Module 6

Lesson 1.2  The Operation of a Voltaic Cell


Key Concepts


Remember that the operation of a voltaic cell is based on a spontaneous redox reaction.

Given the cell notation for a voltaic cell, you are expected to be able to write the half-reactions and the net reaction. To do this, utilize the same procedure that you used to predict redox reactions in the previous module.  As you did in Module 5, consider that oxidizing agents and reducing agents may sometimes occur in combination.  However, before you identify an oxidizing agent or a reducing agent to be in combination, make sure that both entities in the combination are in contact with each other (i.e. in the same half-cell). 

(Click here to review this procedure)

To understand the operation of a voltaic cell, examine the zinc-copper voltaic cell as an example. As shown in Lesson 1.1, the cell can be represented by the following cell notation: 

\( \mathrm { Zn(s)~ |~Zn^{+2}(aq)~ || ~Cu^{+2}(aq)~ |~Cu(s) } \)



  1. Identify the strongest oxidizing and reducing agents in the mixture.


  2. Write the reduction and oxidation half-reactions.

    "An ox ate a red cat."

    Oxidation occurs at the anode.

    Reduction occurs at the cathode
    \( \mathrm { Zn(s) \rightarrow Zn^{2+}(aq) + 2e^- } \)

    \( \mathrm { Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) } \)





  3. To arrive at the net reaction, use coefficients to balance the number of electrons, add the two half reactions together, and cancel electrons. In this example, there is no need to multiply by a co-efficient because equal numbers of electrons are being lost and gained.


    \( \mathrm { Cu^{2+}(aq) + Zn(s) \rightarrow Cu(s) + Zn^{2+}(aq) } \)


    You are also expected to diagram voltaic cells. The zinc-copper cell can be depicted/labelled as shown below:



    Fig. 2   Zinc - copper cell


    • The SOA undergoes reduction at the cathode, which is the positive (+) electrode.
    • The SRA undergoes oxidation at the anode, which is the negative (-) electrode.
    • Electrons flow from the anode to the cathode through the connecting wire.
    • To maintain electrical neutrality, cations flow towards the cathode through the porous boundary (salt bridge or porous cup) and anions flow to the anode through the porous boundary.

Watch

Click this link to watch a video that reviews what you have learned about the zinc-copper cell. Observe the changes that occur as the cell operates. 

Galvanic Cell with Zinc and Copper
© mtchemers
You can also click to watch this video on Youtube  (Actually this is the best video, but I cannot guarantee that the link will always work)

Read pages 622 - 624 in the textbook.

Check Your Understanding


To test your understanding of the zinc-copper cell, answer the following questions. Click the link below to check your answers.


  1. In the zinc-copper voltaic cell studied above, which entity is losing electrons and which entity is gaining electrons?

  2. If the salt bridge is removed from the cell, what will happen? Why is the salt bridge necessary to the functioning of the cell?

  3. Predict observable changes to the electrodes or to the solutions. Use the half reactions stated earlier to assist you in suggesting reasons for the changes.

  1. Zn(s) is losing electrons and copper ions are gaining electrons.


  2. The salt bridge allows ions in the electrolytes to move, completing the flow of charge in the circuit and keeping the cell electrically neutral.

    You can see from the half-reaction equations that copper ions are removed from the solution when they gain electrons to form solid copper. Because the solution started as electrically neutral, removing positively charged ions should result in the solution surrounding the cathode to become negatively charged. However, this does not occur because positively charged ions flow through the salt bridge towards the cathode.

    At the anode, positive ions are being formed - in this case, zinc ions. However, positive charge does not build up at the anode because negatively charged ions move through the salt bridge into the anode compartment.


  3. A reddish-brown sediment should accumulate on the surface of the copper electrode. This is the copper metal being deposited as copper ions in solution gain electrons (as described by the reduction half-reaction). Because the copper ions are being removed from the solution containing the copper electrode, this solution appears to have a slightly less intense blue colour after the cell has been operating for a while. Although no visible changes can be observed on the zinc electrode, this electrode would diminish in size if the voltaic cell continued to operate for a long time.



Voltaic Cells with Inert Electrodes

In voltaic cells containing metals and metal ions, the electrodes are usually the metals themselves. However, voltaic cells can be constructed with half-cells that do not involve a metal as a reactant. If the strongest oxidizing agent or the strongest reducing agent is in solution, then an inert electrode can be used. The term "inert" means that the electrode does not react itself during the cell's operation. Instead, the inert electrode merely provides a surface on which a half-reaction can occur and a location to connect a wire. The material used for an electrode must be an electrical conductor able to transfer electrons in the system. Two common inert electrodes are carbon C(s) and platinum Pt(s).


An example of a voltaic cell that utilizes inert electrodes is shown below.


\( \mathrm { C(s)~|~Fe^{2+}(aq),~ Fe^{3+}(aq)~||~Cr_2O_7^{2-}(aq),~ H^+(aq)~|~C(s) } \)


The half-reactions and the net reaction are as follows:




Fig. 3


Watch

 

  Read pages 625 to 626 in the textbook to better understand how an inert electrode is used in a voltaic cell.

Check Your Understanding


Complete Practice question 6(b) on page 626 of the textbook. Check your work by clicking the link below.