In 1884,Svante Arrhenius proposed that a base is a substance whichdissociates inaqueous solution to formhydroxide ions OH−. These ions can react withhydrogen ions (H+ according to Arrhenius) from the dissociation of acids to form water in anacid–base reaction. A base was therefore a metal hydroxide such asNaOH orCa(OH)2. Suchaqueous hydroxide solutions were also described by certain characteristic properties. They are slippery to the touch, can tastebitter[1] and change the color ofpH indicators (e.g., turn redlitmus paper blue).
Bases and acids are seen as chemical opposites because the effect of an acid is to increase thehydronium (H3O+) concentration in water, whereas bases reduce this concentration. A reaction between aqueous solutions of an acid and a base is calledneutralization, producing a solution of water and asalt in which the salt separates into its component ions. If the aqueous solution issaturated with a given saltsolute, any additional such saltprecipitates out of the solution.
In the more generalBrønsted–Lowry acid–base theory (1923), a base is a substance that can accepthydrogen cations (H+)—otherwise known asprotons. 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 ofammonia (NH3) or its organicderivatives (amines).[2] These bases do not contain a hydroxide ion but nevertheless react with water, resulting in an increase in the concentration of hydroxide ion.[3] Also, somenon-aqueous solvents contain Brønsted bases which react withsolvated protons. For example, inliquid ammonia, NH2− is the basic ion species which accepts protons from NH4+, the acidic species in this solvent.
G. N. Lewis realized that water, ammonia, and other bases can form a bond with a proton due to theunshared pair ofelectrons that the bases possess.[3] In theLewis theory, a base is anelectron pair donor which can share a pair of electrons with an electron acceptor which is described as a Lewis acid.[4] 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-lyingorbital which can accept a pair of electrons. One notable example isboron trifluoride (BF3).
Someother definitions of both bases and acids have been proposed in the past, but are not commonly used today.
Etymology of the term
The concept of base stems from an olderalchemical notion of "the matrix":
The term "base" appears to have been first used in 1717 by the French chemist,Louis Lémery, as a synonym for the olderParacelsian term "matrix." In keeping with 16th-centuryanimism, Paracelsus had postulated that naturally occurring salts grew within the earth as a result of a universal acid or seminal principle having impregnated an earthy matrix or womb. ... Its modern meaning and general introduction into the chemical vocabulary, however, is usually attributed to the French chemist,Guillaume-François Rouelle. ... In 1754 Rouelle explicitly defined a neutral salt as the product formed by the union of an acid with any substance, be it a water-soluble alkali, a volatile alkali, an absorbent earth, a metal, or an oil, capable of serving as "a base" for the salt "by giving it a concrete or solid form." Most acids known in the 18th century were volatile liquids or "spirits" capable of distillation, whereas salts, by their very nature, were crystalline solids. Hence it was the substance that neutralized the acid which supposedly destroyed the volatility or spirit of the acid and which imparted the property of solidity (i.e., gave a concrete base) to the resulting salt.
Concentrated or strong bases arecaustic on organic matter and react violently with acidic substances.
Aqueous solutions or molten bases dissociate in ions and conduct electricity.
Reactions withindicators: bases turn red litmus paper blue, phenolphthalein pink, keep bromothymol blue in its natural colour of blue, and turn methyl orange-yellow.
ThepH of a basic solution at standard conditions is greater than seven.
The following reaction represents the general reaction between a base (B) and water to produce a conjugate acid (BH+) and a conjugate base (OH−):[3]The equilibrium constant, Kb, for this reaction can be found using the following general equation:[3]
In this equation, the base (B) and the extremelystrong base (the conjugate base OH−) compete for the proton.[7] As a result, bases that react with water have relatively small equilibrium constant values.[7] The base is weaker when it has a lower equilibrium constant value.[3]
Bases react with acids to neutralize each other at a fast rate both in water and in alcohol.[8] When dissolved in water, the strong basesodium hydroxide ionizes into hydroxide and sodium ions:
and similarly, in water the acidhydrogen chloride forms hydronium and chloride ions:
When the two solutions are mixed, theH 3O+ andOH− ions combine to form water molecules:
If equal quantities of NaOH and HCl are dissolved, the base and the acid neutralize exactly, leaving only NaCl, effectivelytable salt, 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 assodium hydroxide orpotassium hydroxide, 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 acid. Bothsodium carbonate andammonia are bases, although neither of these substances containsOH− groups. Both compounds accept H+ when dissolved inprotic solvents such as water:
From this, apH, or acidity, can be calculated for aqueous solutions of bases.
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.[8] There are a limited number of elements that have atoms with the ability to provide a molecule with basic properties.[8]Carbon can act as a base as well asnitrogen andoxygen. Fluorine and sometimes rare gases possess this ability as well.[8] This occurs typically in compounds such asbutyl lithium,alkoxides, andmetal amides such assodium amide. Bases of carbon, nitrogen and oxygen withoutresonance stabilization are usually very strong, orsuperbases, which cannot exist in a water solution due to the acidity of water. Resonance stabilization, however, enables weaker bases such as carboxylates; for example,sodium acetate is aweak base.
