# Electrochemistry. Outline

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1 Electrochemistry Outline 1. Oxidation Numbers 2. Voltaic Cells 3. Calculating emf or Standard Cell Potential using Half-Reactions 4. Relationships to Thermo, Equilibrium, and Q 5. Stoichiometry 6. Balancing by Half-Reactions in an Acidified Solution Review 1. Oxidation Numbers 2- Oxidation = charge SO 4 If each O= -2, total O = -8, S + -8 = -2 total, therefore S = +6. Known Oxidation Numbers: Alkali Metals = +1, Alkaline Earth Metals = +2, Halogens = -1, Elemental Form = 0, Oxidation = -2 except in metal peroxides (M 2 O 2 ), all others calculate. 2. Voltaic Cells Oxidation LEO = losing electrons is oxidation ANOX = Anode Reducing Agent Reduction GER = gaining electrons is reduction REDCAT = Cathode Oxidation Agent Electrons flow from Anode to Cathode (alphabetical order)

2 3. Calculating emf or Standard Cell Potential using Half-Reactions Zn Zn +2 Ag + Ag (using standard cell potential chart for a Voltaic or Galvanic Cell) Write Half-Reactions: Zn e- Zn (s) E = -0.76V Ag + + e- Ag (s) E = V Oxidation reaction must be flipped (as well as algebraic sign for the E ) If reaction occurs (spontaneous) E = + If electric charge or electrolysis is applied (non-spont) E = - o Oxidation Zn (s) Zn e- E = +0.76V o Reduction 2Ag + + 2e- 2Ag (s) E = V o Must multiply coefficients to get electrons equal Do not multiply E Overall Reaction Zn (s) + 2 Ag + Zn Ag (s) E = V n= 2 e- transferred 4. Relationships to Thermo, Equilibrium, and Q ΔG = - n F E n = mole e- transferred F = C / 1 mol e- E = ln 25 C, log K = R = 8.31, if spontaneous K>1 products favored Nernst Equation E cell = E - ln 25 C, E cell = E - log Q 5. Stoichiometry Current I (Amps) = Amps x sec = = # g of Zn 6. Balancing by Half-Reaction Method in an Acidified Solution Balance this reaction: MnO Cr + H + MnO 2 + Cr 3+ + H 2 O Oxidation: Cr Cr 3+ - Reduction: MnO 4 MnO 2 1. Balance atoms 2. Balance O using H 2 O, then balance H using H + 3. Find electrons and balance the electrons by multiplying coefficients. Oxidation: Cr Cr e- - Reduction: MnO 4 + 4H + + 3e- MnO 2 + 2H 2 O Balanced reaction: _1_MnO _1_Cr + _4_H + _1_MnO 2 + _1_Cr 3+ + _2_H 2 O

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4 Electrochemistry Problem Set 1. Determine the oxidation number for the indicated element in each of the following substances: a) S in SO 2 b) C in COCl 2 c) Mn in MnO 4 - d) Br in HBrO e) As in As 4 f) O in H 2 O 2 2. Determine the oxidation number of sulfur in each of the following: a) H 2 S b) S 8 c) SCl 2 d) SO 4 2- e) Na 2 SO 3 3. In which of the following species does sulfur have the same oxidation number as it does in H 2 SO 4? a) H 2 SO 3 b) S 2 O 3 2- c) S 2- d) S 8 e) SO 2 Cl 2 4. Sketch the drawing of a voltaic cell that occurs containing one electrode compartment consisting of a silver strip placed in a solution of AgNO 3 and an iron strip placed in a solution of FeCl 2 with a salt bridge which was soaked in a NaNO 3 solution. a) Show the oxidation half-reaction. b) Show the reduction half-reaction. c) Determine the oxidizing and reducing agent. d) Label the anode and the cathode. e) Show the flow of electrons. f) What would happen if the salt bridge was replaced by one soaked in distilled water? g) What is the standard cell potential of this voltaic cell? h) Write the overall net ionic chemical reaction for this electrochemical cell. i) How many electrons are transferred? 5. Sketch the drawing of a voltaic cell that occurs containing one electrode compartment consisting of a aluminum strip placed in a solution of Al(NO 3 ) 3 and an nickel strip placed in a solution of NiSO 4 with a salt bridge which was soaked in a NaCl solution. a) Show the oxidation half-reaction. b) Show the reduction half-reaction. c) Determine the oxidizing and reducing agent. d) Label the anode and the cathode. e) Show the flow of electrons. f) What would happen if the salt bridge was removed? g) What is the standard cell potential of this voltaic cell? h) Write the overall net ionic chemical reaction for this electrochemical cell. i) What are the initial concentrations of the solutions?

