Reducig Reaget ( Reductats ) () Sodium Thiosulfate I I e S O 3 S 4 O 6 e Galvaic lls Compoets Galvaic lls Lie Notatio A galvaic (voltaic) cell uses a spotaeous chemical reactio to geerate electricity. lectrodes (cathode ad aode). Salt bridge: catios move from aode to cathode, aios move from cathode to aode. Lie otatio Cd(s) Cd(NO 3 ) (aq) AgNO 3 (aq) Ag(s) Phase boudary Salt bridge Phase boudary

Cathode Reductio Reactio H ( ) e o 0 V H ( atm) Stadard Reductio (Halfll) Potetials The S.H.. is the cathode. It cosists of a Pt electrode i a tube placed i H solutio. H is bubbled through the tube.

3 Stadard lectrode Potetials Stadard hydroge electrode (S.H..) Stadard reductio potetial ( o ) is the voltage associated with a reductio reactio at a electrode whe all solutes are ad all gases are at atm. Cathode Reductio Reactio H ( ) e o 0 V H ( atm) Stadard Reductio (Halfll) Potetials The S.H.. is the cathode. It cosists of a Pt electrode i a tube placed i H solutio. H is bubbled through the tube. For the S.H.., we assig H (aq, ) e H (g, atm) red of zero. The potetial of a cell ca be calculated from stadard reductio potetials: ( cathode) ( aode) cell red red Stadard Reductio (Halfll) Potetials Cosider Z(s) Z (aq) e. We measure cell relative to the S.H.. (cathode): cell red (cathode) red (aode) 0.76 V 0 V red (aode). Therefore, red (aode) 0.76 V. Stadard reductio potetials must be writte as reductio reactios: Z (aq) e Z(s), red 0.76 V. Stadard Potetials We use hydroge (S.H..) H (aq) e /H( g) We ca measure º for other halfreactios, relative to the hydroge reactio, e.g. for silver: Ag (aq) e Ag(s) Stadard reductio potetials are listed i Ap. H i your book. º for the H reactio is for the reactio at 5 º C Nerst quatio for a HalfReactio The Nerst quatio relates the potetial of the half reactio to reaget cocetratios For the halfreactios: a A e bb RT l F b [ B [ A a a A bb e cc dd RT F l Q # of moles of electros RT l F c d [ C [ D [ A a [ B b 3

Nerst quatio for a HalfReactio Nerst quatio for a Complete Reactio At 98K (5 o C) 0.

4 Nerst quatio for a HalfReactio Nerst quatio for a Complete Reactio At 98K (5 o C) V logq ox red <> red ox xample (Nerst) Write the Nerst equatio for the reductio of phosphoric acid to solid white phosphorous: 3 PO4 5H 5e /4P4 ( s) 4H O.40 H log 5 [ H PO [ H Note that multiplyig the reactio by ay factor does ot affect º or the calculated : H PO4 H e /P4 ( s) 8HO log [ H PO [ 3 4 H.40 xample (Net Reactio) Fid the voltage for the AgCd cell ad state if the reactio is spotaeous if the right cell cotaied 0.50 AgNO 3 (aq) ad if the left cotaied 0.0 Cd(NO 3 ) (aq) ) Ag e Ag( s) V Cd e Cd( s) 0.40 V ) 3) 4) 5) Ag e Ag( s) Cd e Cd( s) log 0.78V [ log 0.46V [ ( 0.46).4 V Cd( s ) Ag Cd Ag( s) Determiatio of the quivalece Poit Redox Titratio Curve A A red [ A log [ A ox A ox B red <> A red B ox At equivalece poit, cell0: A B ad B red red ox A o A A A ox B at the equivalece poit B B [ B log [ B o B B red ox Valid for simple Redox expressios XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. 4

5 XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. Titratio reactio: 4 <> e <> 4 e <> 3 o V o.70 V At 0.00 ml of 4 added, iitial poit o 4 preset; miimal, ukow [ 3 ; thus, isufficiet iformatio to calculate [ log [ XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4.Potetial of saturated calomel electrode is 0.4 V. 4 <> 3 3 At 5.00 ml of 4 added, V > V Buffer regio 3 V [ V V (5.00mL)(0.00 ).308 ( ) ml XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. 4 <> 3 3 At 5.00 ml of 4 added, V > V Buffer regio [ V V [ V V (50.00mL)( ) (5.00mL)(0.00 ).54 ( ) ml XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. 4 <> 3 3 At 5.00 ml of 4 added, V > V Buffer regio Halfreactios: 3 e <> o V 3 [.308 [ [ log [ log V XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. 4 <> 3 3 At 5.00 ml of 4 added, V V, quivalece poit Halfreactios: 3 e <> o V 4 e <> 3 o.70 V V 4 <> 3 3 At 6.00 ml of 4 added, V < V, After equivalece poit 3 V [ V V (50.00mL)( ) 3.9 ( ) ml 5

