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Electrochemistry Electrochemical Cells Voltaic Cells Standard Cell Potentials Effect of Concentration on Cell Potentials Free Energy and Cell Potential.

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Presentation on theme: "Electrochemistry Electrochemical Cells Voltaic Cells Standard Cell Potentials Effect of Concentration on Cell Potentials Free Energy and Cell Potential."— Presentation transcript:

1 Electrochemistry Electrochemical Cells Voltaic Cells Standard Cell Potentials Effect of Concentration on Cell Potentials Free Energy and Cell Potential BatteriesCorrosion Electrolytic Cells Stoichiometry of Electrochemical Reactions Practical Application: pH Electrode

2 Types of electrochemical cells Galvanic or Voltaic The ‘spontaneous’ reaction. Produces electrical energy.Electrolytic Non-spontaneous reaction. Requires electrical energy to occur. For reversible cells, the galvanic reaction can occur spontaneously and then be reversed electrolytically - rechargeable batteries.

3 Types of electrochemical cells Not all reactions are reversible. Examples of non-reversible reactions If a gas is produced which escapes. 2H e - H 2 (g) If one or more of the species decomposes.

4 Voltaic cells There are two general ways to conduct an oxidation-reduction reaction Mixing oxidant and reductant together Cu 2+ + Zn (s) Cu (s) + Zn 2+ This approach does not allow for control of the reaction.

5 Voltaic cells Electrochemical cells Each half reaction is put in a separate ‘half cell.’ They can then be connected electrically. This permits better control over the system.

6 Voltaic cells Cu 2+ + Zn (s) Cu (s) + Zn 2+ Zn Cu Cu 2+ Zn 2+ e-e- e-e- Electrons are transferred from one half-cell to the other using an external metal conductor. Electrons are transferred from one half-cell to the other using an external metal conductor.

7 Voltaic cells e-e- e-e- To complete the circuit, a salt bridge is used. To complete the circuit, a salt bridge is used. salt bridge

8 Voltaic cells Salt bridge Allows ion migration in solution but prevents extensive mixing of electrolytes. It can be a simple porous disk or a gel saturated with a non-interfering, strong electrolyte like KCl. KCl Cl - K+K+ Cl - is released to Zn side as Zn is converted to Zn 2+ K + is released as Cu 2+ is converted to Cu

9 Voltaic cells Zn Zn e - For our example, we have zinc ion being produced. This is an oxidation so: The electrode is - the anode - is positive (+).

10 Voltaic cells Cu e - Cu For our other half cell, we have copper metal being produced. This is a reduction so: The electrode is - the cathode - is negative (-)

11 Cell diagrams Rather than drawing an entire cell, a type of shorthand can be used. For our copper - zinc cell, it would be: Zn | Zn 2+ (1M) || Cu 2+ (1M) | Cu The anode is always on the left. | = boundaries between phases ||= salt bridge Other conditions like concentration are listed just after each species.

12 Cell diagrams Other examples Pt, H 2 (1atm) | H + (1M) || This is the SHE. Pt is used to maintain electrical contact so is listed. The pressure of H 2 is given in atmospheres. Pt, H 2 (1atm) | HCl (0.01M) || Ag+ (sat) | Ag A saturated silver solution (1.8 x M) based on the K SP AgCl and [Cl - ]

13 Electrode potentials A measure of how willing a species is to gain or lose electrons. Standard potentials Potential of a cell acting as a cathode compared to a standard hydrogen electrode. Values also require other standard conditions.

14 Standard hydrogen electrode Hydrogen electrode (SHE) The ultimate reference electrode. H 2 is constantly bubbled into a 1 M HCl solution Pt | H 2 (1atm), 1M H + || E o = V All other standard potentials are then reported relative to SHE. H2H2 1 M HCl Pt black plate

15 Electrode potentials Standard potentials are defined using specific concentrations. All soluble species are at 1 M Slightly soluble species must be at saturation. Any gas is constantly introduced at 1 atm Any metal must be in electrical contact Other solids must also be present and in contact.

