Difference between revisions of "Electrolysis" - New World Encyclopedia

Latest revision as of 04:29, 27 November 2020

An apparatus called a Hoffman voltameter is being used for the electrolysis of water. It is connected to a direct current power source converter.

In chemistry and manufacturing, electrolysis is a method of separating bonded elements and compounds by passing an electric current through them. The process is extremely useful for producing a variety of elements and compounds, including hydrogen, oxygen, chlorine, lithium, sodium, potassium, aluminum, sodium hydroxide, potassium chlorate, and aspirin. The electrolysis of water is used in submarines and space stations to generate oxygen for the crew. If hydrogen were to become an important fuel in the future, a major source would be the electrolysis of water.

Overview

In general terms, the process of electrolysis can be described as follows. An ionic compound is dissolved in a solvent (which may be water or some other liquid) or melted by heating. As a result, ions from the compound become available in the liquid form, which is called the electrolyte. A pair of inert electrodes is immersed in the liquid, and an electrical voltage is applied to them. One electrode, called the cathode, becomes negatively charged; the other, called the anode, becomes positively charged.

Each electrode attracts ions of opposite charge. Positively charged ions (cations) move toward the cathode, while negatively charged ions (anions) move toward the anode. The ions are carriers of electric current through the liquid (electrolyte). The cathode supplies electrons to the cations, and the anode takes up electrons from the anions. As a result, the cations and anions are neutralized at the electrodes, producing the desired elements or compounds.

In most cases, some energy is released in the form of heat. In some cases, such as the high-temperature electrolysis of steam to produce hydrogen and oxygen, heat is absorbed from the surroundings.

Examples

Electrolysis of an aqueous solution of table salt (sodium chloride or NaCl) produces aqueous sodium hydroxide and chlorine, usually in minute amounts. A solution of NaCl can be reliably electrolyzed to produce hydrogen. To produce chlorine commercially, molten sodium chloride is electrolyzed to produce sodium metal and chlorine gas. These products react violently with each other, so a mercury cell is used to keep them apart.

Electrolysis of water

One important use of electrolysis is to produce hydrogen gas from water. The reaction is as follows.

2H₂O → 2H₂ + O₂

The electrolysis of water can be performed by a simple, hands-on project. Platinum electrodes are dipped in a cup of salt water and connected to a battery or low-voltage DC power supply. (The salt is needed to produce ions that carry electricity.) When the power is switched on, hydrogen gas begins to bubble up at the cathode, and oxygen bubbles up at the anode.

If the anode is not made of platinum, the oxygen usually reacts with the anode instead of being released as a gas. For example, if iron electrodes are placed in a solution of sodium chloride, iron oxide is produced at the anode, which reacts with water to form iron hydroxide. When this process is used to produce large quantities of hydrogen, it can significantly contaminate the electrolytic cell. For this reason, iron is not used for commercial electrolysis.

The energy efficiency of water electrolysis varies widely. Some report an energy efficiency of 50–70 percent[1], while others report 80–94 percent[2]. These values refer to the efficiency of converting electrical energy into hydrogen's chemical energy—the energy lost in generating the electricity is not included.

Laws of electrolysis

Upon studying the process of electrolysis, Michael Faraday found two types of relationships that can be enunciated as follows:

The mass of a substance produced at an electrode during electrolysis is proportional to the number of moles of electrons (the quantity of electricity) transferred at that electrode.
The amount (in "Faraday" units) of electric charge required to discharge one mole of substance at an electrode is equal to the number of "excess" elementary charges on that ion.

These two statements are often considered Faraday's laws of electrolysis.

Besides Faraday, scientific pioneers of electrolysis have included Sir Humphry Davy, Paul Héroult, Svante Arrhenius, and Adolph Wilhelm Hermann Kolbe.

Applications

The process of electrolysis is used by industry to manufacture various materials, including aluminum, lithium, sodium, potassium, chlorine, sodium hydroxide, and sodium and potassium chlorate. The production of some organic compounds, such as aspirin and trifluoroacetic acid, also involves electrolysis. Likewise, this process can be used to produce hydrogen for use in hydrogen cars and fuel cells.