XAPL: Derive the titratio curve for 50.00 ml of 0.0500 with 0.00, 5.00, 5.00, 6.00 ml 0.00 4 i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. 4 <> 3 3 At 6.

6 XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. 4 <> 3 3 At 6.00 ml of 4 added, V < V, After equivalece poit 4 V V [ V V [ (6.00mL)(0.00 ) (50.00mL)( ).3 ( ) ml 3 XAPL: Derive the titratio curve for ml of with 0.00, 5.00, 5.00, 6.00 ml i a medium that is.0 i HClO 4. Potetial of saturated calomel electrode is 0.4 V. 4 <> 3 3 At 6.00 ml of 4 added, V > V After equivalece poit Halfreactios: 3 e <> o V 4 e <> 3 o.70 V [ 3.9 [ [ log [ log V Theoretical curve for titratio of 0.0 ml of with i HClO 4. (p.33033) [ log [ log V Galvaic lls A galvaic (voltaic) cell uses a spotaeous chemical reactio to geerate electricity. lectrodes (cathode ad aode). Salt bridge: catios move from aode to cathode, aios move from cathode to aode. produces electricity whe the cell reactio is ot at equilibrium. Free ergy ad lectrochemical Reactio The free eergy chage, G, for a chemical reactio at costat T, P equals the maximum possible electrical work that ca be doe by the reactio o its surroudigs. Work G q G F A spotaeous reactio ( G<0) >0 Thik: how the equilibrium costat ca be related to free eergy? ad quilibrium Costats A galvaic cell produces electricity because the cell reactio is ot at equilibrium At equilibrium, for the et reactio ca thus be related to K aa e cc dd e dd.0596 log c [ C a [ A.0596 log b [ B [ D d log c d [ C [ D a b [ A [ B log K at 5 C logq K Whe the cell is at equilibrium, 0 ad QK at 5 C at 5 C 6

xample: Fid the equilibrium costat Cu( s) 3 Cu This ca be divided ito two half reactios: Cu 3 e ( s) e Cu( s) 0.77V 0.77 0.339 0.43 V 0.0596 ()(0.43) (0.0596) o 0.

V o 0 V The halfreactio that you write must ivolve species that appear i two oxidatio states i the cell.

complex formatio, ad acidbase chemistry () Write halfreactios ad their stadard potetials ()Write Nerst equatio for the et reactio ad put i all the kow quatities.

7 xample: Fid the equilibrium costat Cu( s) 3 Cu This ca be divided ito two half reactios: Cu 3 e ( s) e Cu( s) 0.77V V ()(0.43) (0.0596) o V K 4 4 Two quilibria: lls as Chemical Probes () quilibrium betwee two halfcells () quilibrium withi each halfcell AgCl( s) e Ag( s) Cl ( aq,0. ) H ( aq,? ) e H ( g,.00bar) 0. V o 0 V The halfreactio that you write must ivolve species that appear i two oxidatio states i the cell. The reactio is i equilibrium i the right cell is ot the et cell reactio: AgCl( s) Ag ( aq) Cl ( aq) Survival Tips Importat Biochemical Reactios lectrochemistry, chemical equilibrium, solubility, complex formatio, ad acidbase chemistry () Write halfreactios ad their stadard potetials ()Write Nerst equatio for the et reactio ad put i all the kow quatities. (3) Solve for the ukow cocetratio ad use that cocetratio i the chemical equilibrium equatio to solve the problems. Formal potetial, º, meat to defie potetials uder coditios of biochemistry x. p.8687 Potetiometry Potetiometry: the use of electrodes to measure voltages from chemical reactios. lectroactive species: ca doate or accept electros at a electrode; ca be measured as the part of a galvaic cell (aalyte) Referece electrode: we the coect the aalyte halfreactio to a secod cell with a fixed compositio (kow potetial), the d halfcell is called referece electrode. Idicator electrode: respods to aalyte etal electrodes: iert metals, e.g., Pt, Au Ioselective electrodes: respod to specific aalytes S.H.. (agai) The stadard reductio potetial, o, for each halfcell is measured whe differet halfcells are coected to S.H.. S.H.. Ag (aq. ) Ag(s) Stadard meas that the activities of all species are uity. Not practical for regular use due to the hydroge gas 7

Referece lectrodes Referece lectrodes Detect / 3 i solutio: a Pt wire (idicator electrode) i the halfcell ad coect this half cell to a d halfcell at a costat

77V AgCl( s) e Ag( s) Cl The etire left halfcell cotaiig appropriate solutios ad a salt bridge. 0.

