The plates which carry the electricity into the liquid are called electrodes. The cathode is the electrode connected to the negative terminal of a cell. The anode is the electrode connected to the positive terminal of a cell.
Electrolytes are compounds which when molten or dissolved in water conduct electric current and are decomposed in the process. Here, electricity is passed through the liquid by the movement of ions between the electrodes. Examples of electrolytes are acids, alkalis or salts dissolved in water or molten salts. All these are ionic substances. A non-electrolyte is a liquid which does not allow the passage of electricity, and its examples are water and all covalent substances which do not contain ions.
Process:
When electricity is passed through an electrolyte, it undergoes chemical decomposition. The electrolyte splits up. The ions migrate towards the oppositely charged electrode. At the anode, negative ions (anions) lose their electrons to the anode, which is very ready to accept electron as it is positively charged, and has a lack of electrons. At the cathode, positive ions (cations) gain electrons from the cathode, which has an excess of electrons and therefore an overall negative charge. This release of ions at the electrode results in the chemical decomposition of the electrolyte. It also allows electrons to travel from the cathode to the anode, and the movement of ions during electrolysis allows conduction of electricity.
The process of losing or gaining electrons at the electrodes is called discharge. When ions are discharged at the electrodes, they form atoms or molecules.
Factors Affecting Electrolysis:
In electrolysis, when more than one type of cation or anion is present in a solution, only one cation or one anion are preferentially discharged. This is known as selective discharge of ions.
Position in the Reactivity Series (for cations):
The cations of an element lower in the reactivity series are discharged at the cathode in preference to other cations in the solution. This is because cations of a less reactive element accept electrons more readily.
Relative Ease of Discharge (anions):
The hydroxide ions give up electrons most easily, followed by iodide, bromide and chloride ions. Remember that sulphate and nitrate ions will not be discharged during electrolysis.
Concentration:
If the concentration of a particular ion is high, then this can alter the preferential discharge. The higher concentration makes it more likely to be given off.
Type of Electrode:
Let us take as an example the electrolysis of aqueous copper (II) sulphate solution. If we use carbon electrodes, they will be inert electrodes and will not affect the electrolysis. However, if we use copper electrodes, these are active electrodes and do affect the electrolysis. At the anode, the copper electrode will dissolve into the solution, while at the cathode, the copper ions will be deposited as pink copper metal.
Industrial Application of Electrolysis:
Electrolysis has many varied industrial applications. The extraction of metals from their ores, in particular aluminium, and the purification of metals, especially copper, are important industrial uses.
Another large scale use of electrolysis is in the manufacture of the important alkali, sodium hydroxide. This is produced by the electrolysis of concentrated sea water. The chlorine and hydrogen gases which are produced during this electrolysis are both commercially useful.
Finally, electrolysis is used for electroplating, which is the forming of a thin protective coating of a metal on the surface of another which is likely to corrode.
Chemical to Electrical Energy:
A device which converts chemical energy into electrical energy is called a cell, or battery (a collection of cells). It consists of a pair of dissimilar metals in an electrolyte. The more reactive metal dissolves and turns into ions, thereby producing electrons. These electrons then travel to the less reactive metal electrode, and bubbles of hydrogen are produced at this electrode.
This production or movement of electrons is electricity, so electrical energy has been generated and the bulb that completes the circuit and is connected to the electrodes lights up.
How bright the bulb is depends on the difference in the reactivities of the two metals. If the metals are far apart in the reactivity series, the bulb is bright, but does not stay alight for a very long time. If the metals are closer in the reactivity series, the bulb is less bright, but stays alighted for a longer time.
If the electrolyte in the cells is a paste, as opposed to a liquid, it is called a dry cell. These are the sort of cells most commonly found in the home. They are a convenient, portable energy source and are reasonably cheap to buy.