CHEM 2400/2480. Lecture 19

Transcription

1 Lecture 19 Metal In Indicatr - a cmpund whse clur changes when it binds t a metal in - t be useful, it must bind the metal less strngly than EDTA e.g. titratin f Mg 2+ with EDTA using erichrme black T (In) - very small amunt f In MgIn + EDTA 6 MgEDTA + In red blue - EDTA reacts with free Mg 2+ first, then MgIn - if the metal binds the indicatr mre strngly than EDTA, the metal is said t blck the indicatr and a back titratin must be used e.g. erichrme black T is blcked by Cu 2+, Ni 2+, C 2+, Cr 3+, Fe 3+, Al 3+ -add a knwn excess f EDTA - back titrate excess EDTA with standard Mg 2+ - here Mg 2+ des nt displace Cu 2+ frm EDTA K f (CuEDTA) = 10 19, K f (MgEDTA) = 10 9 ˆ the clur change is frm blue t red. Week 10-1

2 Metal In Indicatrs Equilibrium Equatins f Erichrme Black T and a Metal In M 2+ ( e.g. Ca 2+ ) HIn 2- º In 3- + H + K 3 = 2.8 x M 2+ + In 3- º MIn - K MIn =? M 2+ + HIn 2- º MIn - + H + K =? and K CaIn = 2.5 x 10 5 K 3 K CaIn = 2.8 x x 2.5 x 10 5 = 7.0 x 10-7 Week 10-2

3 Let s lk at the determinatin f hardness f cld tap water frm Guelph. As yu have seen in the lab, impurities (e.g. Cu 2+, Fe 2+, Fe 3+ ) in the tap water frm cmplexes with the erichrme black T and blck the indicatr. In ther wrds, they bind strngly with the indicatr and the metal ins are nt released fr cmplexatin with the EDTA e.g. i. T ml f Guelph cld tap water, buffered t ph 10, was added ml f 0.01 M EDTA. The excess EDTA was titrated with ml 0.01 Mg 2+ slutin ii. T anther ml, als buffered t ph 10, was added an excess f CN - slutin. This slutin was then titrated t the end pint with 7.35 ml f the 0.01M EDTA slutin. Calculate the cncentratins f Ca 2+ (including Mg 2+ ) and f the impurities Week 10-3

4 Titratin Methds with EDTA 1. Direct titratin with EDTA. 2. Back titratin - a knwn excess EDTA added and the excess titrated with anther metal. 3. Displacement Titratin - fr metal ins with n suitable In M n+ + MgY 2-6 MY n-4 + Mg 2+ (+ excess MgY 2- ) 8 8 excess titrated with EDTA K f (MY n-4 ) K f (MgY 2- ) - used fr Hg 2+ e.g. Ag + has n suitable indicatr 2Ag + + Ni(CN) Ag(CN) 2! + Ni 2+ 8 titrated with EDTA 4. Masking - a masking agent is a reagent that prtects a cmpnent f the slutin frm frming a cmplex with EDTA. Week 10-4

5 ELECTROCHEMISTRY In analytical chemistry: 1. electrical wrk using a galvanic cell is nt f interest 2. f interest is the MAXIMUM vltage that can be measured frm a cell Galvanic r Vltaic cells ˆ ˆ ˆ spntaneus chemical reactin t generate electricity half cell ptentials d nt have abslute values instead, they are relative t the standard hydrgen electrde, S.H.E. Week 10-5

6 The Sign f the Electrde Ptential de Reductin at cathde - lss f electrns - gain f electrns Oxi dat in at an Vltage measurements - ptentimeter/electrnic vltmeter - negligible current draw - fr the diagram abve, the reactin is spntaneus. Week 10-6

7 A Cmplete Cell Week 10-7

8 Cmplete Cell and Line Diagram Line Diagram Pt (s) * H 2 (g, " = 1), H + (aq, " = 1) ** Ag + (aq, " = 1)* Ag(s) r S.H.E. ** Ag + (aq, " = 1)* Ag(s) Week 10-8

9 Ptential f a Cell Nrmal = E + E cell cathde E ande 8 8 Reductin Ptential Oxidatin Ptential Cathde - Reductin: 2Ag + + 2e - º 2Ag (s) ADD Ande - Oxidatin: H 2 (g) º 2H + + 2e - 2Ag + + H 2 (g) º 2Ag (s) + 2H + Week 10-9

