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Chemistry LibreTexts

Galvanic or Voltaic Cells

  • Page ID
    3002
  • [ "article:topic", "Fundamental" ]

     

    Chemical Concepts Demonstrated

    • Voltaic/galvanic cells, relative half-cell potentials

    Demonstration

    204.gif 161-pr11.gif

    • One of the dishes is filled with ZnSO4 and the other with HCl.
    • A strip of Zn metal is attached at one end to the posts of the electrochemistry template and is placed at the other end into the dish filled with Zn2+.
    • The hydrogen electrode is attached, placed into the HCl solution, and H2 gas is bubbled in. Insert the salt bridge.
    • The Zn2+/Zn half-cell is replaced with a Cu2+/Cu half-cell.
    • The H+/H2 half-cell is replaced with a Zn2+/Zn. (picture 2)   
       

    Observations

    The potential in the absence of the salt bridge is 0.00 V.  After the salt bridge is inserted, the potential of the first set up is around + 0.76 V and the cell is a galvanic or voltaic cell.  The Zn2+/Zn half-cell is the anode the H+/H2 is the cathode.

    In the second set up, both the magnitude and the sign of the potential change.   The potential is now roughly - 0.34 V.

    Picture 2 shows the third set up.  The potential is now - 1.10 V. If the leads are changed, the cell potential becomes + 1.10 V and the cell becomes a galvanic or voltaic cell.

    Explanations (including important chemical equations)

    With the leads connected so as to produce a cell potential of + 0.76 V, the half reactions are:

    anode: Zn (s) ---> Zn 2+ (aq) + 2 e- E= 0.76 V
    cathode: 2 H + (aq) + 2 e - ---> H2 (g) E= 0.00 V
      Zn (s) + 2 H +  (aq) ---> Zn 2+ (aq) + H2 (g) Eo cell = 0.76 V

    If the standard-state potential for the H+/H2 half-cell is assumed to be 0.00 V, and the potential for the anode half-reaction is equal in magnitude but opposite in sign to the standard-state potential for the Zn2+/Zn couple, then the standard-state reduction potential for the Zn2+/Zn half-cell must be - 0.76 V.

    If the Zn2+/Zn half-cell is replaced with a Cu2+/Cu half-cell without reversing the leads to the voltmeter, the overall cell potential is - 0.34 V and the standard-state reduction potential for the Cu2+/Cu couple is therefore + 0.34 V.

    anode: Cu (s) ---> Cu 2+ (aq) + 2 e- E=  - 0.34 V
    cathode: 2 H + (aq) + 2 e - ---> H2 (g) E=  - 0.00 V
      Cu (s) + 2 H +  (aq) ---> Cu 2+ (aq) + H2 (g) Eo cell  = -0.34 V

    If the H+/H2 half-cell is replaced with a Zn2+/Zn half-cell, the overall cell potential should be - 1.10V. 

    anode: Cu (s) ---> Cu 2+ (aq) + 2 e- E=  - 0.34 V
    cathode: Zn 2+ (aq) + 2 e---> Zn (s) E=  - 0.76 V
      Cu (s) + Zn 2+ (aq) ---> Cu 2+ (aq) + Zn (s) Eo cell  = -1.10 V

    To set up a voltaic cell using these half reactions, one would have to reverse the leads to the voltmeter.

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