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In chemistry, there are three definitions in common use of the word base, known as Arrhenius bases, Brønsted bases and Lewis bases. All definitions agree that bases are substances which react with s as originally proposed by in the mid-18th century. proposed in 1884 that a base is a substance which dissociates in aqueous solution to form hydroxide ions OH. These ions can react with hydrogen ions (H+ according to Arrhenius) from the dissociation of acids to form water in an . A base was therefore a metal hydroxide such as NaOH or Ca(OH)2. Such aqueous hydroxide solutions were also described by certain characteristic properties. They are slippery to the touch, can taste and change the color of s (e.g., turn red blue). In water, by altering the , bases yield solutions in which the hydrogen ion is lower than it is in pure water, i.e., the water has a higher than 7.0 at standard conditions. A soluble base is called an if it contains and releases OH ions . s, hydroxides, and especially s are basic, and s of s are weak bases. Bases and acids are seen as chemical opposites because the effect of an acid is to increase the (H3O+) concentration in water, whereas bases reduce this concentration. A reaction between aqueous solutions of an acid and a base is called , producing a solution of water and a in which the salt separates into its component ions. If the aqueous solution is with a given salt , any additional such salt s out of the solution. In the more general (1923), a base is a substance that can accept s (H+)—otherwise known as s. This does include aqueous hydroxides since OH does react with H+ to form water, so that Arrhenius bases are a subset of Brønsted bases. However there are also other Brønsted bases which accept protons, such as aqueous solutions of (NH3) or its organic derivatives (s). These bases do not contain a hydroxide ion but nevertheless react with water, resulting in an increase in the concentration of hydroxide ion. Also, some contain Brønsted bases which react with protons. For example in , NH2 is the basic ion species which accepts protons from NH4+, the acidic species in this solvent. realized that water, ammonia and other bases can form a bond with a proton due to the of electrons that the bases possess. In the , a base is an donor which can share a pair of electrons with an electron acceptor which is described as a Lewis acid. The Lewis theory is more general than the Brønsted model because the Lewis acid is not necessarily a proton, but can be another molecule (or ion) with a vacant low-lying which can accept a pair of electrons. One notable example is (BF3). Some of both bases and acids have been proposed in the past, but are not commonly used today.


Properties

General properties of bases include: *Concentrated or strong bases are on organic matter and react violently with acidic substances. *Aqueous solutions or molten bases dissociate in ions and conduct electricity. *Reactions with : bases turn red litmus paper blue, phenolphthalein pink, keep bromothymol blue in its natural colour of blue, and turn methyl orange-yellow. *The of a basic solution at standard conditions is greater than seven. *Bases are bitter.


Reactions between bases and water

The following reaction represents the general reaction between a base (B) and water to produce a conjugate acid (BH+) and a conjugate base (OH): : B(aq) + H2O(''l'') ⇌ BH+(aq) + OH(aq) The equilibrium constant, Kb, for this reaction can be found using the following general equation: : Kb = H+OH]/ In this equation, the base (B) and the extremely (the conjugate base OH) compete for the proton. As a result, bases that react with water have relatively small equilibrium constant values. The base is weaker when it has a lower equilibrium constant value.


Neutralization of acids

Bases react with acids to neutralize each other at a fast rate both in water and in alcohol. When dissolved in water, the strong base ionizes into hydroxide and sodium ions: :NaOH → + and similarly, in water the acid forms hydronium and chloride ions: :HCl + → + When the two solutions are mixed, the and ions combine to form water molecules: : + → 2 If equal quantities of NaOH and HCl are dissolved, the base and the acid neutralize exactly, leaving only NaCl, effectively , in solution. Weak bases, such as baking soda or egg white, should be used to neutralize any acid spills. Neutralizing acid spills with strong bases, such as or , can cause a violent exothermic reaction, and the base itself can cause just as much damage as the original acid spill.


Alkalinity of non-hydroxides

Bases are generally compounds that can neutralize an amount of acids. Both and are bases, although neither of these substances contains groups. Both compounds accept H+ when dissolved in s such as water: :Na2CO3 + H2O → 2 Na+ + HCO3 + OH :NH3 + H2O → NH4+ + OH From this, a , or acidity, can be calculated for aqueous solutions of bases. Bases also directly act as electron-pair donors themselves: :CO32− + H+ → HCO3 :NH3 + H+ → NH4+ A base is also defined as a molecule that has the ability to accept an electron pair bond by entering another atom's valence shell through its possession of one electron pair. There are a limited number of elements that have atoms with the ability to provide a molecule with basic properties. can act as a base as well as and . Fluorine and sometimes rare gases possess this ability as well. This occurs typically in compounds such as , s, and metal s such as . Bases of carbon, nitrogen and oxygen without stabilization are usually very strong, or s, which cannot exist in a water solution due to the acidity of water. Resonance stabilization, however, enables weaker bases such as carboxylates; for example, is a .


