is a branch of Chemistry that studies the reactions which take place at the interface of an electronic Conductor (the Electrode composed of a Metal or a Semiconductor , including Graphite ) and an ionic conductor (the Electrolyte ).
If a Chemical Reaction is caused by an external Voltage , or if a voltage is caused by a chemical reaction, as in a Battery , it is an ''electrochemical'' reaction. In general, electrochemistry deals with situations where an Oxidation and a Reduction reaction is separated in space. The direct Charge Transfer from one molecule to another is not the topic of electrochemistry.
See Also: History of Electrochemistry
Physicist Otto Von Guericke beside his electrical generator while conducting experiment.]]
The 16th Century marked the beginning of the electrical understanding. On 1550s English scientist William Gilbert spent 17 years experimenting with Magnetism and, to a lesser extent, electricity. For his work on magnets, Gilbert became known as the ''"Father of Magnetism."'' He discovered various methods for producing and strengthening magnets.
In 1663 German Physicist Otto Von Guericke created the first electric generator, which produced static electricity by applying friction in the machine. The generator was made of a large Sulfur ball cast inside a glass globe, mounted on a shaft. The ball was rotated by means of a crank and a Static Electric Spark was produced when a pad was rubbed against the ball as it rotated. The globe could be removed and used as source for experiments with electricity.
diagram of Galvani's experiment on frog legs.]]
By mid— for ''"glass"''), or positive, electricity; and ''"resinous,"'' or negative, electricity. This was the ''two-fluid theory'' of electricity, which was to be opposed by Benjamin Franklin's ''one-fluid theory'' later in the century.
Charles-Augustin De Coulomb developed the Law Of Electrostatic Attraction in 1781 as an outgrowth of his attempt to investigate the law of electrical repulsions as stated by Joseph Priestley on England.
Physicist Alessandro Volta showing his ''" Battery "'' to French Emperor Napoleon Bonaparte in early 1800s .]]
In late 1700s Italian Physician and Anatomist Luigi Galvani marked the birth of electrochemistry by establishing a bridge between chemical reactions and electricity on his essay ''"De Viribus Electricitatis in Motu Musculari Commentarius"'' (translated from Latin, Commentary on the Effect of Electricity on Muscular Motion) in 1791 where he proposed a ''"nerveo-electrical substance"'' on biological life forms.
On his essay Galvani concluded that animal tissue contained a here-to-fore neglected innate, vital force, which he termed ''"animal electricity,"'' which activated Nerve and Muscle when spanned by Metal Probe s. He believed that this new force was a form of electricity in addition to the ''"natural"'' form that is produced by Lightning or by the Electric Eel and Torpedo Ray and to the ''"artificial"'' form that is produced by Friction (i.e., static electricity).
Galvani's scientific colleagues generally accepted his views, but Alessandro Volta rejected the idea of an ''"animal electric fluid,"'' replying that the frog's legs responded to differences in Metal Temper , composition, and Bulk . Galvani refuted this by obtaining muscular action with two pieces of the same material.
.]]
In 1800 , English chemists William Nicholson and Johann Ritter succeeded in decomposing water into Hydrogen and Oxygen by Electrolysis . Soon thereafter Johann discovered the process of Electroplating . He also observed the amount of metal deposited and the amount of oxygen produced during an electrolytic process that depended on the distance between the Electrodes . By 1801 Ritter observed Thermoelectric Currents and anticipated the discovery of thermoelectricity by Thomas Johann Seebeck .
By 1810s William Hyde Wollaston made improvements to the Galvanic Pile .
Sir Humphry Davy work with electrolysis led to conclude that the production of electricity in simple Electrolytic Cell s resulted from chemical action and that chemical combination occurred between substances of opposite charge. This work led directly to the isolation of Sodium and Potassium from their compounds and of the Alkaline Earth Metals from theirs in 1808 .
Hans Christian Ørsted discovery of the magnetic effect of electrical currents in 1820 was immediately recognized as an epoch-making advance, although he left further work on Electromagnetism to others. André-Marie Ampère quickly repeated Ørsted's experiment, and formulated them mathematically.
.]]
In 1821 , Estonian-German Physicist Thomas Johann Seebeck demonstrated the electrical potential in the juncture points of two dissimilar metals when there is a Heat difference between the joints.
