Understanding galvanic cells

 

The Galvanic cell

A galvanic cell (which is also sometimes referred to as a voltaic or electrochemical cell) consists of two metals that are connected by a salt bridge between the individual half-cells. A galvanic cell generates electricity using the reactions that take place at these two metals, each of which has a different reaction potential.

In the zinc-copper cell the important thing to notice is that the chemical reactions that take place at the two electrodes cause an electric current to flow through the outer circuit. In this type of cell, chemical energy is converted to electrical energy. These are called galvanic cells. The zinc-copper cell is one example of a galvanic cell.

So what is meant by the ’reaction potential’ of a substance? Every metal has a different half reaction and different dissolving rates. When two metals with different reaction potentials are used in a galvanic cell, a potential difference is set up between the two electrodes, and the result is a flow of current through the wire that connects the electrodes.

In the zinc-copper cell, zinc has a higher reaction potential than copper and therefore dissolves more readily into solution. The metal ’dissolves’ when it loses electrons to form positive metal ions. These electrons are then transferred through the connecting wire in the outer circuit.

A galvanic (voltaic) cell is an electrochemical cell that uses a chemical reaction between two dissimilar electrodes dipped in an electrolyte, to generate an electric current.

Note 

It was the Italian physician and anatomist Luigi Galvani who marked the birth of electrochemistry by making a link between chemical reactions and electricity. In 1780, Galvani discovered that when two different metals (copper and zinc for example) were connected together and then both touched to different parts of a nerve of a frog leg at the same time, they made the leg contract. He called this ”animal electricity”. While many scientists accepted his ideas, another scientist, Alessandro Volta, did not. In 1800, because of his professional disagreement over the galvanic response that had been suggested by Luigi Galvani, Volta developed the voltaic pile, which was very similar to the galvanic cell. It was the work of these two men that paved the way for all electrical batteries.

 Uses and applications of the galvanic cell

The principles of the galvanic cell are used to make electrical batteries. In science and technology, a battery is a device that stores chemical energy and makes it available in an electrical form. Batteries are made of electrochemical devices such as one or more galvanic cells, fuel cells or flow cells. Batteries have many uses including in torches, electrical appliances (long-life alkaline batteries), digital cameras (lithium battery), hearing aids (silver-oxide battery), digital watches (mercury battery) and military applications (thermal battery).

The galvanic cell can also be used for electroplating. Electroplating occurs when an electrically conductive object is coated with a layer of metal using electrical current. Sometimes, electroplating is used to give a metal particular properties such as corrosion protection or wear resistance. At other times, it can be for aesthetic reasons for example in the production of jewellery. 

Understanding galvanic cells

Question: For the following cell:

Zn|Zn2+||Ag+|Ag

1. Give the anode and cathode half-reactions.

2. Write the overall equation for the chemical reaction.

3. Give the direction of the current in the external circuit.

Answer Step 1 : Identify the oxidation and reduction reactions In the standard notation format, the oxidation reaction is written on the left and the reduction reaction on the right. So, in this cell, zinc is oxidised and silver ions are reduced.

Step 2 : Write the two half reactions Oxidation half-reaction: Zn → Zn2+ + 2e−

Reduction half-reaction: Ag+ + e− → Ag

Step 3 : Combine the half-reactions to get the overall equation. When you combine the two half-reactions, all the reactants must go on the left side of the equation and the products must go on the right side of the equation. The overall equation therefore becomes: Zn + Ag+ + e− → Zn2+ + 2e− + Ag Note that this equation is not balanced. This will be discussed later.

Step 4 : Determine the direction of current flow A build up of electrons occurs where oxidation takes place. This is at the zinc electrode. Current will therefore flow from the zinc electrode to the silver electrode.

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