Saturday Study Session 1 2 nd Class

1. Saturday Study Session 12nd Class Electrochemistry

2. Galvanic Electrolytic

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.

5. 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 Ecelland Grxn for this reaction? A) Ecell is positive and Grxn is positive. B) Ecellis negative and Grxn is negative. C) Ecellis 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.

8. A current of electricity was applied to a solution of copper chloride for the times shown above.

9. 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. Question 3 Answer D Cathode: X+ + e- X Anode: X X+ + e-

13. 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? • (44.68)(2)(96,500) (63.55)(40)(60) b. (4.66)(2)(96,500) (63.55)(40) c. (4.66)(96,500) (63.55)(40)(60)(2) d. (4.66)(2)(96,500) (63.55)(40)(60)

14. Question 4 Answer D Use dimensional analysis and remember Amp = C/sec

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 96500 C x 1 x 1 min 63.55 g 1 Cu 1 mole e 40 min 60 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. http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swfhttp://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/galvan5.swf

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. Question 6 Answer A Clues: Lead is the anode. A precipitate will form.

21. Free Response 1(7 possible points) • Possible points awarded for: • 3 Cu2+(aq) + 2 Cr(s) 3 Cu(s) + 2 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)

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) • Fe2+ + 2 Cl- Feo + Cl2(2 points)

23. Work for #2 C