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PowerPedia:Electrolysis

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In chemistry and manufacturing, electrolysis is a method of separating bonded There was an error working with the wiki: Code[2]s and There was an error working with the wiki: Code[3]s by passing an Electric current through them.

Overview

An Ionic compound is dissolved with an appropriate There was an error working with the wiki: Code[11], or otherwise melted by heat, so that its Ions are available in the liquid. An electrical current is applied between a pair of inert There was an error working with the wiki: Code[12]s immersed in the liquid. The negatively charged electrode is called the There was an error working with the wiki: Code[13], and the positively charged one the There was an error working with the wiki: Code[14].

Each electrode attracts ions which are of the opposite Electric charge. Therefore, positively charged ions (called There was an error working with the wiki: Code[15]s) move towards the cathode, while negatively charged ions (termed There was an error working with the wiki: Code[16]s) move toward the anode. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply. At the probes, Electrons are absorbed or released by the ions, forming a collection of the desired element or compound.

In electrolysis, the anode is the positive electrode, meaning it has a deficit of electrons species in contact with the anode can be stripped of electrons (i.e., they are oxidized). The cathode is the negative electrode, meaning it has a surplus of electrons. Species in contact with the cathode tend to gain electrons (i.e., they are reduced).

The amount of electrical energy that must be added equals the change in There was an error working with the wiki: Code[4] efficiency equals the There was an error working with the wiki: Code[17] change divided by the free energy change of the reaction. In most cases the electric input is larger than the enthalpy change of the reaction, so some energy is released in the form of heat. In some cases, for instance in the electrolysis of There was an error working with the wiki: Code[18] into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the There was an error working with the wiki: Code[19] of the produced hydrogen is higher than the electric input. In this case the efficiency can be said to be greater than 100%. (It is worth noting that the maximum theoretic efficiency of a Fuel cell is the inverse of that of electrolysis. It is thus impossible to create a Perpetual motion machine by combining the two processes. See Water fuel cell for an example of such an attempt.)

A higher current flow (amperage) through the cell means it will be passing more electrons through it at any given time. This means a faster rate of reduction at the cathode and a faster rate of oxidation at the anode. This corresponds to a greater number of moles of product. The amount of current that passes depends on the conductance of the electrodes and electrolyte, though it also depends on how much current the power source itself can generate.

Current also makes a difference in that it can shift chemical equilibria by sheer mass action. The processes in an electrolytic cell with just two or three reactants can become very, very complex. Most of the time it's best to search the literature to see what current density works best for a desired process. For instance, metals plated at a certain current density might form a durable and shiny coating on the substrate, while some other current density might form an excessively grainy, dull coating.

A higher potential difference (voltage) applied to the cell means the cathode will have more energy to bring about reduction, and the anode will have more energy to bring about oxidation. Higher potential difference enables the electrolytic cell to oxidize and reduce energetically more "difficult" compounds. This can drastically change what products will form in a given experiment. On a practical level, both current and voltage determine what will form in a cell.

The following technologies are related to electrolysis:

There was an error working with the wiki: Code[20]s, including the hydrogen Fuel cell, use the reverse of this process.

There was an error working with the wiki: Code[21] is an electrolysis where the solvent is a gel: it is used to separate substances, such as There was an error working with the wiki: Code[22] strands, based on their electrical charge.

Electrolysis of water

Image:Hoffman voltameter.jpg

This has been suggested as a way of shifting society toward using hydrogen as an There was an error working with the wiki: Code[23] for powering electric motors and internal combustion engines. (See There was an error working with the wiki: Code[24].) Electrolysis of water can be achieved in a simple hands-on project, where electricity from a battery or low-voltage DC power supply (e.g. computer power supply 5 volt rail) is passed through a cup of water (in practice a saltwater solution or other electrolyte will need to be used otherwise no result will be observed). Using There was an error working with the wiki: Code[25] electrodes, hydrogen gas will be seen to bubble up at the cathode, and oxygen will bubble at the anode. If other metals are used as the anode, there is a chance that the oxygen will react with the anode instead of being released as a gas. For example using iron electrodes in a sodium chloride solution electrolyte, iron oxide will be produced at the anode, which will react to form iron hydroxide. When producing large quantities of hydrogen, this can significantly contaminate the electrolytic cell - which is why iron is not used for commercial electrolysis.

