Chapter 20: Electrochemistry. Tentative Schedule Date Topics Problems Video(s) Due One Review tests, introduce redox, identify oxidizing and reducing agents Oxidation numbers / oxidizing and reducing agents worksheet 1 Two Sections 20.12 9, 10, 17a,c,e,g, 19a,b,e,f 2 Three Section 20.34 Voltaic (Galvanic) Cells Cell emf and standard reduction potentials Cell worksheet, 23, 25, 26, 27, 29 3,4 Four Quiz 1 on Galvanic Cell Description Section 20.4 28,31,33,35, 36 5,6 Five Six Quiz 2 on Galvanic Cell Description Oxidizing and Reducing Agents Quiz 3 on Galvanic Cell Description Spontaneity and K 39,41, 43,93 7 47, 49, 51, 53 8, 9 Seven Nernst Equation Section 20.56 55, 57,59,61, 92 and 10, 11 Eight Section 20.68 Review Nernst and peruse Batteries and Corrosion Possible Alair battery lab THQ (due date for THQ t.b.a.) 62,79,99,100 12 Nine Section 20.9 83, 84,85,86, *105 a,b 13 Electrolysis Ten Electrolysis again Electrolytic Cell Worksheet Eleven Lab: Electrochemistry Practice Tests and Take Home Tests Twelve Electrochemistry Review Thirteen Test on Electrochemistry Written and Multiple Choice
Worksheet: Oxidation numbers / oxidizing and reducing agents worksheet 1) Review pages 128129 in your textbook on Assigning Oxidation Numbers. Using these rules, identify the oxidation state of the BOLD element in each of the following: a) NaH b) Cr 2 O 7 2 c) IF 4 d) SO 2 e) SO 3 f) COCl 2 g) MnO 4 h) HBrO i) C 2 O 4 2 2) For each reaction, very neatly: i) identify the oxidation state of each element ii) the element oxidized and the element reduced in each reaction iii) the oxidizing agent and the reducing agent in each reaction (for help with this, and examples, see page 779 in your text) b) 2Fe(NO 3 ) 2 (aq) +2Al(s) 3Fe(s) + 2Al(NO 3 ) 3 (aq) c) Cl 2 (aq) + 2NaI(aq) I (aq) + 2NaCl(aq) 2 d) PbS(s) + 4H 2 O 2 (aq) PbSO 4 (s) + 4H 2 O(l) e) Fe 2 O 3 (s) + 3CO(g) 2Fe(s) + 3CO 2 (g) f) Cr 2 O 72 + Cl (aq) Cr 3+ (aq) + Cl 2 (g) g) CN + MnO 4 (aq) CNO (aq) + MnO 2 (s)
Sample Problems Chapter 20 OIL RIG or LEO GER Oxidizing agent or oxidant Reducing agent or reductant So, how do you know if a reaction is redox or not? What are the oxidation states of the atoms in each substance: CN 1, HNO 3, S 8, SCl 2, SO 4 2 Well, what kinds of reactions are redox? Presenting: The Happy Life of an Electron in a Redox Reaction Mg + CuSO 4 Mg + Cu +2 Cu + MgSO 4 Cu + Mg +2 Can the electrons that are transferred by used for anything?
The nickelcadmium (nicad) battery, a rechargeable dry cell used in battery operated devices, uses the following redox reaction to generate electricity: Cd(s)+NiO 2 (s)+2h 2 O(l) Cd(OH) 2 (s)+ni(oh) 2 (s) Identify the substances that are oxidized and reduced, and indicate which are oxidizing agents and which are reducing agents. Lots and Lots more notes must be taken on your own pieces of paper for the next bunch of videocasts. Enjoy!