In scientific laboratories, a technique called gel electrophoresis relies on electrolysis to separate molecules of different sizes. For instance, if a sample consists of a mixture of DNA strands of different lengths in an aqueous solution, the DNA (being negatively charged) can be made to pass through a porous gel under the influence of an electric field. All the DNA strands move toward the anode, but at different speeds—the shortest strands move fastest through the gel, while the longest strands move slowest. This process separates the strands of different sizes.

Besides producing hydrogen, the electrolysis of water also produces oxygen. In nuclear submarines, seawater is electrolyzed to generate oxygen for the crew to be able to breathe while remaining under water for long periods of time. Likewise, space stations can use electrolysis to produce oxygen from wastewater or from surplus water made by fuel cells. Both these applications depend on an abundant electrical supply, which is provided by the nuclear reactor or solar panels.

Some have suggested that the electrolysis of water be used to shift society toward producing and using hydrogen as a regular fuel for powering such things as electric motors and internal combustion engines. In this manner, they advocate moving toward a "hydrogen economy."

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@@ Line 1: / Line 1: @@
-:''This article is about the chemical process. [[Electrology|Electrolysis]] is also a method of [[depilation]].''
+{{Copyedited}}{{Paid}}{{Images OK}}{{Submitted}}{{Approved}}
+[[Category:Public]]
-In chemistry and manufacturing, '''electrolysis''' is a method of separating bonded [[chemical element|element]]s and [[chemical compound|compound]]s by passing an [[electric current]] through them.
 [[Image:Elektrolyse1.jpg|thumb|200px|right|An apparatus called a Hoffman voltameter is being used for the electrolysis of water. It is connected to a direct current power source converter.]]
+In chemistry and manufacturing, '''electrolysis''' is a method of separating bonded [[chemical element|element]]s and [[chemical compound|compound]]s by passing an [[electric current]] through them. The process is extremely useful for producing a variety of elements and compounds, including [[hydrogen]], [[oxygen]], [[chlorine]], [[lithium]], [[sodium]], [[potassium]], [[aluminum]], [[sodium hydroxide]], [[potassium chlorate]], and [[aspirin]]. The electrolysis of [[water]] is used in [[submarine]]s and [[space station]]s to generate oxygen for the crew. If hydrogen were to become an important fuel in the future, a major source would be the electrolysis of water.
+{{toc}}
 == Overview ==
-In general terms, the process of electrolysis can be described as follows. An ionic compound is dissolved in a solvent (which may be water or some other liquid) or melted by heating. As a result, [[Ion (physics)|ion]]s from the compound become available in the liquid form, which is called the '''electrolyte'''. A pair of inert electrodes are immersed in the liquid, and an electrical voltage is applied to them. One electrode, called the '''cathode''', becomes negatively charged; the other, called the '''anode''', becomes positively charged.
+In general terms, the process of electrolysis can be described as follows. An ionic compound is dissolved in a solvent (which may be water or some other liquid) or melted by heating. As a result, [[Ion (physics)|ion]]s from the compound become available in the liquid form, which is called the '''electrolyte'''. A pair of inert electrodes is immersed in the liquid, and an electrical voltage is applied to them. One electrode, called the '''cathode''', becomes negatively charged; the other, called the '''anode''', becomes positively charged.
-Each electrode attracts ions of opposite [[Electric charge|charge]]*. Positively charged ions ([[Ion (physics)|cation]]s) move toward the cathode, while negatively charged ions ([[Ion (physics)|anion]]s) move toward the anode. The ions are carriers of electric current through the liquid (electrolyte). The cathode supplies electrons to the cations, and the anode takes up electrons from the anions. As a result, the cations and anions are neutralized at the electrodes, producing the desired elements or compounds.
+Each electrode attracts ions of opposite [[Electric charge|charge]]. Positively charged ions ([[Ion (physics)|cation]]s) move toward the cathode, while negatively charged ions (anions) move toward the anode. The ions are carriers of electric current through the liquid (electrolyte). The cathode supplies electrons to the cations, and the anode takes up electrons from the anions. As a result, the cations and anions are neutralized at the electrodes, producing the desired elements or compounds.
 In most cases, some energy is released in the form of heat. In some cases, such as the high-temperature electrolysis of [[steam]] to produce hydrogen and oxygen, heat is absorbed from the surroundings.