16 Electrode potentials The standard potential for: Cu e - Cu (s) is V. This means that: If a sample of copper metal is placed in a 1 M Cu 2+ solution, we’ll measure a value of 0.337V if compared to: 2H + + 2e - H 2 (g) (1 M) (1 atm)

17 Half reactions A common approach for listing species that undergo REDOX is as half-reactions. 2Fe 3+ + Zn o (s) = 2Fe 2+ + Zn 2+ For 2Fe 3+ + Zn o (s) = 2Fe 2+ + Zn 2+ Fe 3+ + e - Fe 2+ (reduction) Zn o (s) Zn e - (oxidation) You’ll find this approach useful for a number of reasons.

18 Half reactions Tables are available which list half reactions as either oxidations or reductions. Will provide Standard E o values to help predict reactions and equilibria. Other species that participate in the reaction. Show the relative ability to gain or loss electrons.

19 Half reactions standard reduction potentials Half reaction E o, V F 2 (g) + 2H + + e - 2HF (aq) Ce 4+ + e - Ce 3+ (in 1M HCl) 1.28 O 2 (g) + 4H + + 4e - 2H 2 O (l) Ag + + e - Ag (s) H + + 2e - H 2 (g) Fe e - Fe (s) Zn e - Zn (s) Al e - Al (s) Li + + e - Li (s)

20 Cell potentials One thing that we would like to know is the spontaneous direction for a reaction. This requires that we determine the E cell. Since our standard potentials (E o ) are commonly listed as reductions, we’ll base our definitions on that. E cell = E half-cell of reduction - E half-cell of oxidation E o cell = E o half-cell of reduction - E o half-cell of oxidation

21 Cell potentials You know that both an oxidation and a reduction must occur. One of your half reactions must be reversed. E cellThe spontaneous or galvanic direction for a reaction is the one where E cell is a positive value. The half reaction with the largest E value will proceed as a reduction. oxidation.The other will be reversed - oxidation.

22 Cell potentials For our copper - zinc cell at standard conditions: E o red Cu e - Cu (s) V Zn e - Zn (s) V E cell 1.03 V Spontaneous reaction at standard conditions. Cu 2+ + Zn (s) Cu (s) + Zn 2+

23 Concentration dependency of E E o values are based on standard conditions. The E value will vary if any of the concentrations vary from standard conditions. This effect can be experimentally determined by measuring E versus a standard (indicator) electrode. Nernst equationTheoretically, the electrode potential can be determined by the Nernst equation.

24 Concentration dependency of E The Nernst equation For A a + ne - B b E = E o + ln where: E o = standard electrode potential R = gas constant, J/ o mol T = absolute temperature F = Faraday’s constant, C n = number of electrons involved a = activity R T n F a Aaa Bb a Aaa Bb

25 Concentration dependency of E If we assume that concentration is proportional to activity and limit our work to 25 o C, the equation becomes: E = E o - log This also includes a conversion from base e to base 10 logs n [B] b [A] a

26 Concentration dependency of E Example Determine the potential of a Pt indicator electrode if dipped in a solution containing 0.1M Sn 4+ and 0.01M Sn 2+. Sn e - Sn 2+ E o = 0.15V E = 0.15V - log = 0.18 V M 0.1M

27 Concentration dependency of E Another example Determine the potential of a Pt indicating electrode if placed in a solution containing 0.05 M Cr 2 O 7 2- and 1.5 M Cr 3+, if pH = 0.00 (as 1 M HCl). Cr 2 O H + + 6e - 2Cr H 2 O (l) E o = 1.36 V

28 Concentration dependency of E E = E o - log = 1.36 V - log = 1.31 V [Cr 3+ ] 2 [Cr 2 O 7 2- ][H + ] (1.5) 2 (0.05)(1) 14

29 Calculation of cell potentials To determine the galvanic E cell at standard conditions using reduction potentials: E cell = E o half-cell of reduction - E o half-cell of oxidation Where E half-cell of reduction E half-cell of reduction - half reaction with the larger or least negative E o value. E half-cell of oxidation E half-cell of oxidation - half reaction with the smaller or more negative E o value.

30 Calculation of cell potentials At nonstandard conditions, we don’t know which will proceed as a reduction until we calculate each E value. Steps in determining the spontaneous direction and E of a cell. Calculate the E for each half reaction. The half reaction with the largest or least negative E value will proceed as a reduction. Calculate E cell

31 Calculation of cell potentials Example Determine the spontaneous direction and E cell for the following system. Pb | Pb 2+ (0.01M) || Sn 2+ (2.5M) | Sn Half reaction E o Pb e - Pb V Sn e - Sn V Note: The above cell notation may or may not be correct.