In 1827 German scientist Georg Ohm expressed his Law in this famous book ''"Die galvanische Kette, mathematisch bearbeitet"'' (The Galvanic Circuit Investigated Mathematically) in which he gave his complete theory of electricity.
In 1832 Michael Faraday 's experiments on Electrochemistry led him to state his two laws of electrochemistry. In 1836 John Daniell invented a primary cell in which Hydrogen was eliminated in the generation of the electricity. Daniell had solved the problem of polarization. In his laboratory he had learned to Alloy the Amalgamated Zinc of Sturgeon with Mercury would produce better voltage.
portrait circa 1880s .]]
William Grove produced the first Fuel Cell in 1839 . Wilhelm Weber developed, in 1846 , the Electrodynamometer . In 1866 , Georges Leclanché patented a new cell which eventually became the forerunner to the world's first widely used battery, the Zinc Carbon Cell .
Svante August Arrhenius published his thesis in 1884 on ''Recherches sur la conductibilité galvanique des électrolytes'' (Investigations on the galvanic conductivity of electrolytes). From his results the author concluded that Electrolyte s, when dissolved in water, become to varying degrees split or dissociated into electrically opposite positive and negative ions.
In 1886 Paul Héroult and Charles M. Hall developed a successful method to obtain Aluminum by using principles described by Michael Faraday.
In 1894 Friedrich Ostwald concluded important studies of the Electrical Conductivity and electrolytic dissociation of Organic Acid s.
portrait in 1910s .]]
Hermann Nernst's developed the theory of the Electromotive Force of the voltaic cell in 1888 . In 1889 , he showed how the characteristics of the current produced could be used to calculate the Free Energy change in the chemical reaction producing the current. He constructed an equation, known as Nernst Equation , which related the voltage of a cell to its properties.
In 1898 Fritz Haber showed that definite reduction products can result on electrolytic process if the potential at the Cathode is kept constant. In 1898 he explained the reduction of Nitrobenzene in stages at the cathode and this became the model for other similar reduction processes.
In 1909 , Robert Andrews Millikan began a series of experiments to determine the electric charge carried by a single Electron .
In 1923 , Johannes Nicolaus Brønsted and Thomas Martin Lowry published essentially the same theory about how acids and bases behave using electrochemical basis.
Arne Tiselius developed the first sophisticated Electrophoretic apparatus in 1937
and some years later the first sophisticated Electrophoretic apparatus was developed in 1937 , who was awarded the 1948 Nobel Prize for his work in protein Electrophoresis .
A year later the International Society Of Electrochemistry (ISE) was founded in 1949
By the 1960s – 1970s Quantum Electrochemistry was developed by Revaz Dogonadze and his pupils.
See Also: Redox reaction
Electrochemical process are redox reactions where Energy is produced by a Spontaneous Reaction which produces electricity, otherwise Electrical Current stimulates a chemical reaction.
In a redox reaction, an atom's oxidation state changes as a result of an Electron Transfer .
The Element s involved in an electrochemical Reaction are characterized by the number of Electron s each has. The ''oxidation state'' of an Ion is the number of electrons it has accepted or donated compared to its neutral state (which is defined as having an oxidation state of 0). If an Atom or ion donates an Electron in a reaction its oxidation state is increased, if an element accepts an electron its oxidation state is decreased.
For example when Sodium reacts with Chlorine , sodium donates one electron and gains an oxidation state of +1. Chlorine accepts the electron and gains an oxidation state of −1. The sign of the oxidation state (positive/negative) actually corresponds to the value of each ion's electronic charge. The attraction of the differently charged sodium and chlorine ions is the reason they then form an Ionic Bond .
The loss of electrons of a substance is called Oxidation , and the gain of electrons is Reduction . This can be easily remembered through the use of Mnemonic devices. Two of the most popular are ''"OIL RIG"'' (Oxidation Is Loss, Reduction Is Gain) and ''"LEO"'' the lion says ''"GER"'' (Lose Electrons: Oxidization, Gain Electrons: Reduction).
The substance which loses electrons is also known as the ''reducing agent'', or ''reductant'', and the substance which accepts the electrons is called the ''oxidizing agent'', or ''oxidant''. The oxidizing agent is always being reduced in a reaction; the reducing agent is always being oxidized.
