The galvanic cell

Electrode

An electrode is an electrical conductor that is used to make contact with a metallic part of a circuit. The anode is the electrode where oxidation takes place. The cathode is the electrode where reduction takes place.

The zinc sulphate and copper sulphate solutions are called the electrolyte solutions.

Electrolyte

An electrolyte is a substance that contains free ions and which therefore behaves as an electrical conductor.

The U-tube also plays a very important role in the cell. In the Zn/Zn ${}^{2+}$ half-cell, there is a build up of positive charge because of the release of electrons through oxidation. In the Cu ${}^{2+}$ /Cu half-cell, there is a decrease in the positive charge because electrons are gained through reduction. This causes a movement of SO ${}_4^{2-}$ ions into the beaker where there are too many positive ions, in order to neutralise the solution. Without this, the flow of electrons in the outer circuit stops completely. The U-tube is called the salt bridge . The salt bridge acts as a transfer medium that allows ions to flow through without allowing the different solutions to mix and react.

Salt bridge

A salt bridge, in electrochemistry, is a laboratory device that is used to connect the oxidation and reduction half-cells of a galvanic cell.

The galvanic cell

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. 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 .

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.

Galvanic cell

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.

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 batteries), hearing aids (silver-oxide batteries), digital watches (mercury batteries) and military applications (thermal batteries). Refer to chapter [link] for more information on batteries.

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. This will be discussed in more detail later in this chapter.

Galvanic cells

1. The following half-reactions take place in an electrochemical cell: $\mathrm{Fe}\to {\mathrm{Fe}}^{3+}+3{\mathrm{e}}^{-}$ ${\mathrm{Fe}}^{2+}+2{\mathrm{e}}^{-}\to \mathrm{Fe}$
   1. Which is the oxidation half-reaction?
   2. Which is the reduction half-reaction?
   3. Name one oxidising agent.
   4. Name one reducing agent.
   5. Use standard notation to represent this electrochemical cell.
2. For the following cell:
   $$Mg|M{g}^{2+}\left|\right|M{n}^{2+}|Mn$$
   1. Give the cathode half-reaction.
   2. Give the anode half-reaction.
   3. Give the overall equation for the electrochemical cell.
   4. What metals could be used for the electrodes in this electrochemical cell?
   5. Suggest two electrolytes for this electrochemical cell.
   6. In which direction will the current flow?
   7. Draw a simple sketch of the complete cell.
3. For the following cell:
   $$Sn|S{n}^{2+}\left|\right|A{g}^{+}|Ag$$
   1. Give the cathode half-reaction.
   2. Give the anode half-reaction.
   3. Give the overall equation for the electrochemical cell.
   4. Draw a simple sketch of the complete cell.