97 V The differece i is due to activity coefficiets.

4V Voltage Coversios betwee Differet Referece Scales Idicator lectrodes etal lectrodes A idicator electrode has a potetial of 0.35 V with respect to a S.C.., what s its potetial with respect to a S.

8 Referece lectrodes Referece lectrodes Detect / 3 i solutio: a Pt wire (idicator electrode) i the halfcell ad coect this half cell to a d halfcell at a costat potetial. 3 e 0.77V AgCl( s) e Ag( s) Cl log Cl log [ 3 [ [ Referece lectrode 0. V Idicator lectrode 3 e 0.77V AgCl( s) e Ag( s) Cl The etire left halfcell cotaiig appropriate solutios ad a salt bridge. 0. V SilverSilver Chloride lectrode Saturated Calomel lectrode (S.C..) Ag AgCl lectrode : AgCl( s) e Ag( s) Cl w/ saturated KCl 0. V 0.97 V The differece i is due to activity coefficiets. A double juctio electrode: avoid to mix Cl with the aalyte Hg Hg Cl lectrode: /Hg Cl ( s) e Hg( l) Cl w/ saturated KCl 0.68 V 0.4V Voltage Coversios betwee Differet Referece Scales Idicator lectrodes etal lectrodes A idicator electrode has a potetial of 0.35 V with respect to a S.C.., what s its potetial with respect to a S.H..? 0.35V 0.4V 0.V etal electrodes: develop potetial i respose to a redox reactio o their surface Pt is mostly iert, ot participatig i reactios It simply allows electro trasfer to/from solutio Platium is the most commo metal idicator electrode Gold is also a iert metal idicator electrode Carbo electrodes are ofte used because may redox reactios are very fast o a carbo surface 8

Idicator lectrodesetal electrodes etal electrodes: develop potetial i respose to a redox reactio o their surface Iert metal

Silver Idicator lectrode Ag ( s) e Ag( s) 0.799 V /Hg Cl( s) e Hg( l) Cl 0.4V 0.799 0.059log 0.4 [ Ag [ 0.558 0.

0 ml? Ag Cl AgCl( s) 0.0 ml V e [ 0.558 0.0596log Ag 0.0 [ Cl (0.350)(0.0 ) 0.0 K [ Ag sp [ Cl.8 0.0 65.0 8.5 9 0.558 0.0596 log (8.

08V 9 Juctio Potetial Io obilities ad Liquid Juctio Potetials Juctio potetial: A voltage differece develops wheever dissimilar

A major (fudametal) source of error i a potetial measuremet.

potetial across the membrae depeds o [aalyte is measured by two referece electrodes 0.

9 Idicator lectrodesetal electrodes etal electrodes: develop potetial i respose to a redox reactio o their surface Iert metal idicator electrode: allows electro trasfer to/from the solutio but ot participatig i reactios, e.g., Pt (the most commo oe), Au. Silver Idicator lectrode Ag ( s) e Ag( s) V /Hg Cl( s) e Hg( l) Cl 0.4V log 0.4 [ Ag [ log Ag xample: 0.0 ml solutio cotaiig 0.0 NaCl was titrated with 0.0 AgNO 3 ad moitored with a S.C.. What voltage readig would be observed after 65.0 ml? Ag Cl AgCl( s) 0.0 ml V e [ log Ag 0.0 [ Cl (0.350)(0.0 ) 0.0 K [ Ag sp [ Cl log (8.5 ) 0.08V 9 Juctio Potetial Io obilities ad Liquid Juctio Potetials Juctio potetial: A voltage differece develops wheever dissimilar electrolyte solutios are i cotact. Happes at the salt bridge/solutio iterface sice differet ios have differet mobilities i water. A major (fudametal) source of error i a potetial measuremet. IoSelective lectrodes (IS) Respods selectively to oe io usig a ioselective membrae Do ot ivolve redox reactios The electric potetial across the membrae depeds o [aalyte is measured by two referece electrodes costat log [ C Out at 5 C charge of aalyte Glass lectrode (ph Combiatio lectrode) Ag(s) AgCl(s) Cl ( aq) H ( aq out The most commo IS A glass membrae selectively bids H Two Ag AgCl referece electrodes measure the potetial differece across the glass membrae A ioexchage equilibrium is o the surface of the glass membrae ) H ( aq ),Cl ( aq) AgCl( s) Ag( s) i 9