The gain of Oxygen , loss of Hydrogen and increase in oxidation number is also considered to be Oxidation , while the inverse is true for reduction.
A reaction in which both oxidation and reduction is occurring is called a . These are very common; as one substance loses electrons the other substance accepts them.
Oxidation requires an oxidant. Oxygen is an oxidant, but not the only one. Despite the name, an oxidation reaction does not necessarily need to involve oxygen. In fact, even fires are often unquenchable, as fluorine is an even stronger oxidant (it has a higher Electronegativity ) than oxygen.
See Also: Chemical equation
Electrochemical reactions in water are better understood by balancing redox reactions using the Ion-Electron Method where H+ , OH- ion, H2O and electrons (to compensate the oxidation changes) are added to cell's Half Reaction s for oxidation and reduction.
In acid medium H atoms and water are added to Half Reaction s to balance the overall reaction.
For example on Manganese reacts to Sodium Bismuthate .
:
:
:
Finally the reaction is balanced by Multiplying the number of electrons from the reduction half reaction to oxidation half reaction and vice versa and adding both half reactions, thus solving the equation.
:
:
Reaction balanced:
:
In basic medium OH- ions and Water are added to half reactions to balance the overall reaction. For example on reaction between Potassium Permanganate and Sodium Sulfite .
:
:
:
The same procedure as followed on acid medium by multiplying electrons to opposite half reactions solve the equation thus balancing the overall reaction.
:
:
Equation balanced:
:
The same procedure as used on acid medium is applied, for example on balancing using electron ion method to Complete Combustion of Propane Gas .
:
:
:
As in acid and basic medium, electrons which were used to compensate oxidation changes are multiplied to opposite half reactions, thus solving the equation.
:
:
Equation balanced:
:
See Also: Electrochemical cell
An electrochemical cell is a device capable of producing electric current by a Spontaneous redox reaction. This kind of cell is also known as Galvanic Cell or Voltaic Cell , named after Luigi Galvani and Alessandro Volta , both scientists conducted several experiments on chemical reactions and electric current during the late 18th Century .
s, a U—Shaped tube is replaced with a porous disk acting as Saline Bridge thus electric current is produced.]]
The Galvanic cell's metals dissolve in the Electrolyte at two different rates, leaving some electrons in the rest of the metal, which charges it negative with respect to the electrolyte. Each metal undergoes a different Half-reaction , giving different dissolving rates, which causes an unequal number of electrons in the two metals. This results in a different electrode potential between the electrolyte and each metal. If an electrical connection, such as a Wire or direct contact, is formed between the two, an electric current appears in the metal.
Electrochemical cell which Electrode s are Zinc and Copper submerged on Zinc Sulfate and Copper Sulfate respectively is known as Daniells Cell .
In a Galvanic cell Anode is defined the electrode where oxidation occurs and Cathode the electrode where the reduction takes place.
Half reactions for a Daniells cell are these:
:
:
In order to avoid positive charges to accumulate on anode's compartment a U—shaped tube inverted filled with an Electrolytic Solution is placed on the cell, thus allowing flow of electrons and producing D.C. electric current.
Cell's Voltage is often defined as an Analogue to difference between Potential Energy in both heights of a Waterfall on which a Voltameter is capable of measuring the change on Electrical Potential between anode and cathode.
Electrochemical cell voltage is also referred to as Electromotive Force or Emf .
A cell diagram traces the path of the electrons in the electrochemical cell. The reduced form of the metal to be oxidized at the anode is written first, followed by its oxidized form, then the oxidized form of the metal to be reduced at the cathode, and finally the reduced form of the metal at the cathode. A vertical line separates both electrodes and the limit between the phases (oxidation changes); a double vertical line represents the saline bridge on the cell.
Daniell's cell diagram:
|
:
Electrochemical cell's
Emf value is used to predict whether redox reaction is a
Spontaneous process or not. A positive sign for overall cell's standard potential is considered to be spontaneous reaction, a negative sign would predict a spontaneous reaction on the opposite direction.
Changes over
Stoichiometric Coefficient s on balanced cell equation will not change
value because standard electrode electrode potential are
Intensive Properties .
See Also: Spontaneous process
On electrochemical cells,
Chemical Energy transforms into
Electrical Energy and is expressed mathematically as the product between cell's emf by
Electrical Charge in
Coulombs .