Identify the oxidizing and reducing agents in the redox reaction: 2H 2 O(l)+Al(s)+MnO 4 (aq) Al(OH)4 (aq)+mno2 (s) Complete and balance this equation by the method of half reactions: 2 Cr 2 O 7 (aq)+cl (aq) Cr 3+ (aq)+cl2 (g) (acidic solution) Complete and balance this equation by the method of half reactions: Cu(s)+NO 3 (aq) Cu 2+ (aq)+no2 (g) (acidic solution) Complete and balance this equation by the method of half reactions: CN (aq)+mno4 (aq) CNO (aq)+mno2 (s) (basic solution)
Electrode A Electrode B 20.3 Why do anions in a salt bridge migrate toward the anode? The redox reaction: 2 Cr 2 O 7 (aq)+14h + (aq)+6i (aq) 2Cr 3+ (aq)+3i2 (s)+7h 2 O(l) is spontaneous. A solution containing K 2 Cr 2 O 7 and H 2 SO 4 is poured into one beaker, and a solution of KI is poured into another. A salt bridge is used to join the beakers. A metallic conductor that will not react with either solution (such as platinum foil) is suspended in each solution and the two conductors are connected with wires through a voltmeter or some other device to detect and electric current. The resultant voltaic cell generates and electric current. Indicate: a) the reaction at the a. anode b. cathode b) the direction of electron migration c) the direction of ion migration d) the signs of the electrodes 20.4 Cell EMF under standard conditions
For the Zn=Cu +2 voltaic cell shown above, we have Zn(s)+Cu 2+ (aq,1m) Zn 2+ (aq,1m)+cu(s) E cell =1.10V Given that the standard reduction potential of Zn 2+ to Zn (s) is 0.76V, calculate the E red for the reduction of Cu 2+ to Cu (s). Line notation is sometimes used to identify each cell in an electrochemical reaction to list its components as follows: anode anode solution cathode solution cathode A double vertical line represents a salt bridge or a porous barrier. A single vertical line represents a change in phase, such as from solid to a solution. Zn Zn 2+ H + H 2 If both aqueous substances are 1.0M. Indicate: e) the reaction at the a. anode b. cathode f) the direction of electron migration g) the direction of ion migration h) the signs of the electrodes i) calculate the cell emf Calculate the standard emf of the voltaic cell based on the reaction: 2 Cr 2 O 7 (aq)+14h + (aq)+6i (aq) 2Cr 3+ (aq)+3i2 (s)+7h 2 O(l)
A voltaic cell is based on: Fe Fe +2 Ag + Ag a) What reaction occurs at the anode? b) What reaction occurs at the cathode? c) Draw this cell and identify the flow of electrons and migration of ions d) What is the standard cell potential? Using the standard reduction potential tables, rank the following ions in order of increasing strength as oxidizing agents: NO 3 (aq), Ag + (aq), Cr2 O 7 2 (aq) Using the standard reduction potential tables, rank the following ions in order of increasing strength as reducing agents: I (aq), Fe (s), Al (s) 20.5 Free Energy and Redox Using standard reduction potentials, determine whether the reactions are spontaneous under standard conditions: a) Cu(s) + 2H + (aq)+>cu 2 + (aq) +H2 (g) b) Cl 2 (g)+2i (aq) 2Cl (aq)+i2 (s)
Which is the stronger reducing agent, Hg (l) or Pb (s)? Calculate the free energy change and the equilibrium constant, K, at 298K for: 4Ag(s)+O 2 (g)+4h + (aq) 4Ag + (aq)+2h 2 O(l) Find E, ΔG, and K when the reaction is written: 2Ag(s)+ 1 O 2 2 (g)+2h+ (aq) 2Ag + (aq)+h 2 O(l)
Calculate the emf at 298K generated by: 2 Cr 2 O 7 (aq)+14h + (aq)+6i (aq) 2Cr 3+ (aq)+3i2 (s)+7h 2 O(l) When the concentrations of each specie are (in order of reaction): 2.0M, 1.0M, 1.0M, 1.0x10 5 M, (I 2(s) and H 2 O (l) do, obviously, not have concentrations) If the voltage of the above cell is 0.45V at 298K when [Zn +2 ]=1.0M and P H2 =1.0atm, what is the concentration of H + (aq)?
What is the ph of the solution in the cathode compartment of the above cell when when [Zn +2 ]=0.10M and P H2 =1.0atm and the cell emf is 0.542V? You will need extra paper of your own to write this down. You need paper of your own to write notes. Electrolysis Sample Problems 1) Calculate the number of grams of aluminum produced in 1.00 h by the electrolysis of molten AlCl 3 if the electrical current is 10.0A. 2) How many seconds would be required to produce 48.6g of Mg from MgCl 2 if the current is 100.0A?
