1. Electrochemical Cells

2. Electrochemical Cells
Electrochemical cells are a way of storing chemical potential energy.
When batteries operate, electrons in high energy states flow through wires, where their energy can be harnessed to do useful work.
For electrons to flow, there must be an acceptor ion that will place the electrons in lower energy states.

3. A voltaic cell spontaneously converts
electrical energy to chemical energy
chemical energy to electrical energy
electrical energy to nuclear energy
nuclear energy to electrical energy

4. Electrochemical Cells
In electrochemical cells, redox reactions are separated into their individual oxidation and reduction components.
Each component is called a half-cell.

5. A Typical Galvanic Cell
Zinc, the active metal, gives electrons to Cu2+, the ion of the less active metal

6. Given the reaction that occurs in an electrochemical cell: Zn(s) + CuSO4(aq) → ZnSO4 (aq) + Cu(s)
During this reaction, the oxidation number of Zn changes from
0 to +2
0 to −2
+2 to 0
−2 to 0

7. The Anode
The oxidation side (the side that loses electrons) is called the anode.
The anode is an electrode, a piece of an active metal.
The active metal loses mass when it is oxidized; ions enter the electrolyte solution.
The anode is the (-) terminal of the battery.

8. The Cathode
The cathode is the place where reduction takes place – electrons are gained by ions of the metal in the cathode electrolyte.
The cathode gains mass as a battery operates.
When the cathode electrolyte is depleted, the battery “dies.”
The cathode is the (+) terminal of a battery.

9. Anode – oxidation an ox
Cathode – reduction red cat

10. Which statement is true about oxidation and reduction in an electrochemical cell?
Both occur at the anode.
Both occur at the cathode.
Oxidation occurs at the anode and reduction occurs at the cathode.
Oxidation occurs at the cathode and reduction occurs at the anode.

11. Which half-reaction can occur at the anode in a voltaic cell?
Ni2+ + 2e- → Ni
Sn + 2e- → Sn2+
Zn → Zn2+ + 2e-
Fe3+ → Fe2+ + e-

12. The Salt Bridge The salt bridge is essential for electrons to flow.
The electrolyte solutions must be kept electrically neutral.
Every time an atom of an active metal is ionized, an anion from the salt bridge enters the electrolyte to balance off the new metal ion.
Likewise, when a metal ion is reduced in the cathode, a cation enters the cathode electrolyte to restore the charge balance.

13. The purpose of a salt bridge in a voltaic cell is to
allow for the flow of molecules between the solutions
allow for the flow of ions between the solutions
prevent the flow of molecules between the solutions
prevent the flow of ions between the solutions

14. The diagram represents a voltaic cell.
Which statement correctly describes the direction of flow for the ions in this cell when the switch is closed?
Ions move through the salt bridge from B to C, only.
Ions move through the salt bridge from C to B, only.
Ions move through the salt bridge in both directions.
Ions do not move through the salt bridge in either direction.

15. Given the statements:
A: The salt bridge prevents electrical contact between solutions of half-cells. B: The salt bridge prevents the direct mixing of one half-cell solution with the other. C: The salt bridge allows electrons to migrate from one half-cell to the other. D: The salt bridge allows ions to migrate from one half-cell to the other.
Which two statements explain the purpose of a salt bridge used as part of a voltaic cell?
A and C
A and D
C and D
B and D

16. The diagram represents a voltaic cell.
When the switch is closed, which group of letters correctly represents the direction of electron flow?
A → B → C → D
A → F → E → D
D → C → B → A
D → F → E → A

17. Voltaic Cells and Table J
The anode is the active metal
The cathode is the less active metal
On table J, the anode is the metal that is higher on table J
Electrons flow from the anode to the cathode.

18. Base your answer to the question on the diagram of the voltaic cell.
Based on the given equation, the balanced half-reaction that occurs in half-cell 1 is
Pb(s) → Pb2+(aq) + 2e−
2Ag(s) → 2Ag+(aq) + 2e−
Pb2+(aq) + 2e− → Pb(s)
2Ag+(aq) + 2e− → Ag(s)

19. Base your answer to the question on the diagram of a voltaic cell and the balanced ionic equation.
What is the total number of moles of electrons needed to completely reduce 6.0 moles of Ni2+(aq) ions?

21. A student collects the materials and equipment below to construct a voltaic cell.
two 250-mL beakers
wire and a switch
one strip of magnesium
one strip of copper
125 mL of 0.20 M Mg(NO3)2(aq)
125 mL of 0.20 M Cu(NO3)2(aq)
Which additional item is required for the construction of the voltaic cell?
an anode
a battery
a cathode
a salt bridge

22. Lab 20: Electrochemical Cells

23. Materials
Materials 1 M CuSO4, 1 M Zn(NO3)2, 1 M Mg(NO3)2, salt solution
24-well plate Mg, Zn, Cu strips
scissor, filter paper
red and black electrical wires with alligator clips
voltmeter

24. Procedure
Place a dropper full of zinc nitrate and copper sulfate in consecutive wells.
Attach a black lead to the black terminal of the voltmeter; attach a red lead to the “5” red terminal.
Take a piece of copper and attach it to the red alligator clip
Take a piece of zinc and attach it to the black alligator clip
Cut a small thin length of filter paper and saturate it with salt solution.

25. Place the “salt bridge” between the two wells, with one side in each well
Place the copper electrode in the copper solution, the zinc electrode in the zinc solution. Record the voltage.

26. Take the salt bridge out and see what happens to the voltage before moving on to the next cell.
Repeat the procedure for Zn/Mg and Mg/Cu cells. Draw pictures of your cells on the following page.
On your diagram, show which electrode is the anode (oxidation electrode) and which is the cathode (reduction electrode)

27. Show the direction of the electron flow (from anode to cathode)
Write the oxidation half reaction.
Write the reduction half reaction.
Write the balanced equation for the reaction that takes place.

28. Zn/Cu cell
_____ V
Zn/Mg cell
Mg/Cu Cell
______V
Ox ½ rxn
Red. ½ rxn
Balanced Equation

29. Questions
Which cell had the largest voltage? ____________________ Based on the activity series, explain why.
What is the function of the salt bridge?
What happens to the voltage when the salt bridge is removed? Give an explanation for this effect.
Using Table J, explain how you can determine the anode and the cathode of an electrochemical cell

30. A student constructed a cell using copper and silver electrodes with copper (II) sulfate, and silver nitrate as electrolytes. Sketch a diagram below and label the following:
a) anode and cathode, with signs (+/-)
b) electrolytes in half cells, with ionic formulas
c) direction of electron flow through the wire
d) salt bridge with direction of (+) and (-) ion flow

31. e) In the electrochemical cell above, the copper electrode loses mass when the cell operates. In the Zn|Cu cell you constructed, the copper electrode gains mass. Explain the difference between these cells.

32. Electolysis
Draw and label a picture of your electrolytic cell. Identify the anode, cathode, show the direction of electron flow.
Explain why copper is both oxidized and reduced in this reaction.
Why are two identical electrodes used?
Compare and contrast voltaic and electrolytic cells (at least 4 similarities/differences)