:
:
Electrochemical cell's total charge is determined by multiplying the number of moles by
Faraday's Constant (F).
:
Faraday's constant is the electrical charge in 1
Mole of
Electrons , it has been measured experimentally and is equivalent to 96 485.3 coulombs.
Cell's emf measured is the maximum voltage produced, this value is used to calculate the maximum electrical energy which is obtained from a
Chemical Reaction , this energy is referred to as
Electrical Work and is expressed on the following equation,
:
:
,thus
Free Energy is the amount of mechanical (or other) work that can be extracted from a system, replacing this value on previous equation with
gives the relation between spontaneity and electrochemical cells.
:
The relation between
Gibbs Free Energy and maximum electrical work may predict (at standard temperature and pressure conditions) whether cell's redox system is a spontaneous process or not.
A
Spontaneous electrochemical reaction can be used to generate an
electrical
Current , in
Electrochemical Cell s. This is the basis of all
Batteries and
Fuel Cell s. For example, gaseous oxygen (O
2) and
hydrogen (H
2) can be combined in a fuel cell to form water and
energy (a combination of heat and electrical energy, typically).
Conversely, non-spontaneous electrochemical reactions can be driven forward by the application of a current at sufficient
Voltage . The
Electrolysis of water into gaseous oxygen and hydrogen is a typical example.
The relation between
Equilibrium Constant and spontaneity based on gibbs free energy terms on electrochemical cells is expressed as follows:
:
:
Solving both equations express cell's mathematical relation between standard potential, and equilibrium constant.
:
Previous equation can use
Briggsian Logarithm as shown below:
:
See Also: Nernst Equation
Calculating cell's potential is not always plausible at standard temperature and pressure conditions. However in
1900s German
Chemist Walther Hermann Nernst proposed a mathematical model to determine electrochemical cell potential where standard conditions cant be reached.
On mid
1800s Willard Gibbs formulated an equation for spontaneous process at any conditions,
:
,
Willard stated Q's dependency over reactants and products activity and designated it as their respective
Chemical Activity .
Walther based on Willard Gibbs work during the mid
19th Century , formulated a new equation where replaced
's value with cell's respective maximum electrical work, on Gibbs equation.
:
Finally he replaced
's value with electrochemical cell potential, thus formulating a new equation which now bears his name.
:
Assuming standard conditions (
) and
R =
the equation above can be expressed on
Base—10 Logarithm as shown below:
:
See Also: Concentration cell
is good example where concentration cells are used in biology to understanding cell's
Metabolism such as
Na+(red) K+(blue) Pump .]]
A concentration cell is an electrochemical cell whose electrodes are from the same material differing in ionic concentrations on both half-cells.
For example an electrochemical cell, where two copper electrodes are submerged on
Blue Vitriol's solution, whose concentrations are 0.05
M and 2.0
M , while connected through wire and saline bridge.
:
Le Chatelier's Principle indicates reaction is favourable to reduction as concentration of
ions increases. Reduction will take place in cell's compartment where concentration is higher and oxidation will occur on the diluted side.
The following cell diagram describes the cell mentioned above:
's value of this kind of cell is zero, as electrodes and ions are the same in both half-cells.
After replacing values from case mentioned is possible to calculate cell's potential:
:
:
However, this value is only approximate, because the potential difference is given from the ratio of activities of the ions, not the ratio of concentrations.
Concentration cell's are often a significant biologist's matter of investigation hence they are present on biological cells where
Membrane Potential is responsible of
Nerve Synapses and
Cardiac Beat .
See Also: Battery (electricity)
A battery is an electrochemical cell or a group of them, where if combined together, may produce
Direct Current at a constant
Voltage . Electrochemical principles which made batteries work are the same as on electrochemical cells, however a battery doesn't need auxiliary components such as saline bridge on Daniell cells.
See Also: Dry cell
Dry cells don't have a
Fluid electrolyte instead they use a moist electrolyte paste.
Leclanché's Cell is a good example of this, where cell's
Anode is a
Zinc Container surrounded by a thin layer of
Manganese Dioxide and a moist electrolyte paste of
Ammonium Chloride and
Zinc Chloride mixed with
Starch to have a pale and flabby consistency and avoiding flees. Cell's cathode is represented by a carbon bar inserted on cell's electrolyte, usually placed in the middle.