3) Calculate the number of kilowatt hours of electricity required to produce 1.0 x 10 3 kg of aluminum by electrolysis of Al +3 if the applied voltage is 4.50V. 4) The compound NaI dissolves in pure water according to the equation NaI(s) Na+(aq) + I(aq). Some of the information in the table of standard reduction potentials given below may be useful in answering the questions that follow. (d) An electric current is applied to a 1.0 M NaI solution. (i) Write the balanced oxidation halfreaction for the reaction that takes place. (ii) Write the balanced reduction halfreaction for the reaction that takes place. (iii) Which reaction takes place at the anode, the oxidation reaction or the reduction reaction? (iv) All electrolysis reactions have the same sign for ΔG. Is the sign positive or negative? Justify your answer.
Electrochemical Cell Quiz Electrode A Electrode B Given the following information, answer the following about the electrochemical cell above: Electrode A is made of lead and the solution is lead (II) nitrate Electrode B is made of manganese and the solution it is in is manganese (II) nitrate 1) Which is the most easily reduced metal? 2) What is the balanced equation showing the spontaneous reaction that occurs? 3) What is the maximum emf the cell can produce if everything exists at standard conditions? 4) What is the direction of flow of electrons in the wire? 5) What is the direction of the positive ion flow in the salt bridge? 6) Which electrode is increasing in size? 7) Which electrode is decreasing in size? 8) What is happening to the concentration of the ions in electrode A s solution? 9) Identify the i) anode ii) cathode iii) positive electrode iv) negative electrode If electrode A were replaced with a(n) S.H.E., what would be 10) emf of the cell 11) direction of electron flow
Electrolytic Cell Worksheet 1) Molten NiBr 2(l) is electrolyzed in an electrolytic cell. a) write the balanced chemical equation for the reaction. b) write the half reaction that occurs at: i) the anode ii) the cathode c) if a solution of aqueous NiBr 2(aq) were electrolyzed in an electrolytic cell i) write the half reaction that occurs at the anode ii) write the half reaction that occurs at the cathode 2) Write and equation that relates Coulombs, amperes and time. 3) How many Coulombs of charge are in one mole of electrons? What is the special name given to this amount of charge? 4) i) Write the half reaction for the formation of magnesium metoal upon electrolysis of molten MgCl 2. ii) Calculate the mass of Mg formed upon passeage of a current of 60.0A for a period of 4.00x10 3 s. iii) How many seconds would be required to produce 50.0g of Mg from MgCl 2 if the current is 100.0A? 5) In an electrolytic cell, a current of 0.250 ampere is passed through a solution of a chloride of iron, producing Fe(s) and Cl 2 (g). (a) Write the equation for the halfreaction that occurs at the anode. (b) When the cell operates for 2.00 hours, 0.521 gram of iron is deposited at one electrode. Determine the formula of the chloride of iron in the original solution. (c) Write the balanced equation for the overall reaction that occurs in the cell. (d) How many liters of Cl 2 (g), measured at 25 C and 750 mm Hg, are produced when the cell operates as described in part (b)? (e) Calculate the current that would produce chlorine gas from the solution at a rate of 3.00 grams per hour. Bonus problem (show all work and units): 20.107 on p.897.
Take Home Quiz 1) A galvanic cell is constructed using a chromium electrode in a 1.00molar solution of Cr(NO 3 ) 3 and a copper electrode in a 1.00molar solution of Cu(NO 3 ) 2. Both solutions are at 25 C. (a) Write a balanced net ionic equation for the spontaneous reaction that occurs as the cell operates. Identify the oxidizing agent and the reducing agent. (b) A partial diagram of the cell is shown below. Cr Cu 1.0 M Cr(NO ) 3 3 1.0 M Cu(NO ) 3 3 (i) Which metal is the cathode? (ii) What additional component is necessary to make the cell operate? (iii) What function does the component in (ii) serve? (c) How does the potential of this cell change if the concentration of Cr(NO 3 ) 3 is changed to 3.00molar at 25 C? Explain. 2) Answer the following questions regarding the electrochemical cell shown below. (a) (b) (c) (d) (e) Write the balanced netionic equation for the spontaneous reaction that occurs as the cell operates, and determine the cell voltage. In which direction do anions flow in the salt bridge as the cell operates? Justify your answer. If 10.0 ml of 3.0molar AgNO 3 solution is added to the halfcell on the right, what will happen to the cell voltage? Explain. If 1.0 gram of solid NaCl is added to each halfcell, what will happen to the cell voltage? Explain. If 20.0 ml of distilled water is added to both halfcells, the cell voltage decreases. Explain.
