1. Chapter A2
2.4 – Voltaic Cells

2. Voltaic cells The rest of the chapter focuses on WHY this occurs.
the focus on metals so far been been what happening on the surface of the metal
tarnishing/ rusting, (aka color change), precipitate formation or bubbles/ gas formation
The rest of the chapter focuses on WHY this occurs.
a focus on the movement of electrons between the two metals

3. Voltaic cells
though commonly a voltaic cell is referred to as a “battery”, technically cells are only referred to as a battery when several are together
when an electronic device is operating, voltaic cells provide a continuous flow (current) of electrons, which is converted into current to power the device

4. Voltaic cells the voltaic cell that you know looks like this:
this is the version of the voltaic cell we will make in the lab.

5. Voltaic cells
an electrode is a solid piece of metal that is suspended in a solution (of the same metal ions as the electrode) and connected to an external circuit.

6. Voltaic cells
the electrode zinc, is immersed into an electrolyte solution, where the zinc electrode acquires an excess of electrons, becoming negatively charged

7. Voltaic cells
the other electrode is usually composed of a different material (copper) and will become positively charged

8. Voltaic cells
once a circuit is closed between the two electrodes, the electrons will repel from the negative zinc electrode, pass through the circuit and flow through to the positive electrode

9. Voltaic cells
the reaction will continue until the negative electrode can no longer be supply electrons.

10. Voltaic cells

11. Voltaic cells
a salt bridge is a glass U-shaped tube that is filled with an ionic solution
this is to allow for free flow of electrons from one solution to the other

12. How the cell works:
because it is the more reactive of the two metals, the zinc electrode will become oxidized, (lose electrons)
these electrons will travel from the electrode, through a metal wire, and then into an electronic device
the device; a voltmeter, measures the quantity of electrons passing through it (= amount of electricity)

13. How the cell works:
the electrons will pass through the device, back through another wire, into the copper electrode
these electrons will be attracted to the Cu2+(aq) ions in the solution and they will be reduced.
over time, the zinc electrode shrinks in size (as Zn  Zn2+) and the copper electrode grows (Cu2+  Cu)

14. How the cell works:
if the two solutions were NOT connected, the zinc would run out of electrons and the cell would stop working
the solution in the salt bridge allows a continuous flow of electrons back into the zinc solution

15. How the cell works:

16. Analyzing a voltaic cell:
Step #1: identify the electrode where oxidation occurs
locate the two metals on the activity series (right side)
the metal closer to the BOTTOM will be OXIDIZED = reducing agent
the electrode that is oxidized is called the anode
the other electrode is reduced, and is called the cathode

17. Analyzing a voltaic cell:
Step #2: describe the oxidation process in the anode
write the oxidation half-reaction
Eg. Pb(s)  Pb2+(aq) +2e-
electrons leave the anode and travel to the external circuit running the electronic device
the voltmeter measures the quantity of electrons (amount of electricity) being produced
because the anode is the electrode where the electrons originate, it is considered the negative electrode

18. Analyzing a voltaic cell:
Step #3: describe the reduction process in the anode
the electrons travel through the voltmeter and into the cathode
the electrons are attracted to the positively-charge metal ions in the cathode solution
the cathode ions will unite with the electrons and form a solid metal, which is deposited on the electrode
Eg. Ag+ (aq) + e-  Ag (s)

19. Analyzing a voltaic cell:
Step #4: describe how the salt bridge completes the circuit
all electrical circuits require a complete circuit in order to function.
the salt bridge connects the cathode back to the anode to replenish the electrons on the anode side
the salt bridge contains a third ionic solution
the positive ions from the salt bridge solution will be attracted to the cathode, while the negative ions from the salt bridge solution will migrate toward the anode.

20. Voltaic Cell- Example in this voltaic cell: anode cathode KCl
zinc is the _________ – it is oxidized
copper is the __________– it is reduced
the solution in the salt bridge is _____________ (aq)
chloride ions are a spectator ion – their job is to replenish the electron supply at the anode
anode
cathode
KCl

21. Cell Notation anode salt bridge cathode
voltaic cells can also be represented using short hand cell notation
Zn(s) | Zn2+(aq) || Cu2+(aq) | Cu(s)
anode salt bridge cathode
 oxidation reduction
the anode is listed on the left, the cathode on the right (think alphabetical order)
the vertical line | represents a boundary between a metal and its solution
the double line || represents the salt bridge

22. Practice Problem #1:
a) Draw a voltaic cell using the following supplies:
two beakers
U-tube & cotton balls
wire & voltmeter
tin and magnesium strips
solutions of SnSO4(aq), MgSO4(aq), and NaNO3(aq)
b) Label the direction of e- flow, the anode, cathode, OA, RA, - and + electrodes, voltmeter and salt bridge
c) Write the short hand cell notation

23. Practice Problem #1 (Solution):

24. Assignment: Practice problem (page 87) Practice problems (page 91)34
Practice problems (page 91)
37 & 39
Practice problem (page 92) 40
2.4 Summary 9page 94)
Q’s 2, 3 & 6 pg 94