Leclanché's simplified half reactions are shown below:
:
:
:
:
The voltage obtained from the
Zinc-carbon Battery is 1.5
V approximately.
See Also: Mercury battery
Mercury battery has many applications on
Medicine and
Electronics . The battery consists on a
Steel —made container with the shape of a cylinder acting as the cathode, where an
Amalgamated anode of mercury and zinc is surrounded by a stronger alkaline electrolyte and a paste of
Zinc Oxide and
Mercury(II) Oxide .
Mercury battery half reactions are shown below:
:
:
:
:
There are no changes on the electrolyte's composition when cell works. Mercurium battery provides 1.35 V of
Direct Current .
See Also: Lead-acid battery
The Lead-acid battery used on
Automobiles , consists on a series of six identical cells in line assembled, each cell has a
Lead anode and a cathode made from
Lead Dioxide packed in a
Metal plaque. Cathode and anode are submerged in a solution of
Sulfuric Acid acting as the electrolyte.
Lead-acid battery half cell reactions are shown below:
:
:
At standard conditions, each cell may produce a
Direct Current of 2
V , hence overall voltage produced is 12 V. Lead-acid batteries, differing from Mercury and Zinc-carbon batteries, are
Rechargeable . If an external voltage is supplied to the battery it will produce an
Electrolysis of the products in the overall reaction (discharge), thus recovering initial components which made the battery work.
See Also: Lithium battery
Most of the batteries work using an
Aqueous electrolyte or a moist electrolyte paste instead, however a solid state battery operates using a solid electrolyte. Solid state
Lithium batteries are an example of this, where a solid Lithium bar acts as the
Anode , a bar of
Lithium Sulfide or
Vanadium Oxide acts as the
Cathode and a
Polymer , allowing the passage of
Ions and not
Electrons , serves as the electrolyte. The advantage of this kind of battery from others is that Lithium possess the highest negative value of standard reduction potential. It is also a
Light Metal and therefore less mass is required to generate 1
Mole Of Electrons . This battery is rechargeable and it can provide a
Direct Current of about 3
V . Although solid state batteries are frowned upon nowadays, it is likely they will someday become a reliable source of
Electricity .
See Also: Flow battery
Most batteries have all of the electrolyte and electrodes within a single housing. A flow battery is unusual in that the majority of the electrolyte, including dissolved reactive species, is stored in separate tanks. The electrolytes are pumped through a reactor, which houses the electrodes, when the battery is charged or discharged.
These types of batteries are typically used for large-scale energy storage (kWh - multi MWh). Of the several different types that have been developed, some are of current commercial interest, including the
Vanadium Redox Battery .
See Also: Fuel cell
Fossil Fuels are used on
Power Plants to supply electrical needs of a certain area, however the conversion of them into electricity is a low efficient process, in fact the most efficient electrical power plant it may convert into electricity about 40
% of the original
Chemical Energy when
Burned or processed.
To enhance electrical production, scientists developed fuel cells where
Combustion reactions are stimulated by electrochemical methods, thus requiring continuous replenishment of the
Reactants consumed.
The most popular is the oxygen-hydrogen fuel cell, where two
Inert–electrodes (
Porous electrodes of
Nickel and
Nickel Oxide ) are placed in an
Electrolytic Solution such as hot
Caustic Potash , in both compartments (anode and cathode) gaseous
Hydrogen and
Oxygen are bubbled into solution.
Oxygen-hydrogen fuel cell reactions are shown bellow:
:
:
:
The overall reaction is some-like to
Hydrogen Combustion , differing on oxidation and reduction took place in
Anode and
Cathode separately, similar to the electrode used in the cell for measuring standard reduction potential having a double function acting as
Electrical Conductors providing a surface required to decomposition of the
Molecules into
Atoms before electron transferring, thus named
Electrocatalyst s.
Platinum ,
Nickel ,
Rhodium are good electrocatalysts.
See Also: Corrosion
Corrosion is the term applied to
Metal Rust caused by an electrochemical process. The most common is the
Iron corrosion, other examples include,
Silver Misted and greenish-like layer may appear over
Brass and
Copper . The cost of replacing metals lost to corrosion is in the multi-billions of
Dollars per year.
over an iron surface. Electrochemical mechanisms involved develop iron rusting process.]]
