Galvanic cell

What is a galvanic cell?

A galvanic cell is an electrochemical cell that derives electrical energy from redox reactions which occur inside the cell.

Any galvanic cell consists of two half cells:
1. In the oxidation half cell ( or the oxidation part) a substance losses one or more electrons. This cell where the oxidation occurs is called the anode
2. In the reduction half cell ( or the reduction part) a substance gains one or more electrons. This cell where the reduction occurs is called the cathode.

Consider the following galvanic cell.

(Photo credit: Wikipedia)



Since oxidation always occurs at the anode, it should be noted that Zinc donates electrons to the cell. In contrast, at the cathode reduction occurs. Hence copper gains electrons.

Thus the redox reactions that occur in these two electrodes are as follows:

At the anode - 

Zinc (Zn) is oxidized to its higher oxidation state (Zn ²⁺ ). Therefore, Zn losses 2 electrons.

Zn _(s)   →  Zn ²⁺_(aq)    + 2e

At the cathode - 

Copper gains two electrons that Zinc donated to the cell and reduces to its metallic form.

Cu ²⁺ _(aq)  + 2e    →  Cu <sub>(s)

Thus, the overall reaction is as follows:</sub>

Cu ²⁺_(aq)  + Zn _(s)   →  Cu _(s)   +  Zn ²⁺_(aq)

Why does Zinc always oxidize?

This can be explained using the cell potentials.

For the reaction that occurs in the anode, the standard cell potential is -0.76V and that in the cathode is + 0.34 V. 

Thus considering the cell potentials we can easily estimate the positions of Zn and Cu in the electrochemical series.

(Photo credit : cikguwong.blogspot.com)


Then it should be clear that Zn should always oxidize.

What is the use of a salt bridge?

_{The salt bridge contains a solution of a salt usually a nitrate salt or a chloride salt of Na or K. However these salts do not contribute towards the chemical reactions inside the cells.}

When oxidation occurs in the anode cell or the oxidation half cell Zn²⁺ ions enter the solution. If the two cells were not connected by a salt bridge, the increasing production of Zn²⁺ would lead to a build up of positive charges in the anode cell. 

When this cell is connected by a salt bridge, cations in the salt bridge, usually chlorides or nitrates flow to the cell and neutralizes the anions.

(Photo credit: tutorvista)

Similarly, the cathode cell or the reduction half cell is neutralized by the cations in the salt bridge (usually K⁺ or Na⁺)

_{All these information can be summarized as follows:}

_{(Photocredit:chem2u.blogspot.com)}