Voltaiccell20
20.4 Galvanic or Voltaic Cells
In electrochemistry, a voltaic cell is a specially prepared system in which an oxidation-reduction reaction occurs spontaneously. This spontaneous reaction produces an easily measured electrical potential. Voltaic cells have a variety of uses. In this experiment, you will prepare a variety of semi-microscale voltaic cells in a 24-well test plate. A voltaic cell is constructed by using two. Voltaic Cells Model. Connect to the wecxadb site: Scroll down to find the diagram of a voltaic cell. Select the metals and solutions for the cell to set up the simulation for each voltaic cell as.
Chemical Concepts Demonstrated: Voltaic/galvanic cells, relative half-cell potentials
Demonstration:
- 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- | Eo = 0.76 V |
| cathode: | 2 H + (aq) + 2 e - ---> H2 (g) | Eo = 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- | Eo = - 0.34 V |
| cathode: | 2 H + (aq) + 2 e - ---> H2 (g) | Eo = - 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.
Voltaic Cell Emf
| anode: | Cu (s) ---> Cu 2+ (aq) + 2 e- | Eo = - 0.34 V |
| cathode: | Zn 2+ (aq) + 2 e- ---> Zn (s) | Eo = - 0.76 V |
| Cu (s) + Zn 2+ (aq) ---> Cu 2+ (aq) + Zn (s) | Eo cell = -1.10 V |
Voltaic Cell Is
To set up a voltaic cell using these half reactions, one would have to reverse the leads to the voltmeter.
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