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6 6. Using the standard reduction potential chart, calculate the standard emf for each of the following reactions. Also, label each reaction as spontaneous or non-spontaneous and indicate how many electrons were transferred. a) Cl 2 (g) + 2 I - (aq) 2 Cl - (aq) + I 2 (s) b) Cu (s) + Ba 2+ (aq) Cu 2+ (aq) + Ba (s) c) 3 Ni (s) + 2 Fe 3+ (aq) 3 Ni 2+ (aq) + 2 Fe (s) d) 2 Al 3+ (aq) + 3 Ca (s) 2 Al (s) + 3 Ca 2+ (aq) 7. By referring to the four reduction half-reactions below, determine which combination of these half-cell reactions would be most likely used to power a battery and why? Once determined write the full oxidation-reduction reaction and calculate the cell emf. (A) Ag + (aq) + e - Ag (s) (B) Cu 2+ (aq) + 2 e - Cu (s) (C) Ni 2+ (aq) + 2 e - Ni (s) (D) Cr 3+ (aq) + 3 e - Cr (s) 8. A 1M solution of Cu(NO 3 ) 2 is placed in a beaker with a strip of Cu metal. A 1M solution of SnSO 4 is placed in a second beaker with a strip of Sn metal. The two beakers are connected by a salt bridge and the two metal electrodes are linked by wires to a voltmeter. a) Which electrode serves as the anode, and which as the cathode? b) Which electrode gains mass and which loses mass as the cell reaction proceeds? c) Write the equation for the overall cell reaction. d) What is the emf generated by the cell under standard conditions? e) What is the free energy, ΔG, under standard conditions? f) If 20 g of tin is used and the reaction runs for 120 seconds, what is the current in Amps for this reaction? 9. A voltaic cell is constructed that uses the following reaction and operates at 298 K: Zn (s) + Ni 2+ (aq) Zn 2+ (aq) + Ni (s) a) What is the emf of this cell under standard conditions? b) How many grams of zinc is needed to product 10 A of current for 180 seconds at standard conditions? c) What is the equilibrium constant, K, for this reaction under standard conditions? d) Show the equilibrium constant expression for this reaction. e) What is the emf of this cell when [Ni 2+ ] = 3.00 M and [Zn 2+ ] = M? f) What is the emf of this cell when [Ni 2+ ] = M and [Zn 2+ ] = M? M(s) + 3 Ag + (aq) --> 3 Ag(s) + M 3+ (aq) Ag + (aq) + e --> Ag(s) E = V E = V 10. According to the information above, what is the standard reduction potential for the half-reaction: M 3+ (aq) + 3 e - M (s) (A) V (B) V (C) 0.06 V (D) 1.66 V (E) 3.26 V

7 11. Complete and balance the following equations (all in acidic solutions) a) Cr 2 O 7 2- (aq) + I - (aq) Cr 3+ (aq) + IO 3 - (aq) b) MnO 4 2- (aq) + CH 3 OH (aq) Mn 2+ (aq) + HCO 2 H (aq) c) NO 2 - (aq) + Cr 2 O 7 2- (aq) Cr 3+ (aq) + NO 3 - (aq) d) MnO 4 - (aq) + Cl - (aq) Mn 2+ (aq) + Cl 2 (aq) Cr 2 O 7 2- (aq) + H 2 S(g) + H + (aq) Cr 3+ (aq) + S(s) + H 2 O(l) 12. When the equation above is correctly balanced and all coefficients are reduced to lowest whole-number terms, the coefficient for H + (aq) is (A) 2 (B) 4 (C) 6 (D) 8 (E) It is observed that when silver metal is placed in aqueous thallium (I) fluoride, TlF, no reaction occurs. When the switch is closed in the cell represented above, the voltage reading is V. a) Write the reduction half-reaction that occurs in the cell. b) Write the equation for the overall reaction that occurs in the cell. c) Identify the anode in the cell. Justify your answer. d) On the diagram above, use an arrow to clearly indicate the direction of electron flow as the cell operates. e) Calculate the value of the standard reduction potential for the Tl + /Tl half-reaction. The standard reduction potential, E, of the reaction Pt e - Pt is 1.20 V. f) Assume that electrodes of pure Pt, Ag, and Ni are available as well as 1.00 M solutions of their salts. Three different electrochemical cells can be constructed using these materials. Identify the two metals that when used to make an electrochemical cell would produce the cell with the largest voltage. Explain how you arrived at your answer. g) Predict whether Pt metal will react when it is placed in 1.00 M AgNO 3 (aq). Justify your answer.

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