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Encyclopedia > Electrolysis of water
Hoffman voltameter used to electrolyze water.

Electrolysis of water is the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an electric current being passed through the water. This electrolytic process is used in some industrial applications when hydrogen is needed. Image File history File links Hoffman_voltameter. ... Image File history File links Hoffman_voltameter. ... Hofmann voltameter as described. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colourless (gas) colourless (liquid) Standard atomic weight 15. ... This article is about the chemistry of hydrogen. ... Electricity (from New Latin Ä“lectricus, amberlike) is a general term for a variety of phenomena resulting from the presence and flow of electric charge. ... In chemistry and manufacturing, electrolysis is a method of separating chemically bonded elements and compounds by passing an electric current through them. ...

An electrical power source is connected to two electrodes, or two plates, (typically made from some inert metal such as platinum or stainless steel) which are placed in the water. Hydrogen will appear at the cathode (the negatively charged electrode, where electrons are pumped into the water), and oxygen will appear at the anode (the positively charged electrode). The generated amount of hydrogen is twice the amount of oxygen, and both are proportional to the total electrical charge that was sent through the water. For other uses, see Electrode (disambiguation). ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ... The 630 foot (192 m) high, stainless-clad (type 304) Gateway Arch defines St. ... Diagram of a copper cathode in a Daniells cell. ... For other uses, see Electron (disambiguation). ... Diagram of a zinc anode in a galvanic cell. ... The word proportionality may have one of a number of meanings: In mathematics, proportionality is a mathematical relation between two quantities. ... Electric charge is a fundamental property of some subatomic particles, which determines their electromagnetic interactions. ...

Electrolysis of pure water is very slow; it is sped up dramatically by adding an electrolyte (such as a salt, an acid or a base). An electrolyte is a substance containing free ions that behaves as an electrically conductive medium. ... This article is about common table salt. ... For other uses, see Acid (disambiguation). ... Acids and bases: Acid-base extraction Acid-base reaction Acid dissociation constant Acidity function Buffer solutions pH Proton affinity Self-ionization of water Acids: Lewis acids Mineral acids Organic acids Strong acids Superacids Weak acids Bases: Lewis bases Organic bases Strong bases Superbases Non-nucleophilic bases Weak bases edit In...

In the water at the negatively charged cathode, a reduction reaction takes place, with electrons (e) from the cathode being given to hydrogen cations to form hydrogen gas: ed|other uses|reduction}} Illustration of a redox reaction Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. ...

Cathode (reduction): 2H2O(l) + 2eH2(g) + 2OH(aq);

At the positively charged anode, an oxidation reaction occurs, generating oxygen gas and giving electrons to the anode to complete the circuit: Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... This article is about the chemistry of hydrogen. ... Hydroxide is a polyatomic ion consisting of oxygen and hydrogen: OHâˆ’ It has a charge of âˆ’1. ... ed|other uses|reduction}} Illustration of a redox reaction Redox (shorthand for reduction/oxidation reaction) describes all chemical reactions in which atoms have their oxidation number (oxidation state) changed. ...

Anode (oxidation): 2H2O(l) → O2(g) + 4H+(aq) + 4e;

Combining these two reactions yields the overall decomposition of water into oxygen and hydrogen: Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colourless (gas) colourless (liquid) Standard atomic weight 15. ... This article is about the chemistry of hydrogen. ...

Overall reaction: 2H2O(l) → 2H2(g) + O2(g)

The number of hydrogen molecules produced is thus twice the number of oxygen molecules. Assuming equal temperature and pressure for both gases, the produced hydrogen gas has therefore twice the volume of the produced oxygen gas. The number of electrons pushed through the water is twice the number of generated hydrogen molecules and four times the number of generated oxygen molecules. Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... This article is about the chemistry of hydrogen. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colourless (gas) colourless (liquid) Standard atomic weight 15. ...

## Spontaneity of the process

Decomposition of pure water into hydrogen and oxygen at standard temperature and pressure is not favorable in thermodynamical terms, as half of the reaction's standard potential are negative values. Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... This article is about the chemistry of hydrogen. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colourless (gas) colourless (liquid) Standard atomic weight 15. ... In chemistry and other sciences, STP or standard temperature and pressure is a standard set of conditions for experimental measurements, to enable comparisons to be made between sets of data. ... The worldâ€™s first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Blackâ€™s prior discovery of latent heat. ...