Strong bases

A strong base is a basic chemical compound that can remove a proton (H+) from (or ') a molecule of even a very weak acid (such as water) in an acid–base reaction. Common examples of strong bases include hydroxides of alkali metals and alkaline earth metals, like NaOH and , respectively. Due to their low solubility, some bases, such as alkaline earth hydroxides, can be used when the solubility factor is not taken into account. One advantage of this low solubility is that "many antacids were suspensions of metal hydroxides such as aluminium hydroxide and magnesium hydroxide." These compounds have low solubility and have the ability to stop an increase in the concentration of the hydroxide ion, preventing the harm of the tissues in the mouth, oesophagus, and stomach. As the reaction continues and the salts dissolve, the stomach acid reacts with the hydroxide produced by the suspensions. Strong bases hydrolyze in water almost completely, resulting in the ." In this process, the water molecule combines with a strong base, due to the water's amphoteric ability; and, a hydroxide ion is released. Very strong bases can even deprotonate very weakly acidic C–H groups in the absence of water. Here is a list of several strong bases: The cations of these strong bases appear in the first and second groups of the periodic table (alkali and earth alkali metals). Tetraalkylated ammonium hydroxides are also strong bases since they dissociate completely in water. is a special case of a species that is exceptionally stable when protonated, analogously to the reason that makes and very strong acids. Acids with a p'' Ka'' of more than about 13 are considered very weak, and their s are strong bases.


Superbases

Group 1 salts of carbanions, amides, and hydrides tend to be even stronger bases due to the extreme weakness of their conjugate acids, which are stable hydrocarbons, amines, and dihydrogen. Usually, these bases are created by adding pure alkali metals such as sodium into the conjugate acid. They are called ''s'', and it is impossible to keep them in water solution because they are stronger bases than the hydroxide ion. As such, they deprotonate conjugate acid water. For example, the ethoxide ion (the conjugate base of ethanol) in the presence of water undergoes this reaction. : + → + Examples of common superbases are: * (n-C4H9Li) * (LDA) CH3)2CHsub>2NLi * (LDEA) * (NaNH2) * (NaH) * Strongest superbases were only synthesised in gas phase: * (C6H4(C2)2)2− (This is the strongest superbase ever synthesized) * (C6H4(C2)2)2− (second strongest superbase) * (C6H4(C2)2)2− (third strongest) * (LiO) was considered the strongest superbase before diethynylbenzene dianions were created.


Weak bases

A weak base is one which does not fully ionize in an , or in which is incomplete. For example, transfers a proton to water according to the equation :NH_3(aq) + H_2O(l) \rightleftharpoons NH_4^+(aq) + OH^-(aq) The for this reaction at 25 °C is 1.8 x 10−5, so that the extent of reaction or is quite small.


Lewis bases

A or ''electron-pair donor'' is a molecule with a high-energy pair of electrons which can be shared with a low-energy vacant orbital in an acceptor molecule to form an . In addition to H+, possible acceptors (Lewis acids) include neutral molecules such as BF3 and metal ions such as Ag+ or Fe3+. Adducts involving metal ions are usually described as es. According to the original formulation of , when a neutral base forms a bond with a neutral acid, a condition of electric stress occurs. The acid and the base share the electron pair that formerly only belonged to the base. As a result, a high dipole moment is created, which can only be destroyed by rearranging the molecules.


Solid bases

Examples of solid bases include: * Oxide mixtures: SiO2, Al2O3; MgO, SiO2; CaO, SiO2 * Mounted bases: LiCO3 on silica; NR3, NH3, KNH2 on alumina; NaOH, KOH mounted on silica on alumina * Inorganic chemicals: BaO, KNaCO3, BeO, MgO, CaO, KCN * Anion exchange resins *Charcoal that has been treated at 900 degrees Celsius or activates with N2O, NH3, ZnCl2-NH4Cl-CO2 Depending on a solid surface's ability to successfully form a conjugate base by absorbing an electrically neutral acid, the basic strength of the surface is determined. "The number of basic sites per unit surface area of the solid" is used to express how much base is found on a solid base catalyst. Scientists have developed two methods to measure the amount of basic sites: titration with benzoic acid using indicators and gaseous acid adsorption. A solid with enough basic strength will absorb an electrically neutral acid indicator and cause the acid indicator's color to change to the color of its conjugate base. When performing the gaseous acid adsorption method, nitric oxide is used. The basic sites are then determined using the amount of carbon dioxide than is absorbed.


Bases as catalysts

Basic substances can be used as heterogeneous s for s. Some examples are metal oxides such as , , and as well as and some s. Many s make good catalysts, many of which form basic substances. Basic catalysts have been used for s, the migration of s, in the , the , and many other reactions. Both CaO and BaO can be highly active catalysts if they are treated with high temperature heat.


Uses of bases

*Sodium hydroxide is used in the manufacture of soap, paper, and the synthetic fiber . *Calcium hydroxide (slaked lime) is used in the manufacture of bleaching powder. *Calcium hydroxide is also used to clean the sulfur dioxide, which is caused by the exhaust, that is found in power plants and factories. *Magnesium hydroxide is used as an 'antacid' to neutralize excess acid in the stomach and cure indigestion. * is used as washing soda and for softening hard water. * (or sodium hydrogen carbonate) is used as baking soda in cooking food, for making baking powders, as an antacid to cure indigestion and in soda acid fire extinguisher. * is used to remove grease stains from clothes


Acidity of bases

The number of (OH-) ions present in one molecule of base is called the acidity of bases. On the basis of acidity bases can be classified into three types: monoacidic, diacidic and triacidic.


Monoacidic bases

When one molecule of a base via complete produces one ion, the base is said to be a monoacidic base. Examples of monoacidic bases are: , , , , etc


Diacidic bases

When one molecule of base via complete produces two ions, the base is said to be diacidic. Examples of diacidic bases are: , , , , , , , , etc.


Triacidic bases

When one molecule of base via complete produces three ions, the base is said to be triacidic. Examples of triacidic bases are: , , ,


Etymology of the term

The concept of base stems from an older notion of "the matrix":


See also

*s *s * * (used in ecology, referring to environments) * * *


References

* * {{DEFAULTSORT:Base (Chemistry)