-A technique called ''[[gel electrophoresis]]'' uses the process of electrolysis to separate molecules of different sizes. For instance, if a sample contains a mixture of [[DNA]] strands of different lengths, the DNA is taken in an aqueous solution and made to pass through a porous gel, under the influence of an electric field. The DNA strands, being negatively charged, move toward the anode a
+==Examples==
+Electrolysis of an aqueous solution of table salt ([[sodium chloride]] or NaCl) produces aqueous [[sodium hydroxide]] and [[chlorine]], usually in minute amounts. A solution of NaCl can be reliably electrolyzed to produce hydrogen. To produce chlorine commercially, molten sodium chloride is electrolyzed to produce sodium metal and chlorine gas. These products react violently with each other, so a [[mercury cell]] is used to keep them apart.
-where the solvent is a gel: it is used to separate substances, such as [[DNA]] strands, based on their electrical charge.
-* [[Electrochemical cell]]s, including the hydrogen [[fuel cell]], use the reverse of this process.
 ==Electrolysis of water==
-[[Image:Hoffman voltameter.jpg|thumb|300px|Diagram of a Hoffman voltameter used for the electrolysis of water to produce hydrogen and oxygen gases.]]
-One important use of electrolysis is to produce hydrogen gas from water. The reaction that occurs is as follows.
+One important use of electrolysis is to produce hydrogen gas from water. The reaction is as follows.
 :2H<sub>2</sub>O &rarr; 2H<sub>2</sub> + O<sub>2</sub>
-This has been suggested as a way of shifting society towards using hydrogen as an [[energy carrier]] for powering electric motors and internal combustion engines. (''See [[hydrogen economy]].'') 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 [[platinum]] electrodes, hydrogen gas will be seen to bubble up at the cathode, and oxygen will bubble at the anode. Using any other electrode for the anode however, 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 quantites of hydrogen, this can significantly contaminate the electrolytic cell - which is why iron is not used for commercial electrolysis.
+The electrolysis of water can be performed by a simple, hands-on project. [[Platinum]] electrodes are dipped in a cup of salt water and connected to a battery or low-voltage DC power supply. (The salt is needed to produce ions that carry electricity.) When the power is switched on, hydrogen gas begins to bubble up at the cathode, and oxygen bubbles up at the anode.
-The [[energy efficiency]] of water electrolysis varies widely.  Some report 50–70%[http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html], while others report 80–94%.[http://www.bellona.no/en/energy/hydrogen/report_6-2002/22871.html]  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]] 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]<!-- See talk page for widely varying values.  Probably needs clarification of what exactly is being measured. —>
+If the anode is not made of platinum, the oxygen usually reacts with the anode instead of being released as a gas. For example, if iron electrodes are placed in a solution of sodium chloride, iron oxide is produced at the anode, which reacts with water to form iron hydroxide. When this process is used to produce large quantities of hydrogen, it can significantly contaminate the electrolytic cell. For this reason, iron is not used for commercial electrolysis.
-== Experimenters ==
+The [[energy efficiency]] of water electrolysis varies widely. Some report an energy efficiency of 50–70 percent[http://www.hyweb.de/Knowledge/w-i-energiew-eng3.html], while others report 80–94 percent[http://www.bellona.no/en/energy/hydrogen/report_6-2002/22871.html]. These values refer to the efficiency of converting electrical energy into hydrogen's chemical energy&mdash;the energy lost in generating the electricity is not included.
-Scientific pioneers of electrolysis included:
-* [[Humphry Davy]]
-* [[Michael Faraday]]
-* [[Paul Heroult|Paul Héroult]]
-* [[Svante Arrhenius]]
-*[[Adolph Wilhelm Hermann Kolbe]]
-More recently, electrolysis of [[heavy water]] was performed by Fleischmann and Pons in [[Fleischmann-Pons experiment|their famous experiment]], resulting in anomalous heat generation and the controversial claim of [[cold fusion]].
 == Laws of electrolysis ==
-'''Faraday's law of electrolysis''' states that:
-*The mass of a substance produced at an [[electrode]] during [[electrolysis]] is proportional to the number of [[Mole (unit)|moles]] of [[electron]]s (the [[quantity of electricity]]) transferred at that electrode
+Upon studying the process of electrolysis, [[Michael Faraday]] found two types of relationships that can be enunciated as follows:
