Electrochemistry. Interconversion of electrical & chemical potential energy

Transcription

1 Electrochemistry Interconversion of electrical & chemical potential energy 1

2 Electrochemistry Always involves redox reactions. 1. Energy released by spontaneous reaction is converted to electrical energy (voltaic or galvanic cell) OR 2. Electricity drives nonspontaneous reaction ( electrolytic cell ) 2

3 Redox Reactions For example: ox # Mg + 2HCl MgCl 2 + H 2 Mg is oxidized (loses e - ) H + is reduced (gains e - ) Cl - is a spectator 3

4 Redox Reactions For you to review: Assigning oxidation numbers Activity series more active metal oxidation more active nonmetal reduction Balancing redox reactions (attached) 4

5 Activity Series Will this reaction occur spontaneously? Mg + Co 2+ Mg 2+ + Co Yes! Mg is more active than Co. 5

6 Redox Balancing: Practice Cu + HNO 3 acid base Cu 2+ + NO + H 2 O CN - + MnO 4 - CNO - + MnO 2 Odd atoms O with H 2 O H with H + Charge with e - 6

7 Galvanic (Voltaic) Cell Zn + CuSO 4 ZnSO 4 + Cu Zn + Cu +2 Zn +2 + Cu Spontaneous half reactions are separated in a galvanic cell (to use the flow of electrons). 7

8 Voltaic Cell Zn + Cu +2 Zn +2 + Cu Oxidation at Zn electrode (anode) Zn Zn e - Reduction at Cu electrode (cathode) Cu e - Cu 8

9 Voltaic Cell Oxidation Zn Zn e - Voltaic Cell video mistry/flash/galvan5.swf Reduction Cu e - Cu (-) (+) 9

10 Electrochemical Cells Electric current (e - ) flows through the wire due to difference in electrical potential, measure by cell voltage or emf. Non-reacting cations and anions flow through the salt bridge to maintain electrical neutrality. 10

11 Standard Electrode Potentials It is impossible to measure the emf (voltage) of a single half cell, so one is arbitrarily set to equal zero. 2H + (1M) + 2e - H 2 (1 atm) Standard hydrogen electrode (SHE) E o SHE = 0 volt (definition) 11

12 S.H.E. 12

13 Standard Reduction Potential All other half-cell potentials are measured relative to SHE, and are assigned E o values if: solutes are 1M gases are 1.0 atm temperature is 25 o C 13

14 Standard Reduction Potential E o for: Zn(s) Zn +2 (aq) + 2e - How do you tell it is oxidation? 0.76V 14

15 Standard Reduction Potential Under these conditions, Zn is found to be the anode (oxidation). Zn(s) Zn +2 (aq) + 2e - And the voltmeter reads: E o cell = 0.76 V 15

16 switch sign for oxidation Standard Reduction Potential E o cell = Eo cathode + Eo anode E o cathode = reduction potential of reduction half-cell (cathode) E o anode = reduction potential of oxidation half-cell (anode) (switch the sign) 16

17 Standard Reduction Potential E o cell = Eo cathode + Eo anode (reduction) (oxidation) = E o H+/H2 + Eo Zn+2/Zn 0.76 V = 0.0 V + E o Zn+2/Zn E o Zn+2/Zn = 0.76 V (oxidation) so reduction potential is -0.76V 17

18 Standard Reduction Potential See problem set for the standard reduction potentials for many half reactions. 18

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20 Some Rules E o s are all written as reductions. Zn e - Zn (switch E o sign if oxidation) E o s are listed for standard conditions. 1M, 1 atm., 25 o C 20

21 Some Rules cont. A redox reaction is thermodynamically favored in the forward direction if E cell is positive. Zn + Cu +2 Zn +2 + Cu E o cell = V 21

22 Some Rules cont. The more positive E o is, the greater tendency for reduction. F 2 + 2e - 2F - Greatest tendency to go in forward direction. 22

23 Some Rules cont. Half cells are reversible: any electrode can act as anode (oxidation) or cathode (reduction) 23

24 Some Rules cont. The Diagonal Rule Draw a diagonal from Li o to F 2. Under standard conditions, a reaction is spontaneous (+E cell ) if the reactants can be connected by this diagonal. Don t forget water at 1.23V & -.83V! 24

25 Diagonal Rule Runs as reduction Diagonal connects reactants of spont. redox reaction Runs as oxidation 25

26 Some Rules cont. Changing the stoichiometric coefficients of a half-cell reaction, does not change E o. I 2 + 2e - 2I - 2I 2 + 4e - 4I - E o = 0.53 V E o = 0.53 V 26

27 Some Rules cont. E o is a thermodynamic function like H, G & S. Reversing the direction of a reaction changes the sign of E o. I 2 + 2e - 2I - 2I - I 2 + 2e - E o = 0.53 V E o = V 27

28 Some Rules cont. These rules apply whether the reactants are separated in a galvanic cell or all mixed together. 28

29 Redox Prediction Write the net ionic reaction that takes place when Br 2 is added to a solution of NaCl and NaI. 29

30 You have: Br 2 (l) Na + (aq) Cl - (aq) I - (aq) H 2 O Look for the longest spontaneous diagonal. It identifies the reactants. 30

