Electrolysis is a method of using an electric current to drive an otherwise non-spontaneous chemical reaction.
Electrolysis is a scientific way of "splitting" substances. Electrolysis means "electric-splitting". Metals above carbon in the reactivity series (potassium, sodium, lithium, calcium, magnesium and aluminum) are extracted by electrolysis.
Uses: Electrolysis is used in the mining industry to split reactive metals from their ores after they are extracted (taken) from the ground. It is also used to plate (cover) things with metal because it costs less than using, for example, 100% pure solid gold to make jewelry. Many beauty companies use electrolysis to remove hair. This is done by "electrocuting" the hair so it splits along its root. This is usually done around the bikini line and genital area.
Process: A metal is melted so that it is easier to get the important part. Then, it is then treated with an electric current so that the metal splits from anything else that was contaminating it, usually a non metal like sulphur or oxygen. Electric current has two "ends", a positive and a negative. The positive end in an electrolysis is called the anode. The impurity goes towards that. The negative end is called the cathode and the metal (the important bit) goes to that side. When "go" is used it does not actually mean that the molten metal, the solution, splits down the middle. The metal part of the solution, called an ion, goes to the anode. They do this because the ions have a charge which is opposite to the end they go to. Metals have a positive charge and most non metals are negatively charged so that they split.
Electrolysing water gives hydrogen gas and oxygen gas. This is strange as water is a liquid.
Topics of Interest
In chemistry and manufacturing, electrolysis is a method of using an electric current to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially highly important as a stage in the separation of elements from naturally-occurring sources such as ores using an electrolytic cell.
- 1800 - William Nicholson and Johann Ritter decomposed water into hydrogen and oxygen.
- 1807 - Potassium, Sodium, Barium, Calcium and Magnesium were discovered by Sir Humphry Davy using electrolysis.
- 1886 - Fluorine was discovered by Henri Moissan using electrolysis.
- 1886 - Hall-Héroult process developed for making aluminium
- 1890 - Castner-Kellner process developed for making sodium hydroxide
Overview: An ionic compound is dissolved with an appropriate solvent, or otherwise melted by heat, so that its ions are available in the liquid. An electrical current is applied between a pair of inert electrodes immersed in the liquid. The negatively charged electrode is called the cathode, and the positively charged one the anode. Each electrode attracts ions which are of the opposite charge. Therefore, positively charged ions (called cations) move towards the cathode, while negatively charged ions (termed anions) 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.
The amount of electrical energy that must be added equals the change in Gibbs free energy of the reaction plus the losses in the system. The losses can (theoretically) be arbitrarily close to zero, so the maximum thermodynamic efficiency equals the enthalpy 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 steam into hydrogen and oxygen at high temperature, the opposite is true. Heat is absorbed from the surroundings, and the heating value 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.)
Electrolysis of water:
One important use of electrolysis is to produce hydrogen. The reaction that occurs is:
2H2O(aq) → 2H2(g) + O2(g)
g = gas; aq = aqueous
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. 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 energy efficiency of water electrolysis varies widely. Some report 50–70%, while others report 80–94%. 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 power plant that converts the heat of nuclear reactions into hydrogen via electrolysis, the total efficiency is more like 25–40%.
Faraday's laws of electrolysis are quantitative relationships based on the electrochemical researches published by Michael Faraday in 1834.
Faraday's 1st Law of Electrolysis - The mass of a substance altered at an electrode during electrolysis is directly proportional to the quantity of electricity transferred at that electrode. Quantity of electricity refers to the quantity of electrical charge, typically measured in coulomb.
Faraday's 2nd Law of Electrolysis - For a given quantity of electricity (electric charge), the mass of an elemental material altered at an electrode is directly proportional to the element's equivalent weight. The equivalent weight of a substance is its molar mass divided by an integer that depends on the reaction undergone by the material.
- Hall-Heroult process for producing aluminium Production of aluminium, lithium, sodium, potassium, magnesium
- 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
- Production of chlorine and sodium hydroxide
- Production of sodium chlorate and potassium chlorate
- Production of perfluorinated organic compounds such as trifluoroacetic acid
- Production of electrolytic copper as a cathode, from refined copper of lower purity as an anode.
