| Electrolysis Of Water |
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Electrolysis of water is an Electrolytic process which decomposes Water into Oxygen and Hydrogen gas with the aid of an Electric Current , where a power source from a 6 Volt battery is commonly used. The electrolysis cell consists of two Electrodes (usually an Inert Metal such as Platinum ) submerged in an Electrolyte and connected to opposite Poles of a source of Direct Current . The electric current disassociates Water Molecule into Hydroxide and Hydrogen Ions . In the electrolytic cell, at the Cathode , hydrogen ions accept Electrons in a Reduction reaction that forms hydrogen gas: : At the Anode , hydroxide ions undergo an Oxidation reaction and give up Electrons to the anode to complete the circuit and form water and oxygen gas: : , hence decompositing water into Oxygen and Hydrogen; : The volume of hydrogen gas produced is therefore twice the volume of oxygen gas. Power Source Converter .]] SPONTANEITY OF THE PROCESS Decomposition of Water into Hydrogen and Oxygen at Standard Temperature And Pressure is not favorable in Thermodynamical terms as half reactions standard potential are negative values, : : , on the other hand Gibbs Free Energy for the process at standard conditions is a higher positive value, about . Those considerations makes the process ''"impossible"'' to occur without adding electrolytes in the Solution . ELECTROLYTE SELECTION See Also: Electrolyte As Pure Water conducts electricity very poorly, a water-soluble Electrolyte must be added to the electrolysis cell to close the circuit. The electrolyte dissolves and disassociates into Cations and Anions (negative and positive Ions ) that carry the current. Electrolytes are normally Acids , Bases , or Salts . Care must be taken in choosing an electrolyte, since an Anion from the electrolyte is in competition with the hydroxide ions to give up an Electron . An electrolyte Anion with less Standard Electrode Potential than hydroxide will be oxidized instead of the hydroxide, and no oxygen gas will be produced. A Cation with a greater Standard Electrode Potential than a hydrogen ion will be reduced in its stead, and no hydrogen gas will be produced. The following +, Rb +, K +, Cs +, Ba 2+, Sr 2+, Ca 2+, Na +, and Mg 2+. Sodium and Lithium are frequently used, as they form inexpensive, soluble salts. If an Acid is used as the Electrolyte , the cation is H+, and there is no competitor for the H+ created by disassociating water. The most commonly used Anion is SO42-, as it is very difficult to oxidize. Standard potential for oxidation of this ion to the peroxydisulfate ion is −2.05 volts. : Strong acids such as Sulfuric Acid (H2SO4) are frequently used as an electrolyte. TECHNIQUES This reaction is simple to replicate. Two leads running from the terminals of a battery into a cup of water and electrolyte is sufficient to produce a visible stream of oxygen or hydrogen bubbles at either Electrode . The presence of Hydroxide (OH-) Ions can be detected with a PH Indicator such as Phenolphthalein or Bromothymol Blue . Hofmann voltameter See Also: Hofmann voltameter The Hofmann voltameter is often used as a small-scale electrolytic cell. It consists of three joined upright cylinders. The inner cylinder is open at the top to allow addition of Water and the Electrolyte . A Platinum electrode is placed inside the bottom of each of the two side cylinders, connected to the positive and negative terminals of a source of Electricity . When current is run through the hofmann voltameter, gaseous Oxygen forms at the Anode and gaseous Hydrogen at the Cathode . Each gas displaces water and collects at the top of the two outer tubes, where it can be drawn off with a stopcock. Industrial electrolysis Many industrial electrolysis cells are very similar to Hofmann Voltameters , with complex platinum plates or honeycombs as electrodes. Hydrogen gas is usually created, collected, and burned on the premises, as its energy density per volume is too low to make transporting or storing it economically feasible. Oxygen gas is treated as a byproduct. High-temperature electrolysis See Also: High-temperature electrolysis High-temperature electrolysis (also HTE or steam electrolysis) is a method currently being investigated for water electrolysis with a Heat Engine . High temperature electrolysis is more efficient than traditional room-temperature electrolysis because some of the energy is supplied as heat, which is cheaper than electricity, and because the electrolysis reaction is more efficient at higher temperatures. APPLICATIONS 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 . A mixture of Oxygen and Hydrogen gasses known as Rhodes Gas or Brown's Gas burns cleanly and is used in Welding and other Metalworking . EFFICIENCY The Energy Efficiency of water electrolysis varies widely. Some report 50–70% while others report 80–94%.[http://www.bellona.no/en/energy/hydrogen/report_6-2002/22871.html These values refer only 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%.[http://www.uic.com.au/nip73.htm] SEE ALSO REFERENCES |
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