| Contents for this page | Related topics |  |
|---|---|---|
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Direct electron transfer The Cu-Zn cell Confusion with electrodes Additional questions |
Electrolysis Standard electrode potentials How to balance redox reactions |
 Data Glossary |
| Learning Outcomes | ||
| After studying this section, you will (a) the principle of the Cu-Zn electrochemical cell, (b) be able to name its constituent parts, and (c) know the processes which takes place at the anode and cathode. | ||
When a piece of metallic zinc is dipped in a solution of copper sulphate, the zinc is coated with metallic copper, and zinc ions pass into the solution:
This is a REDOX REACTION, which is the sum of the two half-reactions shown below:
There is a DIRECT TRANSFER from zinc to copper ions, since the two half-reactions take place at the same site, that is, for example, in the same beaker.
It is possible to build an apparatus where the two half-reactions take place at separate sites, with electrons being transferred indirecty. Such an apparatus is known as an ELECTROCHEMICAL, GALVANIC, OR VOLTAIC CELL. A wide variety of such cells may be constructed. The Cu-Zn or DANIELL cell will illustrates the general principles involved.
As shown in the diagram on the right, the left hand compartment of the cell consists of a copper rod or plate partly immersed in a solution of copper(II) sulphate. The right hand compartment consists of a zinc rod or plate dipping into a solution of zinc sulphate. Contact between the two compartments is made with a "salt bridge" (a glass tube filled with a jelly containing a suitable electrolyte such as K2SO4). Finally, the copper and zinc plates (the ELECTRODES) are linked by a conducting wire, which completes the circuit. |
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The apparatus acts as a battery. Current will flow in the wire, as shown by an ammeter A in the circuit. The copper electrode is found to be the positive pole of the cell, with the zinc being the negative pole. The potential difference of the cell depends, amongst other things, on the concentration of the solutions in the electrode compartments and the temperature. When, in this cell, they are both 1.0 mol.dm-3, the potential difference is about 1.1 volt.
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Have another look at the cell, shown here on the left. The left-hand compartment (with the copper sulphate) is the site for the reduction half-reaction:
The compartment on the right is the site for the oxidation half-reaction:
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Electrons lost by the zinc electrode flow in the wire to the copper electrode where they are passed to the Cu2+ ions.
The zinc electrode, (where oxidation takes place) is called the ANODE, while the copper electrode (where reduction takes place ) is called the CATHODE.
The individual reactions which are taking place in the anode and cathode compartments result in zinc cations (Zn2+) being produced in the anode compartment, and a surplus sulphate anions (SO42-) being left in the cathode compartment. Electrical neutrality is maintained in the electrode compartments by the migration of anions along the salt bridge out of the cathode half of the cell to the anode half. Cations migrate out of the anode compartment to the cathode compartment.
A convention is used to describe this cell in a shorthand form:
When both the electrolyte concentrations are 1.0 mol.dm-3 and the temperature is 25 ºC, the cell is said to be a STANDARD CELL. (Click here for a summary of the electrochemical cell).
It is very easy to be confused about the names CATHODE and ANODE and what their properties are, both with electrochemical cells and electrolytic cells. This, hopefully, will help you!
For BOTH electrochemical cells AND electrolytic cells,
Cathode is the site of reduCtion, or, if you prefer, CCC = Cathode Collects Cations.
Anode is the site of oxidAtion, or, AAA = Anode Attracts Anions.
This makes the table below quite simple:
| Electrochemical cells | Electrolytic cells |  | |
|---|---|---|---|
| CATHODE | Accepts electrons from the external circuit (wire). | Accepts electrons from the external circuit (wire). | |
| Is the site of REDUCTION. | Is the site of REDUCTION. | ||
| Is the conventional POSITIVE pole of the cell. | Is attached to the NEGATIVE pole of the DC source. | ||
| ANODE | Releases electrons to the external circuit (wire). | Releases electrons to the external circuit (wire). | |
| Is the site of OXIDATION. | Is the site of OXIDATION. | ||
| Is the conventional NEGATIVE pole of the cell. | Is attached to the POSITIVE pole of the DC source. |
