4.4 Applications  (Page 17/2)

Applications of electrochemistry

Electrochemistry has a number of different uses, particularly in industry. We are going to look at a few examples.

Electroplating

Electroplating is the process of using electrical current to coat an electrically conductive object with a thin layer of metal. Mostly, this application is used to deposit a layer of metal that has some desired property (e.g. abrasion and wear resistance, corrosion protection, improvement of aesthetic qualities etc.) onto a surface that doesn't have that property. Electro-refining (also sometimes called electrowinning is electroplating on a large scale. Electrochemical reactions are used to deposit pure metals from their ores. One example is the electrorefining of copper.

Copper plays a major role in the electrical reticulation industry as it is very conductive and is used in electric cables. One of the problems though is that copper must be pure if it is to be an effective current carrier. One of the methods used to purify copper, is electrowinning. The copper electrowinning process is as follows:

1. Bars of crude (impure) copper containing other metallic impurities is placed on the anodes .
2. The cathodes are made up of pure copper with few impurities.
3. The electrolyte is a solution of aqueous $CuS{O}_4$ and $H_{2}S{O}_4$ .
4. When current passes through the cell, electrolysis takes place. The impure copper anode dissolves to form Cu ${}^{2+}$ ions in solution. These positive ions are attracted to the negative cathode, where reduction takes place to produce pure copper metal. The reactions that take place are as follows: At the anode :
   $$Cu\left(s\right)\to C{u}^{2+}\left(aq\right)+2e^{-}$$
   At the cathode :
   $$C{u}^{+2}\left(aq\right)+2e^{-}\to Cu\left(s\right)(>99\%\mathrm{purity})$$
5. The other metal impurities (Zn, Au, Ag, Fe and Pb) do not dissolve and form a solid sludge at the bottom of the tank orremain in solution in the electrolyte.

The production of chlorine

Electrolysis can also be used to produce chlorine gas from brine/seawater (NaCl). This is sometimes referred to as the 'Chlor-alkali' process. The reactions that take place are as follows:

At the anode the reaction is:

whereas at the cathode , the following happens:

The overall reaction is:

Chlorine is a very important chemical. It is used as a bleaching agent, a disinfectant, in solvents, pharmaceuticals, dyes and even plastics such as polyvinlychloride (PVC).

Extraction of aluminium

Aluminum metal is a commonly used metal in industry where its properties of being both light and strong can be utilized. It is also used in the manufacture of products such as aeroplanes and motor cars. The metal is present in deposits of bauxite which is a mixture of silicas, iron oxides and hydrated alumina ( $A{l}_2{O}_3$ x $H_{2}O$ ).

Electrolysis can be used to extract aluminum from bauxite. The process described below produces $99\%$ pure aluminum:

1. Aluminum is melted along with cryolite ( $N{a}_{3}Al{F}_6$ ) which acts as the electrolyte. Cryolite helps to lower the melting point and dissolve the ore.
2. The anode carbon rods provide sites for the oxidation of $O^{2-}$ and $F^{-}$ ions. Oxygen and flourine gas are given off at the anodes and also lead to anode consumption.
3. At the cathode cell lining, the $A{l}^{3+}$ ions are reduced and metal aluminum deposits on the lining.
4. The $Al{F}_6^{3-}$ electrolyte is stable and remains in its molten state.