The There was an error working with the wiki: Code[26] of water electrolysis varies widely. The efficiency is a measure of what fraction of electrical energy used is actually contained within the hydrogen. Some of the electrical energy is converted to heat, a useless by-product. Some reports quote efficiencies between 50–70%This efficiency is based on the Lower Heating Value of Hydrogen. The Lower Heating Value of Hydrogen is thermal energy released when Hydrogen is combusted. This does not represent the total amount of energy within the Hydrogen, hence the efficiency is lower than a more strict definition. Other reports quote the theoretical maximum efficiency of electrolysis. The theoretical maximum efficiency is between 80–94%.<pesn type=. The theoretical maximum considers the total amount of energy absorbed by both the hydrogen and oxygen. These values only refer to the efficiency of converting electrical energy into hydrogen's chemical energy. The energy lost in generating the electricity is not included. For instance, when considering a [[nuclear power plant|power plant"></pesn>] that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency is more like 25–40%.http://www.uic.com.au/nip73.htm

About four percent of hydrogen gas produced worldwide is created by electrolysis, and normally used onsite. Hydrogen is used for the creation of ammonia for fertilizer via the Haber process, and converting heavy petroleum sources to lighter fractions via hydrocracking. There is some speculation about future development of hydrogen as an energy carrier.

Experimenters

Scientific pioneers of electrolysis included:

Humphry Davy

Michael Faraday

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More recently, electrolysis of There was an error working with the wiki: Code[6], resulting in anomalous heat generation and the controversial claim of Cold fusion.

First law of electrolysis

In There was an error working with the wiki: Code[7].

Second law of electrolysis

Faraday also discovered that the Mass of the resulting separated elements was directly proportional to the There was an error working with the wiki: Code[29]es of the elements when an appropriate integral divisor was applied. This provided strong evidence that discrete particles of electricity existed as parts of the atoms of elements.

Industrial uses

Manufacture of There was an error working with the wiki: Code[30], There was an error working with the wiki: Code[31], There was an error working with the wiki: Code[32], There was an error working with the wiki: Code[33], There was an error working with the wiki: Code[34].

Manufacture of Hydrogen for There was an error working with the wiki: Code[35]s and Fuel cells.

High-temperature electrolysis is also being used for this.

There was an error working with the wiki: Code[8] techniques can be used to determine the amount of matter transformed during electrolysis by measuring the amount of electricity required to perform the electrolysis.

Manufacture of There was an error working with the wiki: Code[36] and There was an error working with the wiki: Code[37].

Manufacture of sodium and potassium There was an error working with the wiki: Code[38].

Manufacture of perfluorinated organic compounds like There was an error working with the wiki: Code[39].

Military uses

As well as producing hydrogen, electrolysis also produces oxygen. There was an error working with the wiki: Code[40]s are able to generate breathable oxygen from the water around them, so can remain underwater for as long as their fuel lasts.

There was an error working with the wiki: Code[41]s can also use electrolysis to produce amounts of extra oxygen from waste water or surplus water produced from the There was an error working with the wiki: Code[42] fuel cells.

Both these applications depend on having an abundant electrical supply, from either the There was an error working with the wiki: Code[9] or There was an error working with the wiki: Code[43]s.

Examples

Electrolysis of an aqueous solution of table salt (NaCl, or There was an error working with the wiki: Code[44]) produces aqueous There was an error working with the wiki: Code[45] and There was an error working with the wiki: Code[46], although usually only in minute amounts. NaCl(aq) can be reliably electrolysed to produce hydrogen. In order to produce chlorine commercially, molten sodium chloride is electrolysed to produce sodium metal and chlorine gas. These will react violently, so a There was an error working with the wiki: Code[47] (or a There was an error working with the wiki: Code[48]) is used to ensure they do not come into contact with each other.

See also

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The There was an error working with the wiki: Code[50]

Michael Faraday

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