32 Calculation of cell potentials Pb e - Pb V Sn e - Sn V At first glance, it would appear that Pb 2+ would be reduced to Pb. However, we’re not at standard conditions. We need to determine the actual E for each half reaction before we know what will happen.

33 Calculation of cell potentials For lead: E = log = V For tin: E = log = V Under our conditions, tin will be reduced

34 Cell potential, equilibrium and  G We now know that changing concentrations will change E cell. E is a measure of the equilibrium conditions of a REDOX reaction. It can be used to: Determine the direction of the reaction and E cell at non-standard conditions. Calculate the equilibrium constant for a REDOX reaction.

35 Equilibrium constants At equilibrium E A = E B so nm E o A -log [A RED ] n [A OX ] n = nm E o B -log [B RED ] m [B OX ] m E o B - E o A = nm log [A OX ] n [B RED ] m [A RED ] n [B OX ] m K when at equilibrium, Q if not. log K = nm(E o B - E o A ) A - species reduced B - species oxidized

36 Free energy and cell potential Earlier, we explained that  G and the equilibrium constant can be related. Since E cell is also related to K, we know the following. Q  GE Forward change, spontaneous< K -+ At equilibrium= K 00 Reverse change, spontaneous> K + -

37 Batteries Portable voltaic cells These have become important to daily life. Dry cells All chemicals are in the form of a paste or solid. They are not really dry. Wet cells A liquid solution is present.

38 Zinc-carbon dry cell The electrolyte, aqueous NH 4 Cl is made into a paste by adding an inert filler. Electrochemical reaction Zn (s) + 2MnO 2 (s) + 2 NH 4 - (aq) Zn 2+ (aq) + Mn 2 O 3 (s) + 2NH 3 (aq) + H 2 O (l) This cell has a potential of 1.5 V when new.

39 Zinc-carbon dry cell Seal Carbon rod Paste Zinc

40 Lead storage battery These are used when a large capacity and moderately high current is need. It has a potential of 2 V. Unlike the zinc-carbon dry cell, it can be recharged by applying a voltage. Car battery. This is the most common application. Most cars are designed to use a 12 V battery. As a result, six cells connected in a series are needed.

41 Lead storage battery Electrochemical reaction. 2PbSO 4 (s) + 2H 2 O (l) Pb (s) + PbO 2 (s) + 2H + (aq) + 2HSO 4 - (aq)Note. Lead changes from a +2 to 0 and +4 oxidation state when a lead storage battery is discharged. Lead also remains in a solid form.

42 Lead storage battery A series of 6 cells in series are used to produce the 12 volts that most cars require. A series of 6 cells in series are used to produce the 12 volts that most cars require.

43 Corrosion Deterioration of metals by oxidation. Example. Rusting of iron and steel. E o Anode: Anode: Fe (s) Fe e V Cathode: Cathode: O 2 (g) + 2H 2 O (l) + 4e - 4OH V Rusting requires both oxygen and water. The presence of an acid enhances the rate of corrosion - more positive cathode. Cathode: Cathode: O 2 (g) + 4H + (aq) + 4e - 2H 2 O (l) +1.23V

44 Rusting Iron Water drop Fe 2+ Fe Anode Rust Cathode e-e- O 2 from air O2O2

45 Corrosion prevention Another example. Quite commonly a rod of magnesium is placed in a hot water tank. It will be oxidized to Mg 2+ instead of the iron tank rusting. This greatly extends the life of the tank. Sacrificial anode Pieces of reactive metal that are connected to an object to be protected by a conductor.

46 Electrolytic cells With voltaic cells, reactions occur spontaneously. With electrolytic cells, a potential is applied, forcing a reaction to go. - work is done on the system. - polarize the cell. - causes unexpected things to happen. - E cell will change during the reaction.

47 Applying a voltage When we apply a voltage, it can be expressed as the following: E applied = E back + iRWhere E back = voltage required to ‘cancel out’ the normal forward or galvanic reaction. iR = iR drop. The work applied to force the reaction to go. This is a function of cell resistance.

48 Applying a voltage E back Increases as the reaction proceeds Actually consists of: E back = E rev (galvanic) + overpotentialOverpotential An extra potential that must be applied beyond what we predict from the Nernst equation.