Electrochemistry Multiple Choice (Taken from Barron s 3 rd Ed and Cliffs Notes and Arco 2 nd Ed) 1) Balance the following halfreaction in acidic solution: NO 3 NH 4 + When balanced with the smallest wholenumber coefficients possible, the sum of all the coefficients is a) 13 b) 26 c) 15 d) 23 e) 21 2) Which of the following compounds includes and element with an oxidation number of +5? 2 a) ClO 4 b) MnO 4 c) NO 2 d) SO 3 e) NO 3 3) Which of the following metals does NOT react with water to produce hydrogen? a) Zn b) Li c) Ca d) Na e) K 4) The standard voltage, E, is 0.68V for the cell: In In +3 Cu +2 Cu Find the standard reduction potential for the reaction: In +3 + 3e In a) 0.34V b) 0.34V c) 1.02V d) 1.02V e) 0.00V 5) Given the following information, which of the statements is true? Cu 2+ (aq) + e Cu + (aq) E red = 0.34V 2H + (aq) + 2e H 2(g) E red = 0.0V Fe +2 (aq) + 2e Fe (s) E red = 0.44V Ni (s) Ni +2 (aq) + 2e E red = 0.25V a) Cu +2 (aq) is the strongest oxidizing agent b) Cu +2 (aq) is the weakest oxidizing agent c) Ni (s) is the strongest oxidizing agent d) Fe (s) would be the weakest reducing agent e) H + (aq) would be the strongest oxidizing agent 6) Look at the figure below. Which of the following is FALSE. a) The magnesium electrode is being oxidized to Mg +2 b) The aluminum electrode is the cathode c) The magnesium electrode is the anode d) The aluminum ions are being reduced e) The positive ions will flow from the Al compartment to the Mg compartment
7) Given that Zn +2 (aq) + 2e Zn (s) E red = 0.76V Cr 3+ (aq) + 3e Cr (s) E red = 0.74V Calculate the equilibrium constant K for the following balanced reaction: 3Zn (s) + 2Cr +3 (aq) 3Zn +2 (aq) + 2Cr (s) a) e 0.02 b) e 0.02 c) e 4.7 d) e 8.0 e) cannot be determined from the information provided 8) An electric current is applied to an aqueous solution of FeCl 2 and ZnCl 2. Which reaction occurs at the cathode? a) Fe 2+ (aq) + 2e (aq) Fe (s) E red =0.44V b) Fe (s) Fe 2+ (aq) + 2e E ox =0.44V c) Zn +2 (aq) + 2e (aq) Zn (s) E red =0.76V d) Zn (s) Zn 2+ (aq) + 2e E ox =0.76V e) 2H 2 O (l) O 2(g) + 4H + (aq) + 4e E ox =1.23V 9) What is the number of Faradays (moles of electrons) required to produce 9.0g of aluminum by the electrolysis of molten aluminum oxide, Al 2 O 3? a) 9 F b) 4F c) 3F d) 2F e) 1F 10) A voltaic cell is set up using the system Fe Fe +2 Ag + Ag The cathode reaction produces: a) Fe b) Fe 2+ c) Ag + d) Ag e) H 2 11) A voltaic cell has an E cell of 1.56V and the overall reaction: Zn (s) + 2Ag + (aq) Zn 2+ (aq) + 2Ag (s) The [Zn 2 ] is 0.00010M and the [Ag + ] is 0.10M. What is the voltage, E, of this cell? a) 1.65V b) 1.62V c) 1.56V d) 1.50V e) 1.47V
Answers: 1) d 2) e 3) a 4) b 5) a 6) e, 7) c 8) a 9) e 10) d 11) b 12) 1997 B In an electrolytic cell, a current of 0.250 ampere is passed through a solution of a chloride of iron, producing Fe(s) and Cl 2 (g). (a) Write the equation for the halfreaction that occurs at the anode. (b) When the cell operates for 2.00 hours, 0.521 gram of iron is deposited at one electrode. Determine the formula of the chloride of iron in the original solution. (c) Write the balanced equation for the overall reaction that occurs in the cell. (d) How many liters of Cl 2 (g), measured at 25 C and 750 mm Hg, are produced when the cell operates as