For iron rust to occur the metal has to be in contact with
Oxygen and
Water , although
Chemical Reaction s for this process are some complex and not all of them have been completely understood, it is believed the causes are the following:
#Electron transferring (Reduction-Oxidation)
##One surface of the metal acts as the anode where the oxidation occurs.
###:
##
Electrons are transferred from
Iron reducing oxygen in the
Atmosphere into
Water on the cathode, which is placed in another region of the metal.
###:
##Global reaction for the process:
##:
##Standard
Emf for iron rusting:
###:
###:
Iron corrosion takes place on acid medium;
H+ Ions come from reaction between
Carbon Dioxide in the atmosphere and water, forming
Carbonic Acid . Fe
2+ ions oxides, following this equation:
:
Iron(III) Oxide Hydrated is known as rust. Water associated with iron oxide it varies, thus chemical representation is presented as
.
The
Electric Circuit works as passage of electrons and ions occurs, thus if an electrolyte is present it will facilitate
Oxidation , this explains why rusting is quicker on
Salt Water .
Coinage Metal s, such as copper and silver, can also slowly corrode.
At standard temperature and pressure, a
Patina of green-blue
Copper Carbonate forms on the surface of
Copper .
Silver Cutlery that is in contact with food can develop a layer of
Silver Sulfide .
Attempts to save a metal from becoming anodic are of two general types. Anodic regions dissolve and destroy the structural integrity of the metal.
While it is almost impossible to prevent
Anode /
Cathode formation, if a
Non-conducting material covers the metal contact with the
Electrolyte is not possible and corrosion will not occur.
Metals are
Coated on its surface with
Paint or some other non-conducting coating. This prevents the
Electrolyte from reaching the metal surface the coating is complete.
Scratches exposing the metal will corrode with the region under the paint, adjacent to the scratch, to be
Anodic .
Other prevention is called ''
Passivation '' where a metal is coated with another metal such as
Tin Can . Tin is a metal that rapidly corrodes to form a mono-molecular
Oxide coating that prevents further corrosion of the tin. The tin prevents the electrolyte from reaching the base metal, usually
Steel (
Iron ). However, if the tin coating is scratched the iron becomes anodic and the can corrodes rapidly.
A method commonly used to protect a structural metal is to attach a metal which is more anodic than the metal to be protected. This forces the structural metal to be
Cathodic , thus spared corrosion. It is called ''"sacrificial"'' because the
Anode dissolves and has to be replaced periodically.
Zinc bars are attached at various locations on steel
Ship Hulls to render the ship hull
Cathodic . The zinc bars are replaced periodically. Other metals, such as
Magnesium , would work very well but zinc is the least expensive useful metal.
To protect pipelines, buried or exposed an ingot of magnesium (or zinc) is
Buried beside the
Pipeline and
Connected Electrically to the pipe above ground. The pipeline is forced to be a cathode and is protected. The magnesium anode is sacrificed. At intervals new
Ingot s are buried to replace those lost.
See Also: Electrolysis
Spontaneous redox reactions produces electricity, thus passage of electrons through a wire in the
Electric Circuit . Electrolysis requires an external source of
Electrical Energy to induce a chemical reaction, this process takes place in a compartment called
Electrolytic Cell . Principles involved on electrolysis are the same as featured on electrochemical cells.
Sodium Chloride when molten it can be electrolysed to yield metallic form of
Sodium and gaseous
Chlorine . Industrially this process takes place in a special cell named Down's cell. The cell is connected to a battery, allowing
Electrons Migration from the battery to the electrolytic cell.
Reactions that take place at Down's cell are the following:
:
:
:
This process can yield industrial amounts of metallic sodium and gaseous chlorine, and is widely used on
Mineral Dressing and
Metallurgy Industries .
Standard
Emf for this process is approximately -4
V indicating a non-spontaneous process. In order this reaction to occur the battery should provide at least a potential of 4V. However, on mineral refining industry, higher voltages are used, due to low efficiency on the process.