$mbox{Anode (oxidation): }2H_{2}O(l) rightarrow O_{2}(g) + 4H^{+}(aq) + 4e^{-}qquad E^{o}_{ox}=-1.23 V,$
$mbox{Cathode (reduction): }2H_{2}O(l) + 2e^{-} rightarrow H_{2}(g) + 2OH^{-}(aq)qquad E^{o}_{red}=-0.83 V,$

On the other hand, Gibbs free energy for the process at standard conditions is a higher positive value, about $474.4 kJ,$. In thermodynamics, the Gibbs free energy is a thermodynamic potential which measures the useful work obtainable from a closed thermodynamic system at a constant temperature and pressure. ...

Those considerations makes the process "impossible" to occur without adding electrolytes in the solution with necessary energy supplied by an external electrical power source. Making a saline water solution by dissolving table salt (NaCl) in water This article is about chemical solutions. ...

Hoffman voltameter connected to a direct current power source converter.

Image File history File links Elektrolyse1. ... Image File history File links Elektrolyse1. ... Direct current (DC or continuous current) is the continuous flow of electricity through a conductor such as a wire from high to low potential. ... A switching-mode power supply for laboratory use. ...

## Electrolyte selection

If the above described processes occur in pure water, H+ cations will accumulate at the anode and OH anions will accumulate at the cathode. This can be verified by adding a pH indicator to the water: the water near the anode is acidic while the water near the cathode is basic. These charged ions will repel the further flow of electricity until they have diffused away, a slow process. This is why pure water conducts electricity poorly and why electrolysis of pure water proceeds slowly. A cation is an ion with positive charge. ... An anion is an ion with negative charge. ... Acids and bases: Acid-base extraction Acid-base reaction Acid dissociation constant Acidity function Buffer solutions pH Proton affinity Self-ionization of water Acids: Lewis acids Mineral acids Organic acids Strong acids Superacids Weak acids Bases: Lewis bases Organic bases Strong bases Superbases Non-nucleophilic bases Weak bases edit A... diffusion (disambiguation). ...

If a water-soluble electrolyte is added, the conductivity of the water rises considerably. The electrolyte disassociates into cations and anions; the anions rush towards the anode and neutralize the buildup of positively charged H+ there; similarly, the cations rush towards the cathode and neutralize the buildup of negatively charged OH there. This allows the continued flow of electricity.[1] An electrolyte is a substance containing free ions that behaves as an electrically conductive medium. ... A cation is an ion with positive charge. ... An anion is an ion with negative charge. ...

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. An anion is an ion with negative charge. ... For other uses, see Electron (disambiguation). ... An anion is an ion with negative charge. ... It has been suggested that Electrode potential be merged into this article or section. ... A cation is an ion with positive charge. ... It has been suggested that Electrode potential be merged into this article or section. ...

If an acid is used as the electrolyte, the cation is H+, and there is no competitor for the H+ created by disassociating water. For other uses, see Acid (disambiguation). ... An electrolyte is a substance containing free ions that behaves as an electrically conductive medium. ...

The most commonly used anion is sulfate (SO42-), as it is very difficult to oxidize, with the standard potential for oxidation of this ion to the peroxydisulfate ion being −0.22 volts. An anion is an ion with negative charge. ... The sulfate anion, SO42âˆ’ The structure and bonding of the sulfate ion In inorganic chemistry, a sulfate (IUPAC-recommended spelling; also sulphate in British English) is a salt of sulfuric acid. ...

Strong acids such as Sulphuric acid (H2SO4), and strong bases such as potassium hydroxide (KOH), and sodium hydroxide (NaOH) are frequently used as electrolytes. Sulfuric acid (British English: sulphuric acid), H2SO4, is a strong mineral acid. ... The chemical compound potassium hydroxide, (KOH) sometimes known as caustic potash, potassa, potash lye, and potassium hydrate, is a metallic base. ... Flash point Non-flammable. ...

