The galvanic cell  (Page 2/3)

Note: A voltmeter can also be used in place of the ammeter. A voltmeter will measure the potential difference across the cell.

Results:

During the experiment, you should have noticed the following:

• When the U-tube containing the Na ${}_2$ SO ${}_4$ solution was absent, there was no reading on the ammeter.
• When the U-tube was connected, a reading was recorded on the ammeter.
• After the plates had been connected directly to each other and left for a day, there was a change in their mass. The mass of the zinc plate decreased, while the mass of the copper plate increased.
• The direction of electron flow is from the zinc plate towards the copper plate.

Conclusions:

When a zinc sulphate solution containing a zinc plate is connected by a U-tube to a copper sulphate solution containing a copper plate, reactions occur in both solutions. The decrease in mass of the zinc plate suggests that the zinc metal has been oxidised. The increase in mass of the copper plate suggests that reduction has occurred here to produce more copper metal. This will be explained in detail below.

Half-cell reactions in the zn-cu cell

The experiment above demonstrated a zinc-copper cell. This was made up of a zinc half cell and a copper half cell .

Half cell

A half cell is a structure that consists of a conductive electrode surrounded by a conductive electrolyte. For example, a zinc half cell could consist of a zinc metal plate (the electrode) in a zinc sulphate solution (the electrolyte).

How do we explain what has just been observed in the zinc-copper cell?

• Copper plate At the copper plate, there was an increase in mass. This means that Cu ${}^{2+}$ ions from the copper sulphate solution were deposited onto the plate as atoms of copper metal. The half-reaction that takes place at the copper plate is: $C{u}^{2+}+2e^{-}\to Cu$ (Reduction half reaction) Another shortened way to represent this copper half-cell is Cu ${}^{2+}$ /Cu.
• Zinc plate At the zinc plate, there was a decrease in mass. This means that some of the zinc goes into solution as Z ${}^{2+}$ ions. The electrons remain on the zinc plate, giving it a negative charge. The half-reaction that takes place at the zinc plate is: $Zn\to Z{n}^{2+}+2e^{-}$ (Oxidation half reaction) The shortened way to represent the zinc half-cell is Zn/Zn ${}^{2+}$ . The overall reaction is: $Zn+C{u}^{2+}+2e^{-}\to Z{n}^{2+}+Cu+2e^{-}$ or, if we cancel the electrons: $Zn+C{u}^{2+}\to Z{n}^{2+}+Cu$ For this electrochemical cell, the standard notation is:
  $$Zn|Z{n}^{2+}\left|\right|C{u}^{2+}|Cu$$
  where
  $$\begin{array}{ccc}\hfill |& =& \mathrm{a}\mathrm{phase}\mathrm{boundary}(\mathrm{solid}/\mathrm{aqueous})\hfill \\ \hfill \left|\right|& =& \mathrm{the}\mathrm{salt}\mathrm{bridge}\hfill \end{array}$$

In the notation used above, the oxidation half-reaction at the anode is written on the left, and the reduction half-reaction at the cathode is written on the right. In the Zn-Cu electrochemical cell, the direction of current flow in the external circuit is from the zinc electrode (where there has been a build up of electrons) to the copper electrode.

Components of the zn-cu cell

In the zinc-copper cell, the copper and zinc plates are called the electrodes . The electrode where oxidation occurs is called the anode , and the electrode where reduction takes place is called the cathode . In the zinc-copper cell, the zinc plate is the anode and the copper plate is the cathode.