Electrolysis

Electrolytic Cell

A cell is better called an electrochemical cell. It acts as a pump which pumps electrons around a circuit it is connected in. Since electrons are negatively charged, the flow of electrons will always be from the negative terminal to the positive terminal.

If the cell is connected to a molten ionic compound then the charge will flow through the ions since no electrons are available. This cannot be applied to solid ionic compounds because the ions are in a crystel lattice and are not mobile.

In molten state or in aquous solution the ions become mobile and according to the kinetic particle theory have weaker forces of attraction between them causing the crystel lattice to derail or lose its shape.

An electrolytic cell is basically a container in which the process of electrolysis occurs. For electrolysis two electrodes are used which are connected to the oppositely charged terminal of a battery or cell. The other component is the electrolyte which is the substance needed for the process of electrolysis.

The electrode connected to the negative terminal is called the cathode and the ion which is attracted to the cathode is called the cation. It is attracted to the cathode because it is positively charged due to loss of electron and oppositely charged particles are attracted to each other.

The electrode connected to the positive terminal is called the anion which attracts the anion which are negatively charged due to gaining extra electrons.

One thing to notice here is that when anode forms ions, electrons which are produced are used to reduce the ions on cathode to atoms.

Two processes are commonly involved in this process. The oxidation and reduction. Oxidation happens when electrons are removed or oxygen is added into a substance. Reduction occurs due to gain in electrons or when oxygen is removed from the substance.

Electrolysis is easier to describe as the process which seperates a pair ions to form two atoms of different elemant. There are three main uses of electrolysis which are most commonly used in the industrial sector around the world.

Electroplating

Some things, like a spoon, are covered with seperate metals which make these kind of things more attractive and also makes it more lasting without any rust. To cover an object with a more valuable metal is called electroplating.

The object which needs to be electroplated is first dipped in acid to remove any traces of oxides. This object is then made the cathode and the metal needed to cover it is made the anode.

The electrolyte will be a soluble salt. At the anode metal ions will be formed and the electrons will be attracted to the cathode. The ions will then form atoms of the same metal on the cathode. Soon the object will be covered with the metal.

Purification of Copper

Copper is extracted from sulphide ores and is mainly extracted in impure form and must be purified. To purify it, it is first smelted and then turned into a copper ingot and is used as the anode. A small piece of pure copper ingot is used as the cathode and the copper (II) sulphide is used as the electrolyte.

When a charge is passed, at the anode copper is oxidized to copper ions: Cu → Cu(²+) + 2e(-). Simultaneously the copper ions are reduced to copper atoms at the cathode: Cu(²+) + 2e(-) → Cu.

The mass of the pure copper increases with time and the mass of impure copper decreases, the impurities in the impure copper are also gradually deposited at the bottom of the electrolytic cell. This way the copper is purified.

Aluminum extraction

Aluminum is extracted from the bauxite ore. Since this is an impure form of aluminum, it is smelted and through chemical reactions is turned into alumina which is the molten form of aluminum.

The alumina is then mixed in cryolite so that its melting point is decreased. The mixture is then poured in a Hall-Héroult, and then heated. Inert graphite electrodes are used in this case. At the cathode oxygen gas is given off which combines with the carbon (since graphite is an allotrope of carbon). This forms the carbon dioxide gas. The molten aluminum then seperates from the cryolite and then drains off.

At anode: 6O(²-) → 3O² + 12e(-).

At Cathode: 4Al(²-) + 12e(-) → 4Al

Hoffman Voltameter

Hoffman Voltameter is an expensive apparatus necessary to use for electrylosis if it results in formation of gases. For example, to electrolyse water, sulfuric acid is added in it and mixed because water is a covalent compound which does not have any ions. Inert platinum electrodes are used for electrolysis.

When the charge is passed, oxygen is produced at the anode and hydrogen gas is produced at the cathode:

At anode: 4OH(-) → O² + H2O.

At cathode: 4H(+) + 4e(-) → 2H².

A thing to know about this apparatus is that the term “voltameter” is not in any way linked to the word voltmeter, which is the device used to measure voltage in a circuit.

Pereferential Discharge

Pereferential discharge means that in the process of electrolysis some elements are discharged in preference to other elements. In these cases, the concentration of substances, the ease of discharge and the natures of electrodes matters. For example, a solution of sodium chloride has the ions; Na(+), Cl(-), H(+), OH(-).

Which ions are discharge at the cathode will depend upon the 3 factors written above. If Na(+) and H(+) is involved then H(+) will be discharged due to its ease of discharge. If Na is in higher concentration than H then Na will be discharged. The nature of the electrodes, whether inert or active, matters as well becuase if it is active, like in the case of copper anode and cathode, a different chemical reaction occurs than intended.