Electrochemistry (Galvanic and Electrolytic Cells) Exchange of energy in chemical cells

Size: px

Start display at page:

Download "Electrochemistry (Galvanic and Electrolytic Cells) Exchange of energy in chemical cells"

Horace Bryan
6 years ago
Views:

Electrochemistry (Galvanic and Electrolytic Cells) Exchange of energy in chemical cells Oxidation loss of electrons (oxidation number increases) OIL RIG Reduction gain of electrons (oxidation number

1 Electrochemistry (Galvanic and Electrolytic Cells) Exchange of energy in chemical cells Oxidation loss of electrons (oxidation number increases) OIL RIG Reduction gain of electrons (oxidation number decreases) LEO GER Galvanic Cell Electrolytic Cell Thermodynamically favored energy produced (Spontaneous) Not thermodynamically favored energy source must be present (nonspontaneous) (-) ΔG (+) E cell + G (-) E cell 2 containers to separate the half reactions 1 container-both half reactions in same container anode (-) electrode where oxidation occurs an ox anode (+) electrode where oxidation occurs an ox cathode (+) electrode where reduction occurs red cat cathode (-) electrode where reduction occurs red cat Salt bridge used to balance the charge EPA electrolytic positive anode Electrons flow from anode to cathode ) Electrons flow from anode to cathode Electrode made of metal sitting in solution of the same metal nitrate Energy source is necessary to force electrons toward the negative cathode Inert electrode (platinum or graphite) is used when a solution or gas is involved, ex. Fe 2+ Fe 3+ Inert electrodes are used (platinum or graphite) Electroplating-metal used Is a battery Needs a battery Voltmeter wire Cd Ag 1 M Cd(NO ) 3, 2 salt bridge 1 M AgNO, 3 Standard reduction potentials (standard conditions: 1 atm, 1.0 M, 25⁰C) Each ½ reaction has a cell potential; most positive gets reduced Flip the other ½ reaction and flip the sign on the cell potential E⁰ cell = E⁰ oxicdation + E⁰ reduction or E o cell = E o cathode E o anode Write ½ reactions and combine to get the overall reaction Concentration cells Write the overall reaction and then look at the change in concentration (not an equilibrium therefore Le Chatelier is not a proper justification) Greater product [ ] voltage decrease Greater reactant [ ] voltage increase Q calculation for the ions Q = [products] [reactants] Favored K > 1 E cell > 0 ΔG < 0 ΔG = R T ln K ΔG = n F E Group IA and IIA cations are not easily reduced Polyatomic ions will not undergo oxidation If water is present, always consider: Oxidation of water: 2 H 2O O H + + 4e - E⁰ = 0.83 V Reduction of water: 2 H 2O + 2e - H 2 + OH _ E⁰ = 1.23 V If you have an aqueous solution, evaluate to see what is oxidized or reduced; most positive happens Faraday s Law Used to calculate mass produced, time or amperage Conversion factors 1 amp = Coulomb/second 1 Faraday = coulomb/1 mole of electron 1 volt = 1 Joule/coulomb # coulombs = It (current x time) When working a problem using Faraday s Law, most will begin with either mass or time

2 Multiple Choice 1. The galvanic cell based on the reactions: Ag e - Ag E⁰ = 0.80 V Fe e _ Fe 2+ E⁰ = 0.77 V A) Has electrons lost by Ag + and gained by Fe 2+ B) Has the mass of the silver electrode decreasing as this reaction proceeds C) Has an overall cell potential difference (voltage) of V D) Has electrons flowing from Fe 2+ to Ag + Given the following data: Ca e - Ca E⁰ = 2.87 V Co e - Co E⁰ = 0.28 V Pb e - Pb E⁰ = 0.13 V Sn e - Sn E⁰ = 0.14 V Zn e - Zn E⁰ = 0.76 V 2. Which of the following correctly describes the ease of oxidation of the substances listed under standard state conditions? A) Ca 2+ > Zn 2+ > Co 2+ > Sn 2+ > Pb 2+ B) Pb 2+ > Sn 2+ > Co 2+ > Zn 2+ > Ca 2+ C) Ca > Zn > Co > Sn > Pb D) Pb > Sn > Co > Zn > Ca A galvanic cell is composed of the following: Ag e - Ag E⁰ = 0.80 V Al e - Al E⁰ = 1.66 V 3. What would happen to the cell emf if NH 3 were added to the silver cell and Ag(NH 3) + 2 forms? A) No change B) Increased C) Decreased D) Cannot be determined without additional information 4. How many moles of electrons are transferred between the substance being oxidized and the substance being reduced in the reaction given below? 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2O (l) A) 5 B) 10 C) 16 D) In the reaction, 2 KMnO H 2SO H 2S 5 S + 2 MnSO 4 + K 2SO H 2O the oxidation number of sulfur changes from A) 2 to 0 B) 0 to 2 C) +5 to 5 D) +6 to +4