Strong bases
A strong base is a base that is quantitatively protonated upon exposure to water. This complete protonation is a result of theleveling effect. The term "strong base" can lead to confusion, since in this case "strong" is a category of base rather than a qualitative description. For example,guanidine is a very basic molecule, but it does not meet the definition of a strong base because it is not fully protonated by water.
One advantage of this low solubility is that "manyantacids weresuspensions of metal hydroxides such asaluminium hydroxide andmagnesium hydroxide";[10] compounds with low solubility and 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.[10] As the reaction continues and the salts dissolve, thestomach acid reacts with the hydroxide produced by the suspensions.[10]
Strong bases hydrolyze in water completely due to theleveling effect.[8] In this process, the water molecule acts as an acid to protonate the base, resulting in the formation of a hydroxide anion.[8] Under anhydrous conditions, some strong bases can even deprotonate weakly acidic C–H bonds. 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.Guanidine is a special case of a species that is exceptionally stable when protonated, analogously to the reason that makesperchloric acid andsulfuric acid very strong acids.
Group 1 salts ofcarbanions,amide ions, andhydrides 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 or throughmetal halogen exchange. They are calledsuperbases, and it is impossible to keep them in aqueous solutions because they are stronger bases than thehydroxide ion and would therefore immediately react with water to form hydroxide and their conjugate acid (see theleveling effect). For example, the ethoxide ion (conjugate base of ethanol) undergoes this reaction quantitatively in the presence of water.[11]
A weak base is one which does not fully ionize in anaqueous solution, or in whichprotonation is incomplete. For example,ammonia transfers a proton to water according to the equation[12]
ALewis base orelectron-pair donor is a molecule with one or more high-energylone pairs of electrons which can be shared with a low-energy vacant orbital in an acceptor molecule to form anadduct. In addition to H+, possibleelectron-pair acceptors (Lewis acids) include neutral molecules such as BF3 and high oxidation state metal ions such as Ag2+, Fe3+ and Mn7+. Adducts involving metal ions are usually described ascoordination complexes.[14]
According to the original formulation ofLewis, when a neutral base forms a bond with a neutral acid, a condition of electric stress occurs.[8] The acid and the base share the electron pair that formerly belonged to the base.[8] As a result, a high dipole moment is created, which can only be decreased to zero by rearranging the molecules.[8]
Charcoal that has been treated at 900 degrees Celsius or activates with N2O, NH3, ZnCl2-NH4Cl-CO2[15]
Depending on a solid surface's ability to successfully form a conjugate base by absorbing an electrically neutral acid, basic strength of the surface is determined.[16] The "number of basic sites per unit surface area of the solid" is used to express how much basic strength is found on a solid base catalyst.[16] Scientists have developed two methods to measure the amount of basic sites: one, titration with benzoic acid using indicators and gaseous acid adsorption.[16] A solid with enough basic strength will absorb an electrically neutral acidic indicator and cause the acidic indicator's color to change to the color of its conjugate base.[16] When performing the gaseous acid adsorption method,nitric oxide is used.[16] The basic sites are then determined by calculating the amount of carbon dioxide that is absorbed.[16]
Sodium hydroxide is used in the manufacture of soap, paper, and the synthetic fiberrayon.
Calcium hydroxide (slaked lime) is used in the manufacture of bleaching powder.
Calcium hydroxide is also used to clean thesulfur dioxide, which is caused by the exhaust, that is found in power plants and factories.[10]
Magnesium hydroxide is used as an 'antacid' to neutralize excess acid in the stomach and cure indigestion.
Sodium carbonate is used as washing soda and for softening hard water.
Sodium bicarbonate (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.
Bases with only oneionizablehydroxide (OH−) ion per formula unit are calledmonoprotic since they can accept one proton (H+). Bases with more than one OH- per formula unit arepolyprotic.[17]
The number ofionizablehydroxide (OH−) ions present in one formula unit of a base is also called theacidity of the base.[18][19] On the basis of acidity bases can be classified into three types: monoacidic, diacidic and triacidic.
When one molecule of a base via completeionization produces onehydroxide ion, the base is said to be a monoacidic ormonoprotic base. Examples of monoacidic bases are:
When one molecule of base via completeionization produces twohydroxide ions, the base is said to be diacidic ordiprotic. Examples of diacidic bases are:
When one molecule of base via completeionization produces threehydroxide ions, the base is said to be triacidic ortriprotic. Examples of triacidic bases are:
^Johlubl, Matthew E. (2009).Investigating chemistry: a forensic science perspective (2nd ed.). New York: W. H. Freeman and Co.ISBN978-1429209892.OCLC392223218.
^"10.4.1. Alkoxide Ions".Chemistry Libretexts. LibreText. 16 July 2015. Retrieved28 October 2022.
^Whitten, Kenneth W.; Gailey, Kenneth D.; Davis, Raymond E. (1992).General Chemistry (4th ed.). Saunders College Publishing. p. 358.ISBN0-03-072373-6.
^Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002).General Chemistry. Principles and Modern Applications (8th ed.). Prentice Hall. p. 678.ISBN0-13-014329-4.
^Miessler, Gary L.; Tarr, Donald A. (1999).Inorganic Chemistry (2nd ed.). Prentice-Hall. pp. 157–159.ISBN0-13-841891-8.
