5.0 Electrochemical Cell

5.1 Salt Bridge

Consider the galvanic cell in which reaction occurs between zinc metal and cupric ion. The cell consists of two beakers. In one beaker, there is a $Z{n^{2 + }}$ solution and a zinc rod and in other beaker or compartment there are $C{u^{2 + }}$ solution and copper rod. And these two compartments are connected to each other through salt bridge.

Salt bridge is generally a tube which contains a solution of electrolyte, generally $N{H_4}N{O_3}$ or $KCl$. There are chances of flow of solution from the salt bridge. So the ends of a bridge are plugged with the glass wool to prevent the flow of solution from salt bridge or we can use a salt dissolved in gelatious material as in the bridge electrolyte. When two metallic rods or electrodes are connected through an ammeter, there will be some deflection, which is an evidence of current flowing through them and the occurrence of reaction.

Separate compartment are known as half cells and reactions occurring in them are called half cell reactions. Each half cell is a part of electro-chemical cell in which half cell reaction takes place. A simple half cell can be made by dipping a metal strip into its solution. Eg. Zinc-Zinc Ion half cell which is often called zinc electrode which consists of zinc strip dipped in zinc salt. Similarly Copper strip dipped in copper salt solution. Consider a redox reaction:

$$Zn(s) + C{u^{2 + }}(aq) \to Z{n^{2 + }}(aq) + Cu(s)$$

Electrical Potential is found to be $1.1V$ when the concentration of $C{u^{2 + }}$ and $Z{n^{2 + }}$ are unity ($1$mol$dm{}^{ - 3}$). Such a device is called voltaic cell. When an external opposite potential is applied and is increasing slowly, we found that reaction continues till the opposing voltages become $1.1\ V$, then reaction stops and no current flow in the cell. And now if the external potential is increases further, then the reaction occurs but in the opposite direction. And now it is termed as electrolytic cell. So in the same arrangement we can get the galvanic cell as well as electrolytic cell depending on the potential.

So we can conclude the following points:

$(i)$ ${E_{ext}} = 1.1$

Then no flow of electrons and hence no current. No reaction occurs.

$(ii)$ ${E_{ext}} > 1.1$

Electrons flow from Cu to Zn and current flows from Zn to Cu as direction of electrons is opposite to direction of flow of current.

Zn is deposited at the zinc electrode and Cu dissolves at copper electrode.

$(iii)$ ${E_{ext}} < 1.1$

Ammeter indicates that electrons flow from $Zn$ to $Cu$ and current flows from $Cu$ to $Zn$ as direction of electrons is opposite to direction of flow of current.It will be continuous as long as electrical connection is made and salt bridge is maintained.

$Zn$ dissolves at anode and $Cu$ deposits at cathode.