The Glass embrae of a ph lectrode Cross sectio of the glass membrae of a ph electrode. H ca diffuse ito the membrae to replace the metal ios through bidig to oxyge i glass (ioexchage equilibrium).

Juctio potetial (~0.0 ph) 3. Juctio potetial drift (recalibrate every hrs) 4. Sodium error (whe [H is low ad [Na is high) 5. Acid error (strog acid, the glass surface is saturated with H ) 6.

Cleaig (remove hydrophobic liquid) k K, Na Selectivity Coefficiet respose to X k A, X respose to A 5 The selectivity coefficiet: the relative respose of the electrode to differet species k 0.

10 The Glass embrae of a ph lectrode Cross sectio of the glass membrae of a ph electrode. H ca diffuse ito the membrae to replace the metal ios through bidig to oxyge i glass (ioexchage equilibrium). Resposeof the electrode: costat β (0.059)log costat β (0.059)pH [ H β: ~.00, electromotive efficiecy measured durig calibratio. Out Out O at 5 C rrors i ph easuremet. Calibratio stadards (±0.0 ph). Juctio potetial (~0.0 ph) 3. Juctio potetial drift (recalibrate every hrs) 4. Sodium error (whe [H is low ad [Na is high) 5. Acid error (strog acid, the glass surface is saturated with H ) 6. quilibratio time (~30s with adequate stirrig) 7. Hydratio of glass (A dry electrode requires several hours of soakig) 8. Temperature (calibrate at same T as measuremet) 9. Cleaig (remove hydrophobic liquid) k K, Na Selectivity Coefficiet respose to X k A, X respose to A 5 The selectivity coefficiet: the relative respose of the electrode to differet species k 0.44 K, Cs The smaller k is, the k.8 K, Rb less iterferece there is due to io X Respose of io selective electrode : costat ± β ( ) log [ a (k a at 5 C ) A A,X x x O Specificatios for lectrochemical Techiques Advatages Liear respose to aalyte over wide dyamic rage Nodestructive Short respose times Uaffected by color/turbidity (limited matrix effects) Cheap Disadvatages Sesitivity (High detectio limits) Not uiversal Voltammetry A collectio of techiques i which the relatio betwee curret ad voltage is observed durig electrochemical process. Ca be used to () Study electroactivity of ios ad molecules at the electrode/solutio iterface () Probe coupled chemical reactios ad measure electro trasfer rates (3) xamie electrode surfaces i p A electrochemical cell cosists of a workig (aalyzig) electrode, a auxiliary (couter) electrode, ad a referece electrode. The cotrol device is a potetiostat. [A Calibratio Curve

11 Strippig Aalysis Aodic strippig voltammetry (ASV): aalytes are reduced ad deposited ito (oto) a electrode. They are reoxidized durig the strippig step. e.g. Cd (aq) e Cd(Hg) Depositio Step Cd(Hg) e Cd (aq) Strippig Cathodic strippig voltammetry (CSV): typically aios are oxidized ad deposited oto a electrode with subsequet strippig via a egative potetial sca. e.g. I Hg e Hg I at a Hg electrode Depositio step Hg I e Hg I cathodic strippig, reductio Trace aalysis (ehaced sesitivity) ca be realized sice sample aalytes are precocetrated from a largevolume dilute solutio ito (oto) a smallvolume electrode uder forced covectio.

Standard Potentials. Redox Reaction - the basics. Ch. 14 & 16 An Introduction to Electrochemistry & Redox Titrations. Standard Electrode Potentials

Standard Potentials. Redox Reaction - the basics. Ch. 14 & 16 An Introduction to Electrochemistry & Redox Titrations. Standard Electrode Potentials Red Reactio - the basics Ch. 4 & 6 Itroductio to lectrochemistry & Red Titratios Reduced Oxidizig get Reducig get Oxidized Red reactios: ivolve trasfer of electros from oe species to aother. Oxidizig