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Saturday Study Session 1 2nd Class

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1 Saturday Study Session 1 2nd Class
Electrochemistry

2 Galvanic Electrolytic
Use venn diagram to compare and contrast electrolytic and galvanic cells. The students should be able to provide many points to insert into the chart. Use this as a point of review before beginning the MC questions.

3 1. A student is assigned the task of creating a galvanic cell with the highest voltage using two of the metals shown at 1.0 M and 1 atm pressure. Which two metals should the student utilize? Cr and Pb Pb and Cu Cu and Cr Ag and Cr

4 Question 1 Answer D Clue: Two cells with the greatest difference in cell potential will produce the highest voltage. Some teachers say “cathode – anode.” I teach my students to change the sign on the anode and then add the values together. Be aware of these different teaching methods and perhaps ask the students what they do before working the problem.

5 2. A redox reaction has an equilibrium constant of K = 2. 6 x 105
2. A redox reaction has an equilibrium constant of K = 2.6 x 105. Which of the following statements best represents the algebraic signs of Ecell and Grxn for this reaction? A) Ecell is positive and Grxn is positive. B) Ecell is negative and Grxn is negative. C) Ecell is positive and Grxn is negative. D) Ecell is negative and Grxn is positive.

6 Question 2 Answer C Use K (greater than 1) to determine the sign on G and E.

7 + - =1 K ΔG E >1 (positive exponent) <1 (negative exponent)
Take this opportunity to discuss spontaneity as well as the equations that relate these 3 variables and equilibrium.

8 Mass of electrode Time 40.02 grams 0 min 42.34 grams 20 min 44.68 grams 40 min A current of electricity was applied to a solution of copper chloride for the times shown above.

9 Mass of electrode Time 40.02 grams 0 min 42.34 grams 20 min 44.68 grams 40 min 3. Which of the following best identifies the reason why the electrode is changing mass. The electrode shown must be the anode. Metal atoms are being converted to metal ions which add mass to the anode. The electrode shown must be the cathode. Metal atoms are being converted to metal ions which add mass to the anode. The electrode shown must be the anode. Metal ions are being converted to metal atoms which often adhere to the electrode. The electrode shown must be the cathode. Metal ions are being converted to metal atoms which often adhere to the electrode.

10 D Cathode: X+ + e-  X Anode: X X+ + e- Question 3 Answer
Reduction takes place at the cathode and oxidation at the anode, not matter if it is an electrolytic or galvanic cell.

11 This website provides great images of electroplating, including flow of electrons, etc. (http://media.pearsoncmg.com/bc/bc_0media_chem/chem_sim/electrolysis_fc1_gm_ /main.html) You may want to load it before hand and have it ready to demonstrate.

12 Another example of electroplating that may assist

13 Mass of electrode Time 40.02 grams 0 min 42.34 grams 20 min 44.68 grams 40 min c. (4.66)(96,500) (63.55)(40)(60)(2) 4. A student completed the experiment above to plate out copper solid. Which of the following expressions could be used to determine the number of amps applied to the system for 40 minutes? d. (4.66)(2)(96,500) (44.68)(2)(96,500) (63.55)(40)(60) b. (4.66)(2)(96,500) (63.55)(40)

14 Use dimensional analysis and remember
Question 4 Answer D Use dimensional analysis and remember Amp = C/sec The next slide shows the set-up for this question, but I think it would be better to show it on the board step-by-step.

15 Amps = C/sec Beginning with grams, use dimensional analysis to find Coulombs, then divide by time (converted to seconds) 4.66 g x 1 mole x 2 e x C x x min 63.55 g 1 Cu mole e min sec

16 5. When the switch on the voltaic cell to the right is closed, the mass of the lead electrode decreases. Which of the following is true? Electrons flow from the silver electrode to the lead electrode. The lead electrode is the cathode. Silver ions are being reduced. Electrons flow to silver through the salt bridge.

17 Question 5 Answer C

18 The link is for a great/quick animation that shows the flow of electrons as well as the flow of anions/cations in the salt bridge. It allows you to zoom in to specific areas of the cell.

19 6. Which of the following occurs if a 50-milliliter sample of a 1-molar NaCl solution is added to the left beaker? Voltage increases. Voltage decreases. Voltage becomes zero and remains at zero. No change in voltage occurs.

20 Clues: Lead is the anode. A precipitate will form.
Question 6 Answer A Clues: Lead is the anode. A precipitate will form. Here you can use Nernst equation to explain, but it may be more beneficial to explain using the balanced equation and equilibrium concepts (lead concentration goes down, causing the reaction to shift to the right and therefore resulting in an increase in voltage).

21 Free Response 1 (7 possible points)
Possible points awarded for: 3 Cu2+(aq) + 2 Cr(s)  3 Cu(s) Cr3+(aq) (2 points) b. Copper (1 point) c. Salt bridge—maintains neutrality in the two half-cells (2 points) d. i. Increase voltage (1 point) ii. Decrease voltage (1 point) If time, ask the students to calculate the voltage, gibbs free energy, and equilibrium constant for the reaction.

22 Free Response 2 (10 points possible)
Possible points awarded for: Electrons from from right to left. Cathode on the left; anode on the right (where gas is being produced). (3 points) 2 Cl-  Cl2 + 2 e (1 point) Charge on Fe is +2 (so FeCl2)—work is shown on next slide (3 points) Fe Cl-  Feo Cl (2 points) C will likely be quite a challenge for students. Review the work on the next slide as you may have to guide them through it. The points are just a guestimation. I would award 1 point for finding moles of e on c. 1 point for finding moles of Fe. 1 point for the ratio.

23 Work for #2 C


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