10 By Cnventin S We use this methd fr ur calculatins: = E E cell cathde E ande cell + E = E E 8 8 Reductin Ptential Reductin Ptential Cathde - Reductin: 2Ag + + 2e - º 2Ag (s) SUBTRACT Ande - Reductin 2H + + 2e - º H 2 (g) 2Ag + + H 2 (g) º 2Ag (s) + 2H + Week 10-10

11 Lecture 20 NERNST EQUATION expresses the net driving frce fr a reactin E and cncentratin are related t free energy i.e. ) G = -nfe and ) G = -nfe Fr a half reactin, aa + ne - º bb the Nernst equatin is, E = E - RT ln Q nf where Q B b A a = α α = reactins nt at equilibrium and " = activities. Nte: we will use species cncentratins [ ] instead f activities, ", in ur slutins t prblems. The difference between these tw units are negligible fr very dilute slutins. The Usable Frm f the Nernst Equatin at K is: E = E lg Q n Week 10-11

12 E and E are Independent f Hw the Cell Reactin is Written A cell can be written as: 2H + + 2e - º H 2 (g) E = E lg P 2 [H + ] H 2 2 OR H + + e - º ½ H 2 (g) E 1 2 H = E lg P 1 [H + ] withut affecting the values f E r E E = wrk dne per culmb f charge - cnstant fr a specific reactin E = ptential fr a specific cell - will change ONLY if species cncentratins are altered. Week 10-12

13 Calculatin f Half-Cell Ptentials Let s lk at a half cell cnsisting f a silver rd immersed in AgNO 3 (aq). What is the half cell ptential? Nernst equatin is: Ag + + e - º Ag (s) E + = V Ag E = E Ag lg 1 [ Ag ] + + Different descriptin f the same reactin e.g. what happens if an excess f NaCl is added t the half cell? S AgCl(s) is precipitated ut and we have excess Cl - (aq) and [Ag + ] = K sp [Cl - ] S Ag + is still being reduced at the electrde t Ag (s) and the Nernst equatin can nw be written as: E = E + Ag Cl lg [ ] Ksp Week 10-13

14 But if [Cl - ] = 1.0 M i.e standard state cnditins, then E = E + + lg K Ag 1 = ( ) = E / sp AgCl Ag where E AgCl / Ag refers t: AgCl (s) + e - º Ag (s) + Cl - Thus, the tw electrdes are related via K sp. Week 10-14

15 E H 2 O / H 2 H 2 O + e - º ½ H 2 (g) + OH - = V E H + / H 2 2H e - º H 2 (g) = V The abve 2 half cells are related via K w E H + / H 2 i.e. E = [H 2 ] lg 2 2 [ H + ] H 2 O º H + + OH - and K w = [H + ] [OH - ] [H + ] = K w / [OH - ] ˆ E = lg 2 [H 2 ][ OH ] 2 K w 2 at standard state cnditins f K and [H 2 ] / [OH - ] / 1.0 E = lg 2 K 2 w lg K 2 w E = 2 = E H 2 O / H 2 = lg = x (-28) and E H 2 O / H 2 = V Week 10-15

16 E ZnY 2 / Zn ZnY e - º Zn (s) + Y 4- = V where Y 4- is the cmpletely deprtnated frm f EDTA E Zn 2+ / Zn Zn e - º Zn (s) = V Shw the abve 2 half cells are related via K f Week 10-16

17 Nernst Equatin fr a Cmplete Cell Find the ptential f the cell: Cd (s) * CdCl 2 (aq, M) * AgCl (s) * Ag (s) 2AgCl () s + 2e 2Ag () s + 2Cl E = V 2 + Cd + 2e Cd () s E = V 2 + 2AgCl () s + Cd () s 2Ag () s + Cd + 2Cl Ecell = V + Cathde Ptential E+ = E+ lg [ Cl ] 2 = lg[?? ] = V Ande Ptential E lg 2 [ 2 Cd ] = E + = lg?? = = V Week 10-17

18 Cell Ptential E cell =? E cell is psitive ˆ What? Frm the net cell reactin abve, ptential difference can als be calculated i.e Ecell = Ecell lg [ Cd ][ Cl ] 2 Week 10-18