## Techniques

### Fundamental Demonstration

Two leads, running from the terminals of a battery, are placed in a cup of water with a quantity of electrolyte added to establish conductivity. Hydrogen and Oxygen gases will stream from the oppositely charged electrode. Oxygen will collect at the anode and hydrogen will collect at the cathode. A wire is a single, usually cylindrical, elongated strand of drawn metal. ... For other uses, see Electrode (disambiguation). ... Diagram of a zinc anode in a galvanic cell. ... Diagram of a copper cathode in a Daniells cell. ...

Match test used to detect the presence of hydrogen gas.

Image File history File links Elektrolyse2. ... Image File history File links Elektrolyse2. ...

### Hofmann voltameter

Main article: 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 the addition of water and the electrolyte. A platinum electrode is placed at 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. Hofmann voltameter schema Figure of the apparatus from Hofmanns 1866 book. ... Impact from a water drop causes an upward rebound jet surrounded by circular capillary waves. ... An electrolyte is a substance containing free ions that behaves as an electrically conductive medium. ... General Name, Symbol, Number platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10, 6, d Appearance grayish white Standard atomic weight 195. ... Electricity (from New Latin Ä“lectricus, amberlike) is a general term for a variety of phenomena resulting from the presence and flow of electric charge. ... General Name, symbol, number oxygen, O, 8 Chemical series nonmetals, chalcogens Group, period, block 16, 2, p Appearance colourless (gas) colourless (liquid) Standard atomic weight 15. ... Diagram of a zinc anode in a galvanic cell. ... This article is about the chemistry of hydrogen. ... Diagram of a copper cathode in a Daniells cell. ...

### 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 and collected on site for use in other chemical processes, although in case of refineries it then makes more sense to produce it from natural gas. It can also be produced as a by-product, for example in brine electrolysis. Hofmann voltameter schema Figure of the apparatus from Hofmanns 1866 book. ... This article is about the chemistry of hydrogen. ...

### High-temperature electrolysis

Main article: 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. High-temperature electrolysis schema. ... A heat engine is a physical or theoretical device that converts thermal energy to mechanical output. ...

## Applications

About four percent of hydrogen gas produced worldwide is created by electrolysis, and normally used on site. Hydrogen is used for the creation of ammonia for fertilizer via the Haber process, and for converting heavy petroleum sources to lighter fractions via hydrocracking. This article is about the chemistry of hydrogen. ... For other uses, see Ammonia (disambiguation). ... The Haber process (also known as Haberâ€“Bosch process) is the reaction of nitrogen and hydrogen, over an iron-substrate, to produce ammonia [1] [2] [3]. The Haber process is important because ammonia is difficult to produce, on an industrial scale. ... Petro redirects here. ... In petroleum geology and chemistry, cracking is the process whereby complex organic molecules (e. ...

There is some speculation about future development of hydrogen as an energy carrier in a hydrogen economy, although the rapid evolution of electric battery technology makes overall efficiency a major consideration. Hydrogen fuel injection is a concept having a long history; its popularity has reemerged in recent years. A hydrogen economy is a hypothetical economy in which the energy needed for motive power (for automobiles or other vehicle types) or electricity (for stationary applications) is derived from reacting hydrogen (H2) with oxygen. ... A battery is of one or more electrochemical cells, which store chemical energy and make it available in an electrical form. ... Hydrogen Fuel Injection, or HFI, is a system to reduce exhaust emissions of internal combustion engines and improve fuel economy. ...

## Efficiency

Water electrolysis does not convert 100% of the electrical energy into the chemical energy of hydrogen. The process loses energy because ions in the water need to move to carry electricity, and this movement ultimately heats up the water.

The energy efficiency of water electrolysis varies widely. Some report 50–70%[1], while others report 80–94%.[2] 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 may be closer to 25–45%.[3] tytytrtyty This article is about energy efficiency as a ratio. ...

English chemists John Daniell (left) and Michael Faraday (right), both credited to be founders of electrochemistry as known today. ... In chemistry and manufacturing, electrolysis is a method of separating chemically bonded elements and compounds by passing an electric current through them. ... Hydrogen production is done in bulk today from hydrocarbon fossil fuels via a chemical path. ... A gas cracker is any device that splits the molecules in a gas or liquid, usually by electrolysis, into atoms. ...

## References

1. ^ Linus Pauling, General Chemistry, Section 15-2. San Francisco, 1970

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