3 A standard Fe 2+ /Fe 3+ half-cell is connected to a standard Pb 0 /Pb 2+ half-cell. Pb e - Pb 0 E⁰ = 0.13 volts (electrode 1) Fe 3+ + e - Fe 2+ E⁰ = volts (electrode 2) 6. Which gives the changes in masses of the electrodes that occur during operation of this cell? Mass of electrode 1 Mass of electrode 2 A) decreases increases B) decreases remains the same C) increases decreases D) increases remains the same 7. When the value of E⁰ for a standard galvanic cell is greater than zero, which ranges apply to ΔG⁰ and K eq for the cell reaction? ΔG⁰ K eq A) <0 >1 B) <0 <0 C) >0 >1 D) >0 <0 8. Copper may be used for electroplating, with a half-reaction of Cu e - Cu. What is the mass of copper plated if a current of 10.0 A is applied to a Cu 2+ solution for 60.0 minutes? A) g B) 11.8 g C) 23.7 g D) 47.4 g 9. Approximately what minimum quantity of charge, measured in coulombs, is needed to produce 0.15 moles of chromium metal in an electrolytic cell that contains 0.75 M solution of Cr(NO 3) 3? A) 5,000 coulombs B) 15,000 coulombs C) 25,000 coulombs D) 45,000 coulombs 10. Suppose a solution in an electrolytic cell contains the ions Zn 2+, Cu 2+, Cd 2+, and Ag +. If the voltage is initially very low and is gradually turned up, in which order will the metals be plated out onto the cathode? Ag + + e - Ag E⁰ = 0.80V Zn e - Zn E⁰ = 0.76V Cu e - Cu E⁰ = 0.34V Cd e - Cd E⁰ = 0.40V A) Zn 2+, Cd 2+, Cu 2+, Ag + B) Cu 2+, Ag +, Zn 2+, Cd 2+ C) Ag +, Cu 2+, Cd 2+, Zn 2+ D) Cd 2+, Zn 2+, Ag +, Cu 2+

4 Free Response 1. A galvanic cell is constructed using a chromium electrode in a 1.00 molar solution of Cr(NO 3) and a copper electrode in a 1.00 molar solution of Cu(NO 3) 2. Both solutions are at 25 C. Cr e - Cr Cu e - Cu E⁰ = 0.74 V E⁰ = 0.34 V (a) Write a balanced net ionic equation for the spontaneous reaction that occurs as the cell operates. (b) A partial diagram of the cell is shown. (i) (ii) (iii) (iv) Which metal is the cathode? What observation would indicate this? What additional component is necessary to make the cell operate What function does the component in (iii) serve? (c) How does the potential of this cell change if the concentration of Cr(NO 3) 3 is changed to 3.00 molar at 25 C? Explain.

5 2. a) Balance this equation IO I -1 I 2 3. Label the cell, including: o Anode/cathode o the solution in each beaker o the directions the electrons move in the circuit o assuming the salt bridge contains NaNO 3, the direction the ions move. o Write a balanced equation. Assume the metal ions are in 1 M aqueous solution o Is this cell spontaneous? Justify your answer. Fe 3+ + e - Fe 2+ Cu e - Cu E⁰ = 0.77 V E⁰ = 0.34 V Voltmeter wire Cd Ag 1 M Cd(NO ) 3, 2 salt bridge 1 M AgNO, 3

6 4. An unknown metal M forms a soluble compound, M(NO 3) 2. A solution of M(NO 3) 2 is electrolyzed. When a constant current of 2.50 amperes is applies for 35.0 minutes, 3.06 grams of the metal M is deposited. Calculate the molar mass of M and identify the metal. 5. In an electrolytic cell, a current of ampere is passed through a solution of a chloride of iron, producing Fe (s) and Cl 2 (g). a) Write the equation for the reaction that occurs at the anode. b) When the cell operates for 2.00 hours, 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 25⁰C and 750 mm Hg, are produced when the cell operates as described in part (b)?

AP CHEMISTRY NOTES 12-1 ELECTROCHEMISTRY: ELECTROCHEMICAL CELLS

AP CHEMISTRY NOTES 12-1 ELECTROCHEMISTRY: ELECTROCHEMICAL CELLS Review: OXIDATION-REDUCTION REACTIONS the changes that occur when electrons are transferred between reactants (also known as a redox reaction)