Electrolysis

Electrolysis is a process that involves forcing electricity through a liquid or solution to cause a reaction to occur. Electrolysis reactions will not run unless energy is put into the system from outside. In the case of electrolysis reactions, the energy is provided by the battery.

Electrolysis and mechanism of electrolysis

Formation of electrolytic cell is same as Galvanic cell, but in this case in making of cell the source of electricity (e.g. battery) is connected to cell instead of electricity absorbing substance (e.g. electric bulb). Electrolytic cell may be of one compartment or of two compartments. The formation of two compartments cell is same as Daniel cell. In the fig (Fig-Electrolytic cell) one compartment electrolytic cell is shown. When electricity is supplied to the cell there creates a positive pole electrode (anode) and negative pole electrode (cathode). For this reason the ion present in electrolytic solution are attracted to the electrode according to their charge.

Electrolytic cell

That means the negative charge ion attracts to the anode and the positive charge ion attract to the cathode. Negative ions release electron to anode (oxidation) and form new substance. On the other hand positive charge ion accept electron from cathode (reduction) and form new compound. In this way in electrolytic cell the produced electron in anode by the oxidation reaction pass through the cell to the cathode to meet the demand of electron for reduction reaction.

Anode reaction: X^- → X + e^-

Cathode reaction: M⁺ + e^-→ M

It is necessary to mention here that, it is not correct that on supplying current to the cell only charged ion attract to the electrode. The neutral compound present in the solution may be attracted by anode or cathode depending upon their tendency to be oxidized or reduced. In fact in electrolytic cell oxidation–reduction of neutral compound is also possible as ions. Now discuss the physical uses of electrolytic cells.

Application of Electrolysis

Although in ancient period electrolysis was only used to coating a metal with another metal, but now in modern time the use of electrolysis is wide. The process of coating a metal with another metal by electrolysis is called the electroplating. In modern chemistry production of new chemical compound, extraction of metal from ore, production of chemical energy (fuel cell), analysis of chemical substance in laboratory, recycling and purification of matter, pollutant management etc. is done by electrolysis technique. Some apparatus used in medical science is also electrolysis technique dependent.

Coating on iron or silver with gold is possible by using electrolysis technique. Water is purified by removing unwanted ion through the use of electrolysis technique. Electricity can be produced from hydrogen fuel cell, where hydrogen molecule oxidized in anode and oxygen molecule reduced in cathode and produced water. As a result in cell electron flows from anode to cathode and we get current, even motor vehicles can run by this electricity (Fig-Bus run by fuel cell).

Bus run by fuel cell

In laboratory determination of amount of one substance in different compound is possible by electrolysis such as determination of amount of arsenic in water. By treatment of waste in electrolysis cell we can protect the environment. Electrolysis dependent sensors are used to determine the amount of sugar/glucose in blood of a diabetic patient.

Electrochemical glucose sensor

In the figure (Fig-Electrochemical glucose sensor) determination of amount of glucose in human blood using electrolysis dependent sensor is shown. Anode and cathode is placed in the small parts touched at the finger of left hand. In fact anode and cathode is a fine layer of metal in plastic, which is made by screen printing technology. There is a small space/channel between the anode and cathode. The big part in the right hand is mainly the source of electricity (battery) and the machine to calculate the molecules taking part in reaction, occurred due to the electric supply. Thus if we think omitting the machine of calculation the rest parts are anode and cathode connect with current source. Now if we compare the parts with an electrolytic cell we will find that in making the electrolytic cell only the electrolytic solution is absent, is not it? We know different electrolytic substance such as- ion, protein etc. are present in human blood. If blood is filled in the space between the anode and cathode thus a complete electrolytic cell will be formed. In fact, filling blood in apace and supplying current from the attached source oxidized the glucose molecules in anode. On the other hand with the calculating machine attached with the cell determine the number of electron derived from the oxidation of glucose and show in the digital screen the amount of glucose in blood. The interesting matter is that it will take only one minute to determine the glucose in blood by this technique.

Electrolysis of Water

We know, water molecule is composed of 2 hydrogen atom and 1 oxygen atom. The chemical reaction of water formation is given below.