- Electrometallurgy is the process of reduction of metals from metallic compounds to obtain the pure form of metal using electrolysis. For example, sodium hydroxide in its molten form is separated by electrolysis into sodium and oxygen, both of which have important chemical uses. (Water is produced at the same time.)
- Anodization is an electrolytic process that makes the surface of metals resistant to corrosion. For example, ships are saved from being corroded by oxygen in the water by this process. The process is also used to decorate surfaces.
- A battery works by the reverse process to electrolysis. Humphry Davy found that lithium acts as an electrolyte and provides electrical energy.
- Production of oxygen for spacecraft and nuclear submarines.
- Electroplating is used in layering metals to fortify them. Electroplating is used in many industries for functional or decorative purposes, as in vehicle bodies and nickel coins.
- Production of hydrogen for fuel, using a cheap source of electrical energy.
- Electrolytic Etching of metal surfaces like tools or knives with a permanent mark or logo.
- Electrolysis is also used in the cleaning and preservation of old artifacts. Because the process separates the non-metallic particles from the metallic ones, it is very useful for cleaning old coins and even larger objects.
A gas cracker is any device that splits the molecules in a gas or liquid, usually by electrolysis, into atoms. The end product is usually a gas. A hydrocracker is an example of a gas cracker. In nature, molecules are split often, such as in food digestion and microbial digestion activity. A gas cracker device splits the molecule at a rate much greater than that normally found in nature. In science and industry, gas crackers are used to separate two or more elements in a molecule. For example, liquid water, or H2O, is separated into hydrogen and oxygen gases.
The Hall-Héroult process is the major industrial process for the production of aluminium. It involves dissolving alumina in molten cryolite, and electrolysing the solution to obtain pure aluminium metal.
High pressure electrolysis (HPE) is the electrolysis of water by decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water. The difference with a standard proton exchange membrane electrolyzer is the compressed hydrogen output around 120-200 Bar (1740-2900 psi) at 70 °C. By pressurising the hydrogen in the electrolyser the need for an external hydrogen compressor is eliminated, the average energy consumption for internal differential pressure compression is around 3%.
High-temperature electrolysis (also HTE or steam electrolysis) is a method currently being investigated for the production of hydrogen from water with oxygen as a by-product.
The Castner-Kellner process is a method of electrolysis on an aqueous alkali chloride solution (usually sodium chloride solution) to produce the corresponding alkali hydroxide, invented by American Hamilton Castner and Austrian Karl Kellner in the 1890s.
Faraday Efficiency (also called faradaic effiency, faradaic yield, coulombic efficiency or current efficiency) describes the efficiency with which charge (electrons) are transferred in a system facilitating an electrochemical reaction. The word "faraday" in this term has two interrelated aspects. First, the historic unit for charge is the faraday, but has since been replaced by the coulomb. Secondly, the related faraday's constant correlates charge with moles of matter and electrons. This phenomenon was originally understood through Faraday's work and expressed in his Faraday's laws of electrolysis.
An electrolytic cell decomposes chemical compounds by means of electrical energy, in a process called electrolysis; the Greek word lysis means to break up. The result is that the chemical energy is increased. Important examples of electrolysis are the decomposition of water into hydrogen and oxygen, and bauxite into aluminium and other chemicals.
Electrology is the practice of electrical depilation to permanently remove human hair. The actual process of removing the hair is referred to as electrolysis. The practitioner slides a solid hair-thin metal probe into each hair follicle. Proper insertion does not puncture the skin. Electricity is delivered to the follicle through the probe, which causes localized damage to the areas that generate hairs, either through the formation of caustic lye (galvanic method), overheating (thermolysis method), or both (blend method).
Hydrogen production is usually the term for the industrial methods for generating hydrogen. Currently the dominant technology for direct production is steam reforming from hydrocarbons. Hydrogen is also produced as a by product in other processes and managed with hydrogen pinch. Many other methods are known including electrolysis and thermolysis. The discovery and development of less expensive methods of production of bulk hydrogen is relevant to the establishment of a hydrogen economy.
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