See Also: Electrolysis of water
Water at standard temperature and pressure conditions doesn't decompose into
Hydrogen and
Oxygen Spontaneously as the
Gibbs Free Energy for the process at standard conditions is a higher positive value, about
However a special
Laboratory Glassware has been designed for this purpose called
Hofmann Voltameter where a pair of inert
Electrodes usually made of
Platinum acts as anode and cathode in the electrolytic process. After the water (if pure) has been placed in the
Apparatus , nothing happens, hence there are not enough
Ions to let the passage of electrons occur. To start the electrolysis an electrolyte should be placed in, usually
Sodium Chloride or
Sulfuric Acid (most used 0.1
M ).
Bubbles from the gases will be seen near both electrodes. The following half reactions describe the process mentioned above:
:
:
:
Although strong acids may be used in the apparatus, the reaction will not net consume the acid.
An aqueous solution electrolysis is a similar process as mentioned in electrolysis of water, however is considered to be a complex process due to contents in solution had to be studied in
Half Reactions whether reduced or oxidized.
The presence of water in a solution of
Sodium Chloride has to be examined over how is reduced and oxidized in both electrodes. Usually water is electrolysed as mentioned in electrolysis of water yielding ''gaseous
Oxygen in the anode'' and gaseous
Hydrogen in the cathode. On the other hand, sodium chloride in water
Dissociates in Na
+ and Cl
- ions,
Anion will be attracted to the cathode thus reducing
Sodium ion, and the
Cation will be attracted to the anode oxidizing
Chloride ion.
The following half reactions describes the process mentioned:
:
:
:
:
Reaction 1 is discarded as it has the most
Negative value on standard reduction potential thus making it less thermodynamically favorable in the process.
When comparing the reduction potentials in reactions 2 & 4, the reduction of chloride ion is favored. Thus, if the Cl
- ion is favored for
Reduction , then the water reaction is favored for
Oxidation producing gaseous oxygen, however experiments shown gaseous chlorine is produced and not oxygen.
Although the initial analysis is correct, there is another effect that can happen, known as the
Overvoltage Effect . Additional voltage is sometimes required, beyond the voltage predicted by the
. This may be due to
Kinetic rather than
Thermodynamic considerations. In fact it has been proved the
Activation Energy for chlorine ion is very low, hence favorable in
Kinetic Terms . In other words, although the voltage applied is thermodynamically sufficient to drive electrolysis, the rate is so slow that to make the process proceed in a reasonable time frame, the
Voltage of the external source has to be increased (hence, overvoltage).
Finally reaction 3 is favorable due to describes the proliferation of
OH- ions thus letting a probable reduction of
H+ ions less favorable option.
The overall reaction for the process according to the analysis would be the following:
:
:
:
The overall reaction indicates, the
Concentration of chloride ions is reduced in comparison to OH
- ions which concentration increases, the reaction also shows the production of gaseous
Hydrogen ,
Chlorine and aqueous
Sodium Hydroxide .
See Also: Faraday's law of electrolysis
Quantitative aspects of electrolysis were originally developed by
Michael Faraday in
1834 . Faraday is also credited to have coined the terms ''
Electrolyte '', electrolysis, among many others while he studied quantitative analysis of electrochemical reactions. Also he was an advocate of the
Law Of Conservation Of Energy .
metals a process named
Electroplating is used (diagram shows
Nickel refining); the process has its bases on the first and the second law of electrolysis stated by Faraday in the
19th Century .]]
Faraday concluded after several experiments on
Electrical Current in
Non-spontaneous Process , the
Mass of the products yielded on the electrodes was proportional to the quantity of current supplied to the cell and the molar mass of the substance analyzed.
In other words, the amount of a substance deposited on each electrode of an electrolytic cell is directly proportional to the
Quantity Of Electricity passed through the cell.
Below a simplified equation of Faraday's first law:
:
Where,
See Also: Electroplating
Faraday devised the laws of chemical electrodeposition of metals from solutions in
1857 . He formulated the second law of electrolysis stating ''"the amounts of bodies which are equivalent to each other in their ordinary chemical action have equal quantities of electricity naturally associated with them."'' In other terms, the quantities of different elements deposited by a given amount of electricity are in the
Ratio of their chemical
Equivalent Weight s.
An important aspect of the second law of electrolysis is
Electroplating which together with the first law of electrolysis, has a significant number of applications in the industry, as when used to protect
Metal s to avoid corrosion.