25. Homework Check-up Zn(s) / Zn2+(aq) // Ni2+ (aq) / Ni(s)
Use the cell notation below to answer the following questions. Assume the salt bridge contains a solution of potassium nitrate (KNO3(aq) )
Zn(s) / Zn2+(aq) // Ni2+ (aq) / Ni(s)
Identify which metal will be oxidized and which would be reduced
Identify the anode and the cathode
Write the half reactions that occur at each electrode
Draw the voltaic cell. Label the direction of the electron flow and the anions of the salt bridge.
Homework Check-up

26. Zn(s) / Zn2+(aq) // Ni2+ (aq) / Ni(s)
According to the activity series, zinc is the more reactive metal- so Zn(s) is oxidized and the Ni2+(aq) are reduced.
Anode = Oxidation= zinc metal Cathode = Reduction = nickel metal
Oxidation: Zn(s) Zn2+(aq) + 2e–
Reduction: Ni2+(aq) + 2e–  Ni(s)
Identify which metal will be oxidized and which would be reduced (1 mark)
Identify the anode and the cathode (1 mark)
Write the half reactions that occur at each electrode

27. Zn(s) / Zn2+(aq) // Ni2+ (aq) / Ni(s)
d) Draw the voltaic cell (2 marks) Label the direction of the electron flow (1 mark) and the anions of the salt bridge. (1 mark)
Cell must have 2 beakers, (one with Zn(s) and Zn2+(aq), one with Ni(s) and Ni 2+ (aq)), connected by wires to a voltmeter, and a salt bridge
Electrons must leave the Zn anode and flow towards the Ni cathode
The negative NO3- anions are attracted to the anode. The positive K+ cations are attracted to the cathode
(think alphabetical…Anion-Anode. Cation= Cathode)

28. Assignment:
Complete the pre-lab assignment for the Voltaic Cells Lab
Your pre-lab MUST BE done, in order to participate in the lab! ? ?

29. Chapter A2
2.5 – Electrolytic Cells

30. Electrolytic vs. Voltaic
an electrolytic cell is a system where a non-spontaneous redox reaction is forced to occur
a reaction that is non-spontaneous will only occur if energy is added
in an electrolytic cell, energy is added in the form of electricity
Voltaic
Electrolytic
spontaneous?
yes no
requires energy?
produces voltage?
use
energy source
electroplating
change in energy
exothermic
endothermic

31. Electroplating
metals, like gold and silver, that are the most stable and corrosion-resistant are also the most expensive
to manufacture a metal object that is resistant to corrosion it would NOT be cost-effective to make the whole thing out of gold
instead, a thin coating of gold is applied to the surface of a more affordable metal

32. Electroplating
the object to be coated is submerged in a solution of the metal ions (e.g. silver ions for objects that are to be coated in silver metal)
an external energy source (a battery) supplies energy forcing electrons to flow into the object
the negatively charged electrons will attract the positively charged metal ions from the solution, and turn them back into metal atoms, which will accumulate on the surface of the object to be plated.

33. Electrolytic cells Step #1:
electrons from the plating (the expensive) metal cathode are attracted to the + electrode of the power source
by removing electrons from the metal atoms, ions are formed and added into the solution

34. Electrolytic cells Step #2: Step #3:
once removed from the metal, the free electrons flow through the power source.
Step #3:
electrons are forced out of the - end of the power source and accumulate on the surface of the object to be plated

35. Electrolytic cells Step #4:
positive Au+(aq) from the solution are attracted to the negative electrons in the object to be plated
the Au+(aq)ions gain the electrons, and turn back into solid gold coating the object.

36. Electroplating
electroplating is a good way to protect metals that are easily oxidized, like iron
metals that work as good electroplaters (coatings) are chromium (aka chrome), platinum, silver and gold

37. Gold jewelry solid gold gold plated
two types of gold jewelry exist - that which is made out of solid gold, and that which is gold plated
solid gold
karats - pure gold is 24K
gold is a soft metal, so it is often combined with other metals like brass (copper and zinc) and nickel to make it more durable
the number of karats in the gold refers to how many 1/24th of gold it contains
gold plated
if you have a piece of gold plated jewelry, care must be taken to avoid any deep scratches
deep scratches will expose the oxidizable metal underneath

38. Other uses for electrolytic cells
refining metals
a sample of impure metal (anode), pure metal (cathode)
ions of the pure metal will travel from the anode to the cathode to build up the atoms of pure metal
electrolysis
decomposition of a compound by means of an electric current
e.g. electrolysis of water makes it decompose into O2 and H2

39. Other uses for electrolytic cells
producing non-metals
non-metals, especially the halogens, are difficult to obtain in pure form because they are so reactive
non-metal atoms will accumulate around the anode of an electrolytic cell
recharging voltaic cells
when you use a battery recharger, you are using an electrolytic cell to reverse the process that occurs normally in the voltaic cell
you are literally re-charging the voltaic cell with a new supply of electrons

40. Assignment:
Complete the Voltaic & Electrolytic cells Worksheet

41. Assignment:
Prepare for your Chapter A2 Exam