-*The number of [[Faraday constant|Faradays]] of electric charge required to discharge one mole of substance at an electrode is equal to the number of "excess" elementary charges on that ion
+*The mass of a substance produced at an electrode during electrolysis is proportional to the number of [[Mole (unit)|moles]] of [[electron]]s (the quantity of electricity) transferred at that electrode.
+*The amount (in "Faraday" units) of electric charge required to discharge one mole of substance at an electrode is equal to the number of "excess" elementary charges on that ion.
-These two statements are often considered as separate laws: Faraday's 1st and 2nd laws of electrolysis.
+These two statements are often considered Faraday's laws of electrolysis.
-7777777777777
-===First law of electrolysis===
+Besides Faraday, scientific pioneers of electrolysis have included Sir [[Humphry Davy]], Paul Héroult, [[Svante Arrhenius]], and [[Adolph Wilhelm Hermann Kolbe]].
-In [[1832]], [[Michael Faraday]] reported that the quantity of elements separated by passing an electrical current through a molten or dissolved [[salt]] was proportional to the quantity of electric charge passed through the circuit.  This became the basis of [[Faraday's law of electrolysis|the first law of electrolysis]].
+== Applications ==
-===Second law of electrolysis===
+The process of electrolysis is used by industry to manufacture various materials, including [[aluminum]], [[lithium]], [[sodium]], [[potassium]], [[chlorine]], [[sodium hydroxide]], and sodium and potassium [[chlorate]]. The production of some organic compounds, such as [[aspirin]] and [[trifluoroacetic acid]], also involves electrolysis. Likewise, this process can be used to produce [[hydrogen]] for use in [[hydrogen car]]s and [[fuel cell]]s.
-Faraday also discovered that the [[mass]] of the resulting separated elements was directly proportional to the [[atomic mass]]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.
+In scientific laboratories, a technique called ''[[gel electrophoresis]]'' relies on electrolysis to separate molecules of different sizes. For instance, if a sample consists of a mixture of [[DNA]] strands of different lengths in an aqueous solution, the DNA (being negatively charged) can be made to pass through a porous gel under the influence of an electric field. All the DNA strands move toward the anode, but at different speeds&mdash;the shortest strands move fastest through the gel, while the longest strands move slowest. This process separates the strands of different sizes.
-== Industrial uses ==
+Besides producing hydrogen, the electrolysis of water also produces oxygen. In nuclear [[submarine]]s, seawater is electrolyzed to generate oxygen for the crew to be able to breathe while remaining under water for long periods of time. Likewise, [[space station]]s can use electrolysis to produce oxygen from wastewater or from surplus water made by fuel cells. Both these applications depend on an abundant electrical supply, which is provided by the nuclear [[reactor]] or [[solar panel]]s.
-*Manufacture of [[aluminum]], [[lithium]], [[sodium]], [[potassium]], [[aspirin]].
-*Manufacture of [[hydrogen]] for [[hydrogen car]]s and [[fuel cell]]s.
-*[[High-temperature electrolysis]] is also being used for this.
-*[[Coulometry|Coulometric]] 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 [[chlorine]] and [[sodium hydroxide]].
-*Manufacture of sodium and potassium [[chlorate]].
-*Manufacture of perfluorinated organic compounds like [[trifluoroacetic acid]].
-== Military uses ==
+Some have suggested that the electrolysis of water be used to shift society toward producing and using hydrogen as a regular fuel for powering such things as electric motors and internal combustion engines. In this manner, they advocate moving toward a "hydrogen economy."
-As well as producing hydrogen, electrolysis also produces oxygen. [[Nuclear submarine]]s are able to generate breathing oxygen from the water around them, so can remain underwater for as long as their fuel lasts.
-[[Space station]]s can also use electrolysis to produce amounts of extra oxygen from waste water or surplus water produced from the [[Space Shuttle]] fuel cells.
+==See also==
-Both these applications depend on having an abundant electrical supply, from either the [[reactor]] or [[solar panel]]s.
+* [[Humphry Davy]]
-==Examples==
-Electrolysis of an aqueous solution of table salt (NaCl, or [[sodium chloride]]) produces aqueous [[sodium hydroxide]] and [[chlorine]], 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 [[mercury cell]] is used to ensure they do not come into contact with each other.
-==See also==
-* [[Faraday's law of electrolysis]]
-* The [[Faraday constant]]
 * [[Michael Faraday]]
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 [[Category:Chemistry]]
 [[Category:Electrochemistry]]
-[[Category:Chemical processes]]
-[[Category:Industrial processes]]
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