49 Overvoltage or overpotential polarized A cell is polarized if its potential is made different than its normal reversible potential - as defined by the Nernst equation. The amount of polarization is called the overpotential or overvoltage.  = E - E rev

50 Overvoltage or overpotential There are two types of . Concentration overpotential. This occurs when there is a difference in concentration at the electrode compared to the bulk of the solution. This can be observed when the rate of a reaction is fast compared to the diffusion rate for the species to reach the electrode.

51 Overvoltage or overpotential Concentration overpotential. Assume that we are electroplating copper. As the plating occurs, copper is leaving the solution at the electrode. This results in the [Cu 2+ ] being lower near the electrode. [Cu 2+ ] bulk [Cu 2+ ] electrode

52 Overvoltage or overpotential Activation overpotential Results from the shift in potential at the electrode simply to reverse the reaction. This effect is at its worst when a reaction becomes nonreversible. Effect is slight for deposition of metals. Can be over 0.5V if a gas is produced. Occurs at both electrodes making oxidations more ‘+’ and reductions more ‘-’.

53 Electrolytic cells In electrolytic cells The reaction requiring the smallest applied voltage will occur first. As the reaction proceeds, the applied E increases and other reactions may start. Lets look at an example to determine if a quantitative separation is possible.

54 Electrolytic example Can Pb 2+ be quantitatively be separated from Cu 2+ by electrodeposition? Assume that our solution starts with 0.1M of each metal ion. We’ll define quantitative as only 1 part in cross contamination (99.99%) Cu e = CuE o = V Pb e = PbE o = V

55 Electrolytic example Copper We start with 0.1 M and begin our deposition. We don’t want any lead to deposit until at least 99.99% of the copper has been removed M Cu 2+ E = log E = V

56 Electrolytic example Lead Pb would start depositing at: E = log E = V The separation is possible but our calculations neglect any overpotential

57 Stoichiometry of electrochemical reactions Faraday determined that the the amount of product formed was proportional to the quantity of electricity transferred. A coulomb (C) is a quantity of electricity. Current is the rate of electrical flow coulombs of electricity are are equivalent to one mole of electrons coulombs = 1 Faraday ( F ) coulombs = 1 Faraday ( F ) Current = Amps = i = C / sCurrent = Amps = i = C / s

58 Stoichiometry of electrochemical reactions The number of equivalents deposited can be found by: coulombs grams gram equivalent weight g x e in transfer formula weight equivalents = i t = = =

59 ( ) The number of grams deposited then is: g deposited = Wherei = current in amps t = time in seconds FM = formula mass n = number of electrons transferred per species Stoichiometry of electrochemical reactions i t FM n equivalent weight

60 Example Determine the number of grams of Cu that could be converted to Cu 2+, if a current of 6 A is applied for 5 minutes. Half reaction Cu 2+ (aq) + 2 e - Cu (s) g = = g (6 A) (5 min x 60 ) (63.55 ) (96 500) ( 2e - ) s min g mol

61 Electrogravimetry One practical application of electrolysis is the method of electrodeposition. A quantitative analysis based on weight gain. It relies on the production of a metal or metal oxide on an electrode. The weight of the electrode is measured both before and after the material is deposited. The amount of material is determined by difference.

62 Electrogravimetry R - potentiometer A - ammeter V - Voltmeter Pt cathode Anode Stirbar

63 Electrogravimetry

64 Electrogravimetry Only a limited number of species work well with electrodeposition. Cathode electrodepositions. Deposited from simple cations: Cu, Ni, Zn Deposited from cyanide complexes: Ag, Cd, Au Anode electrodepositions Deposited as oxides. Pb 2+ PbO 2 Mn 2+ MnO 2

65 pH electrode Reference electrode The part of the cell that is held constant Indicator electrode The part of the cell that contains the solution we are interested in measuring We can use one half of an electrochemical cell to measure properties of the other half.

66 pH electrode The earlier example would be too difficult for routine use. We can ‘repackage’ a half cell in the form of an electrode. pH electrode - first discovered - still the most significant - relies on a glass wall or membrane. Ag wire 0.1M HCl AgCl thin glass wall

67 pH electrode Combination pH electrode A reference electrode is inside the pH electrode.

68 How a pH electrode works H 3 O + partially populates both the inner and outer SiO 2 surfaces of the glass membrane. The concentration difference results in a potential across the glass membrane. A special glass is used: 22% Na 2 O, 6% CaO, 72% SiO 2


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