H_2(g) + ½ O_2(g) → H₂O_(l) + Heat

One molecule of hydrogen and half molecule oxygen combine to form one molecule of water. Thus on breaking of water molecule, we will get hydrogen and oxygen gas by reverse reaction.

2H₂O_(l) → 2H_2(g) + O_2(g)

The above reaction is not spontaneous. That means energy supply is required to occur the reaction. Water can be broken by electrochemical cell. The electrochemical cells are used to break the water consist of anode and cathode of chemically inert metal. Generally platinum (Pt) metal sheet is used as anode and cathode. Making some acidic solution of water with a small quantity of sulfuric acid and supplying electricity by platinum anode and cathode, the following half cell reaction occurs.

Anode reaction: 2H₂O_(s) → O_2(g) + 4H⁺ _(aq) + 4e^-

Cathode reaction: 4H⁺ _(aq) + 4e^- → 2H_{2 (g)}

2H₂O_(l) → 2H_2(g) + O_2(g)

In anode, water molecule is oxidized and produces oxygen gas, hydrogen ion and electron. On the other hand in cathode hydrogen ion is reduced and produces hydrogen gas. In fact hydrogen produced in anode reach to cathode by solution and the electron reach to cathode by the wire. It is mention-able that there is no change of sulfuric acid in the reaction; it only acts as conveyor of electricity through the solution.

Electrolysis of water

Electrolysis of Sodium Chloride Solution

Saturated water solution of sodium chloride is called brine. Mainly chlorine gas is produced from the electrolysis of sodium chloride solution. Chlorine is commercially produced by electrolysis of sea water. In this case sea water is considered as sodium chloride solution because there is a huge amount of salt in sea water. For electrolysis dipping the anode and cathode in sodium chloride solution as usual, electricity is supplied. Mention-able that as a result of supplying electricity the anode and cathode reaction is slightly complex. As water is an electrolyte that is why when electricity is supplied to the sodium chloride solution oxidation reduction in water happen as well as sodium chloride. The main oxidation-reduction occurred in anode and cathode is discussed below.

Anode reaction: 2Cl^- _(aq) → Cl_2(g) + 2e^-

Cathode reaction: 2H₂O_(l) + 2e^- → H_{2 (g)} + 2OH^- _(aq)

2H₂O_(l) + 2Cl^- _(aq) → H_2(g) + Cl_2(g) + 2OH^-_(aq)

In electrolysis of water sulfuric acid is added to convey current through the solution, in this case no such addition is required. Because sodium ion (Na⁺) and chloride ion (Cl^-) present in solution acts as conveyor of electricity. On supply of electricity in anode chloride ion oxidized and form chlorine gas and electron. On the other hand in cathode water molecule on reduction converts to hydroxyl ion (OH^-) and hydrogen gas.

Electrolysis of sodium chloride solution

In fact electrons produced in anode reach to cathode by wire and supply the required electron for reduction of water. Hydroxyl ions produced in cathode and join together with sodium ion and remain in solution as sodium hydroxide (NaOH). Thus sodium hydroxide found as byproduct with the chlorine and hydrogen gas on electrolysis of sodium chloride.

Commercial use of electrolysis product

Different metals as sodium, aluminum, copper, zinc, iron, lead etc. are extracted from ore by electrolysis. In modern world uses of these metals are infinite. Commercial use of iron is spread everywhere. Can we think the construction of building, house, rail line, road, bridge, vehicles, airplane, ship industries furniture without iron? Besides this steel, alloy of iron is well-regarded /valued as rust preventive metal. Commercially steel is used in exchange of iron. Electric wire made of copper is widely used. Copper wire is commercially valued due to its low resistance to electricity. Aluminum is used to make airplane due to its low weight. Besides this house hold cooking wares and cooking pans are made of aluminum.

Commercially iron is made rust preventive by coating with zinc and magnesium by electroplating. Stability of iron is increased by this. When one metal is coated with another metal by electroplating it becomes very smooth. Different kinds of attractive ornaments are made by coating an easily available metal with a costly metal. Such as, brightness of silver ornament is increased by coating with gold.