31 Redox Prediction A Cr electrode in 1M Cr(NO 3 ) 3 is connected to a silver electrode in 1M AgNO 3. What is the anode and cathode? Write the net ionic reaction and calculate E o cell. 31

32 You have: Cr Cr 3+ Ag Ag + H 2 O 32

33 For spontaneous reaction: (+) (-) (>1) E o cell DG o K eq DG o, the amount of energy available to do work, is given by: DG o = -n F E o cell n = number of moles of e - F = Farady constant = 96,500 J/V mol e - = 96,500 C/mol e - 33

34 E o cell DG o K eq Also, since DG o = -n F E o cell DG o = -RT lnk then E o cell = RT nf lnk 34

35 E o DG o DG o = -RT lnk K 35

36 E o cell DG o K eq DG o K E o Rxn. at Std. State - > 1 + Spontaneous 0 = 1 0 At equilibrium + < 1 - Non-spontaneous 36

37 Show Your Stuff 1.Calculate K eq for the following reaction at 25 o C: Sn(s) + 2Cu +2 Sn Cu + 2.Calculate DG o for the following reaction at 25 o C: 2Au(s) + 3Ca +2 2Au Ca(s) 37

38 Effect of Concentration on E o If concentrations are not standard (1.0M), use LeChatelier s principle to see the effect on voltage: e.g. If a reactant concentration is greater than 1.0M, reaction will go in forward direction to a greater extent, so cell voltage will increase. 38

39 Effect of Concentration on E o Co +2 (aq) + Fe(s) Co(s) + Fe +2 (aq) Calculate E o cell when: [Co +2 ] = [Fe +2 ] = 1.0M Does E o cell increase or decrease when: 1. [Co +2 ] = 0.5M 2. [Fe +2 ] = 1.5M 3. More Co(s) is added 39

40 Batteries Portable electrochemical cell used as a source for electricity. Dry cell Car battery (Pb) Read!!! Fuel cell 40

41 Electrolysis Use of electrical energy to cause a non-spontaneous chemical reaction to occur. This process is carried out in an electrolytic cell. The opposite of a galvanic cell. Find the shortest opposite diagonal. 41

42 Electrolysis: NaCl(l) Anode 2Cl - Cl 2 + 2e - Cathode 2Na + + 2e - 2Na Overall 2Na + + 2Cl - 2Na + Cl 2 Since E o = -4 V, at least 4V must be supplied to cause the reaction to occur. 42

43 Electrolysis: NaCl(aq) What are the two half-reactions and the voltage required for this electrolysis? Use the table of reduction potentials and find the shortest diagonal. 43

44 You have: Na + Cl - H 2 O 44

45 Electrolysis: H 2 O What are the two half reactions for the electrolysis of acidified water to produce hydrogen gas and oxygen gas? Find shortest opposite diagonal. 45

46 You have: H + H 2 O 46

47 Electrolysis: H 2 O Anode 2H 2 O O 2 + 4H + +4e - Cathode 4H + + 4e - 2H 2 Overall 2H 2 O 2H 2 + O 2 E o = V H 2 SO 4 is added as electrolyte & source of H +. 47

48 Hall Process 48

49 Electrolysis of KI(aq) Lab 49

50 Electrolysis demo m_sim/electrolysis_fc1_gm_ /main.html Quantitative Electrolysis stoichiometry with electrons! 1 F = charge of 1 mol e - = 96,500 C/mol e - and current = charge/time ampere = coulomb/sec. 1A = 1 C/s 50

51 Quantitative Electrolysis In an electrolytic cell, Ca +2 is converted to Ca at the cathode. How much Ca is produced by A (current) in 1.50 hours? Ca e - Ca 51

52 Quantitative Electrolysis A = C/s 1.5 hr x 3600s hr x C 1 s x 1 mol e- 96,500 C x 1 mol Ca 2 mol e - x g Ca 1 mol Ca = g Ca 52

53 Time to Rock n Roll In the electrolytic reduction of acidified water, what volume of hydrogen gas at STP is produced using a current of 1.26 A for 7.44 hours? 53

54 You Survived! 54

55 Warm-up What is the difference in meaning between DG & DG o? For a given reaction, when does DG = DG o? What is DG o and DS o when water condenses at 100 o C? (DH vap = 40.6 kj/mol) 55

56 Warm-up Sketch and label the Ni and Cd half-cells running as a voltaic cell. Calculate the cell voltage. Write the balanced equation. 56

57 Warm-up Into water you toss Al, AlCl 3, Ag and AgNO 3. Write the net ionic equation for what actually takes place. Calculate DG o and K eq. 57

58 Warm-up For the reaction 2Ag + Pb +2 Pb + 2Ag + what [Pb +2 ] is needed for the reaction to be spontaneous if [Ag + ] = 0.10 M? Is this possible? Write a balance equation for the electrolysis of pure water. Is this possible? 58

59 Warm-up What are the products of electrolysis of NaCl(aq)? What is E cell? 59