In chemistry, bases are substances that, in aqueous solution, release
hydroxide (OH−) ions, are slippery to the touch, can taste bitter if
an alkali, change the color of indicators (e.g., turn red litmus
paper blue), react with acids to form salts, promote certain chemical
reactions (base catalysis), accept protons from any proton donor,
and/or contain completely or partially displaceable OH− ions.
Examples of bases are the hydroxides of the alkali metals and the
alkaline earth metals (NaOH, Ca(OH)2, etc.).
These particular substances produce hydroxide ions (OH−) in aqueous
solutions, and are thus classified as Arrhenius bases. For a substance
to be classified as an Arrhenius base, it must produce hydroxide ions
in an aqueous solution. In order to do so, Arrhenius believed the base
must contain hydroxide in the formula. This makes the Arrhenius model
limited, as it cannot explain the basic properties of aqueous
solutions of ammonia (NH3) or its organic derivatives (amines).
There are also bases that do not contain a hydroxide ion but
nevertheless react with water, resulting in an increase in the
concentration of the hydroxide ion. An example of this is the
reaction between ammonia and water to produce ammonium and
hydroxide. In this reaction ammonia is the base because it accepts
a proton from the water molecule.
2 Reactions between bases and water
3 Neutralization of acids
6 Neutral bases 7 Bases as catalysts 8 Solid bases 9 Weak bases 10 Uses of bases 11 Acidity of bases
11.1 Monoacidic bases 11.2 Diacidic bases 11.3 Triacidic bases
12 Etymology of the term 13 See also 14 References
Properties General properties of bases include:
Concentrated or strong bases are caustic on organic matter and react violently with acidic substances. Aqueous solutions or molten bases dissociate in ions and conduct electricity. Reactions with indicators: bases turn red litmus paper blue, phenolphthalein pink, keep bromothymol blue in its natural colour of blue, and turn methyl orange yellow. The pH of a basic solution at standard conditions is greater than seven. Bases are bitter in taste.
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 = [BH+][OH−]/[B]
In this equation, both the base (B) and the extremely strong base (the conjugate base) compete with one another 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 sodium hydroxide ionizes into hydroxide and sodium ions:
NaOH → Na+ + OH−
and similarly, in water the acid hydrogen chloride forms hydronium and chloride ions:
HCl + H 2O → H 3O+ + Cl−
When the two solutions are mixed, the H 3O+ and OH− ions combine to form water molecules:
H 3O+ + OH− → 2 H 2O
If equal quantities of NaOH and HCl are dissolved, the base and the
acid neutralize exactly, leaving only NaCl, effectively table salt, in
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 sodium hydroxide or potassium hydroxide, can cause a
violent exothermic reaction, and the base itself can cause just as
much damage as the original acid spill.
Na2CO3 + H2O → 2 Na+ + HCO3− + OH− NH3 + H2O → NH4+ + OH−
From this, a pH, 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
The cations of these strong bases appear in the first and second groups of the periodic table (alkali and earth alkali metals). Acids with a pKa of more than about 13 are considered very weak, and their conjugate bases are strong bases. Superbases Main article: Superbase 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 superbases, and it is impossible to keep them in water solution because they are stronger bases than the hydroxide ion. As such, they deprotonate the conjugate acid water. For example, the ethoxide ion (conjugate base of ethanol) in the presence of water undergoes this reaction.
CH 3CH 2O− + H 2O → CH 3CH 2OH + OH−
Examples of superbases are:
Neutral bases 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. Bases as catalysts Basic substances can be used as insoluble heterogeneous catalysts for chemical reactions. Some examples are metal oxides such as magnesium oxide, calcium oxide, and barium oxide as well as potassium fluoride on alumina and some zeolites. Many transition metals make good catalysts, many of which form basic substances. Basic catalysts have been used for hydrogenations, the migration of double bonds, in the Meerwein-Ponndorf-Verley reduction, the Michael reaction, and many other reactions. Both CaO and BaO can be highly active catalysts if they are treated with high temperature heat. Solid bases Examples of solid bases include:
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. Weak bases Main article: weak base A weak base is one which does not fully ionize in an aqueous solution, or in which protonation is incomplete. Uses of bases
Acidity of bases The number of ionizable hydroxide (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 ionization produces one hydroxide ion, the base is said to be a monoacidic base. Examples of monoacidic bases are: Sodium hydroxide, potassium hydroxide, silver hydroxide, ammonium hydroxide, etc. Diacidic bases When one molecule of base via complete ionization produces two hydroxide ions, the base is said to be diacidic. Examples of diacidic bases are:
Barium hydroxide, magnesium hydroxide, calcium hydroxide, zinc hydroxide, iron(II) hydroxide, tin(II) hydroxide, lead(II) hydroxide, copper(II) hydroxide, etc. Triacidic bases When one molecule of base via complete ionization produces three hydroxide ions, the base is said to be triacidic. Examples of triacidic bases are: Aluminium hydroxide, ferrous hydroxide, Gold Trihydroxide, Etymology of the term The concept of base stems from an older alchemichal 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 older Paracelsian term
"matrix." In keeping with 16th-century animism,
Acids Acid-base reactions Base-richness (used in ecology, referring to environments) Conjugate base Titration
^ Johll, Matthew E. (2009). Investigating chemistry : a forensic science perspective (2nd ed ed.). New York: W.H. Freeman and Co. ISBN 1429209895. OCLC 392223218. CS1 maint: Extra text (link) ^ Chemistry, 9th Edition. Kenneth W. Whitten, Larry Peck, Raymond E. Davis, Lisa Lockwood, George G. Stanley. (2009) ISBN 0-495-39163-8. Page 363 ^ a b c d e f g Zumdahl, Steven; DeCoste, Donald (2013). Chemical Principles (7th ed.). Mary Finch. p. 257. ^ Chemistry. Page 349 ^ a b c d e f g h i j Lewis, Gilbert N. (1938). "Acids and Bases" (PDF). Journal of the Franklin Institute. pp. 293–313. Retrieved 19 February 2015. ^ a b Jensen, William B. (2006). "The origin of the term "base"" (PDF). The Journal of Chemical Education. 83 (8): 1130. Bibcode:2006JChEd..83.1130J. doi:10.1021/ed083p1130. Archived from the original (PDF) on 4 March 2016. ^ http://www.merriam-webster.com/dictionary/base ^ a b Zumdahl, Steven; DeCoste, Donald (2013). Chemical Principles (7th ed.). Mary Finch. p. 258. ^ Zumdahl, Steven; DeCoste, Donald (2013). Chemical Principles (7th ed.). Mary Finch. p. 255. ^ a b c d Zumdahl, Steven; DeCoste, Donald (2013). Chemical Principles (7th ed.). Mary Finch. p. 256. ^ a b c d e f g Tanabe, K.; Misono, M.; Ono, Y.; Hattori, H. New Solid Acids and Bases: their catalytic properties. p. 234. Retrieved 19 February 2015. ^ a b c d e Tanabe, Konzo. Solid Acids and Bases: their catalytic properties. Academic Press. p. 2. Retrieved 19 February 2015. ^ "ELECTROPHILE- NUCLEOPHILE - BASICITY - ACIDITY - pH SCALE". www.citycollegiate.com. Retrieved 2016-06-20.