Hydrogen gas that is produced from the electrolysis of water is a valuable and environment friendly fuel. The essential water and heat for the environment is produced on burning of hydrogen. Hydrogen gas is the best fuel of present time for fuel cells. Chlorine gas that is produced from the electrolysis of sea water is used commercially as antiseptic and sodium hydroxide alkali is used as raw material in different industries.

Nuclear Reaction and Electricity Generation

We have seen in case of chemical reactions that, chemical bond is formed by accepting donating or sharing of electrons of outermost energy level. No changes of nucleus occur that means no new atom is formed rather the atoms join together and form a new compound by rearrangement of electrons of outermost energy level. In this case consider the mechanism of formation of sodium chloride (NaCl) and water (H₂O). In this section we will learn about a particular reaction where the issue of electron is absent, here new elements are formed from reactions.

We know nucleus of all elements except hydrogen is composed of two fundamental particles. The particles are neutron and proton. The nucleus of heavy elements particularly of atomic number more than 83, dissociate into smaller nucleus spontaneously. On dissociation of heavy nucleus into smaller nucleus a large amount of energy is evolved as light rays. The phenomenon is called radioactivity. Such aspollonium- 210 (Po) dissociates spontaneously and form lead-206 (Pb) and uranium-238 (U). In every case a-particles (die-positive helium-4) are produced. Again a heavy nucleus can be formed by associating smaller nucleus, such as- at high temperature (15 million ^oC) nucleus of two hydrogen atoms join together and form helium nucleus that is helium atom. Such types of reactions occur in sun. Thus we have understood that, in nuclear reaction heavy nucleus dissociate into smaller nucleus, which is called nuclear fission reaction. Again heavy nucleus can be formed by joining of smaller nucleus, which is called nuclear fusion reaction.

Radioactivity is a nuclear fission reaction. The rate of radioactivity of an element can be increased for many times. If any radioactive element is heated by a high energetic neutron, then the nucleus of the radioactive element will dissociate and form instantly many small nucleuses. Such as- 30 different elements are formed from fission reaction by heating of uranium-235 with high energetic neutron. In this reaction first produced strontium-90 (Sr-90) and xenon-143 (Xe-143) are produced first and also two high energetic neutrons. These two neutrons will again heat the new uranium-235 (U-235) atom or strontium-90 (Sr-90) & xenon-143 (Xe-143) and form new atoms and neutrons. In this way nuclear reaction continue like chain till the atom present there to dissociate by neutron. This is called nuclear chain reaction. In this reaction large amount of energy is produced as well as new nucleus. In fact fission reaction is an exothermic reaction. One mole uranium-235 (U-235) produces 2.0 x 1013 Joule of energy by nuclear fission reaction.

In nuclear fission reaction uranium-235 dissociate into smaller nucleus that is in atom by absorbing neutron

So it is clear that, a large amount of energy can be produced by nuclear reaction using small amount of chemical substance. Now let us highlight a relative diagram of evolved energy by nuclear reaction and chemical reaction. 891000 Joule of energy is produced on burning 1 mole methane gas. Thus to get the energy equal to the energy produced by nuclear fission reaction of uranium-235 (2.0 x 10¹³ / 891000) = 2.2 x 10⁷ mole methane gas is to be burnt.

Besides this, it will be possible to realize the extent of harm caused to the environment by the evolved carbon-die-oxide. In many countries of the world electricity is produced by using the energy evolved from the fission reaction in nuclear reactor. North America produces 20% electricity of their total demand from nuclear reactor. Different kinds of nuclear reactors are used to produce electricity. Among them lighter water reactor, heavy water reactor and braider reactor is used generally.

Production of electricity by nuclear reactor. The reactor is situated in Lorraine in France

Though the production of electricity by nuclear reactor is cheap but its risk is very high. Some substances produced by fission reaction are highly radioactive; they can emit radioactivity for many years which are very harmful to environment. Besides this, accidents in nuclear reactors cause dangerous harm to environment as well as animals. Recent accident in nuclear reactor of Fukuyama in Japan is remarkable one. Tsunami (flood), due to the earthquake under the sea caused great damage to the nuclear reactor of Fukuyama in Japan and radioactivity spread in the environment.

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