Electrolytic Cell
General Electrolytic cell
Electrolysis is endothermic!
The cell in which electrolysis occurs is called an electrolytic cell. Electrolysis is a non-spontaneous, endothermic redox reaction which occurs by supplying external energy to force one specie to oxidise and the other to reduce. Applications of electrolysis is everywhere from the tin cans to the elegant jewelries, they all exploit the very useful concept of electrolysis. It is also evident in metal extraction as well.
Depending on the nature of the type of species used the solution to electrolysis can be molten or in an aqueous form.
(Lisachem 12, pp 69-70)
Depending on the nature of the type of species used the solution to electrolysis can be molten or in an aqueous form.
(Lisachem 12, pp 69-70)
Electrolytic cell structure
Figure 9. The basics of electrolytic cell.
Electrolytic cells are composed of two electrode which is either a metal or an inert electrode like carbon or graphite. These serve as the interface for the conduction of electrons. There are distinctive half-cells and each one has its own polarity associated with it. This time only the cathode is (-) while the anode is (+). This is because of the battery (the external source of energy) which drains the electrons from the anode and give it to the cathode.
Note: oxidation still occurs at the anode and reduction still occurs at the cathode.
Also since electrolysis is non-spontaneous e.g. the reverse of the galvanic cell, it will revert back to being spontaneous if the two species (the oxidant and reductant) are in direct contact. This is why care is take to separate the two reacting species so that they do not react again by the use of permeable membranes or separators but at the same time it should also allow the ions to move through it.
Note: oxidation still occurs at the anode and reduction still occurs at the cathode.
Also since electrolysis is non-spontaneous e.g. the reverse of the galvanic cell, it will revert back to being spontaneous if the two species (the oxidant and reductant) are in direct contact. This is why care is take to separate the two reacting species so that they do not react again by the use of permeable membranes or separators but at the same time it should also allow the ions to move through it.
Reactions
The electrochemical series is used to predicate any reaction occurs in electrolytic cells as well.
If there are more than two species present it is always the stronger reductant with the stronger oxidant. Consider the following lists of metals:
Reactions: Anode(+)
Remember again that the anode is where oxidation occurs however this time its polarity is (+). If we want to deposit metal Y to an object from a solution that contains Y ions and X ions how would we setup a cell and what to put in it? (Assume you have the metal electrodes as well)
Of course we cannot use a galvanic approach to this is problem hence we setup an electrolytic cell like this:
Of course we cannot use a galvanic approach to this is problem hence we setup an electrolytic cell like this:
Metal X would be used as the anode and it would react according to:
Reactions: Cathode(-)
Likewise, the object is placed on the cathode and here it will be reduced but the polarity of the terminal will be (-)
(Lisachem pp 74-78)
Specific Electrolytic cell - Membrane cell
Figure 11. The basic structure of membrane cell.
The membrane cell is about the electrolysis of concentrated salt. It uses a specially developed polymer sheet or cation exchange membrane to stop the direct contact between the two species. The membrane only allows positive ions to pass through it but not negative ions. The electrode used are inert and they do not react whatsoever in the reaction.
(Heinemann Chemistry 2, pp 457-459)
The species present are sodium ions chloride ions and water.
(Heinemann Chemistry 2, pp 457-459)
The species present are sodium ions chloride ions and water.
Reaction:Anode
At the anode there are two possibilities of reaction and these are:
However because the chloride ions are stronger reductants they are oxidised in preference to water molecules.
Reaction:Cathode
At the cathode there are also two reactions which can occur, these are:
However water is a stronger oxidant than sodium ions hence the first half cell will proceed.
Overall reaction
The overall reaction is:
Uses
Two very useful chemicals are produced from this reaction, they are Chlorine gas and sodium hydroxide.
Chlorine gas:
Sodium hydroxide
Hydrogen gas is also produced which is either used as a fuel or as a reactant in ammonia production.
Chlorine gas:
- Used to manufacture polyvinyl chloride (PVC)
- For pesticides
- Water treatment and paper bleaching
- Disinfectant and purifier in plastics, polymers, solvents, and pharmaceuticals
Sodium hydroxide
- Caustic soda
- Drain cleaners
- Paint strippers
- Oven cleaners
- For production of soap and paper.
Hydrogen gas is also produced which is either used as a fuel or as a reactant in ammonia production.