described in part (b)? (e) Calculate the current that would produce chlorine gas from the solution at a rate of 3.00 grams per hour. Answer: (a) 2 Cl 2 e Cl 2 (b) 0.250 amp 7200 sec = 1800 coulomb 1 mol e 1800 coul 96500 coul 1 mol Fe 0.521 g Fe 55.85 g Fe 0.0187 mol e 0.00933 mol Fe = 2 1 Fe 2+ + 2e Fe ; FeCl 2 (c) Fe 2+ + 2 Cl Fe + Cl 2 = 0.0187 mol e = 0.00933 mol Fe 1 mol Cl (d) 0.0187 mol e 2 2 mol e = 0.00933 mol Cl 2! L atm (0.00933 mol) 0.0821 # nrt " V = P = mol K$ (298K) ( 750 760 )atm (e) = 0.231 L 3.00 g Cl 2 1 hr OR 1 hr 3600 sec 1 mol Cl 2 70.906 g Cl 2 2 mol e 96500 amp sec 1 mol Cl 2 1 mol e = 2.27 amp
0.00933 mol Cl 2 = 0.662 g Cl 2 2 hrs. 2 hrs. 0.250 amp = 0.331 g Cl 2 = 0.331 g Cl 2 1 hr. X 3.00 g Cl 2 ; X = 2.27 amp Answer the following questions regarding the electrochemical cell shown. (a) Write the balanced netionic equation for the spontaneous reaction that occurs as the cell operates, and determine the cell voltage. (b) In which direction do anions flow in the salt bridge as the cell operates? Justify your answer. (c) If 10.0 ml of 3.0molar AgNO 3 solution is added to the halfcell on the right, what will happen to the cell voltage? Explain. (d) If 1.0 gram of solid NaCl is added to each halfcell, what will happen to the cell voltage? Explain. (e) If 20.0 ml of distilled water is added to both halfcells, the cell voltage decreases. Explain. Answer (a) 2 Ag + + 2 e 2 Ag E = +0.80 v Cd 2 e Cd 2+ E = +0.40 v 2 Ag + + Cd 2 Ag + Cd 2+ E = +1.20v (b) (c) (d) Anions flow into the cadmium halfcell. As the cell operates, Cd 2+ cations increase in number and need to be balanced by an equal number of anion charges from the salt bridge. Cell voltage will increase. An increase in silver ion concentration will result in faster forward reaction and a higher cell potential. Cell voltage will decrease. As the salt dissolves, the Cl ion will cause the Ag + ion to precipitate as AgCl and decrease the [Ag + ]. This will result in a slower forward reaction and a decrease in cell potential. Since cadmium chloride is a soluble salt, it will not affect the cadmium halfcell. (e) E cell = 1.20v 0.0592 log [Cd2 + ] 2 [Ag + ] 2 1.20v. But if each solution s concentration is cut in half, then, E cell = 1.20v ; while both concentrations are 1.0M, the cell potential is 0.0592 log [.5] 2 [.5] 2 = 1.19v
1993 D A galvanic cell is constructed using a chromium electrode in a 1.00molar solution of Cr(NO 3 ) 3 and a copper electrode in a 1.00molar solution of Cu(NO 3 ) 2. Both solutions are at 25 C. (a) Write a balanced net ionic equation for the spontaneous reaction that occurs as the cell operates. Identify the oxidizing agent and the reducing agent. (b) A partial diagram of the cell is shown below. Cr Cu 1.0 M Cr(NO ) 3 3 1.0 M Cu(NO ) 3 3 (i) Which metal is the cathode? (ii) What additional component is necessary to make the cell operate? (iii) What function does the component in (ii) serve? (c) How does the potential of this cell change if the concentration of Cr(NO 3 ) 3 is changed to 3.00molar at 25 C? Explain. Answer: (a) 2 Cr + 3 Cu 2+ > 2 Cr 3+ + 3 Cu Cr = reducing agent; Cu 2+ = oxidizing agent (b) (i) Cu is cathode (ii) salt bridge (iii) transfer of ions or charge but not electrons (c) E cell decreases. use the Nernst equation to explain answer