Editing Acid–base reaction (section)

===Arrhenius definition===
{{Anchor|Arrhenius theory}}
[[File:Arrhenius2.jpg|thumb|130px|[[Svante Arrhenius]]]]
The first modern definition of acids and bases in molecular terms was devised by [[Svante Arrhenius]].<ref name="miessler_154">Miessler G.L. and Tarr D.A. ''Inorganic Chemistry'' (2nd ed., Prentice-Hall 1999) p. 154 {{ISBN|0-13-841891-8}}</ref><ref>Whitten K.W., Galley K.D. and Davis R.E. ''General Chemistry'' (4th ed., Saunders 1992) p. 356 {{ISBN|0-03-072373-6}}</ref> A hydrogen theory of acids, it followed from his 1884 work with [[Friedrich Wilhelm Ostwald]] in establishing the presence of ions in [[aqueous solution]] and led to Arrhenius receiving the [[Nobel Prize in Chemistry]] in 1903.

As defined by Arrhenius:
* An ''Arrhenius acid'' is a substance that [[ionization|ionises]] in water to form hydrogen ions ({{chem2|H+}});<ref name="miessler_165">{{harvnb|Miessler|Tarr|1991|p=165}}</ref> that is, an acid increases the concentration of H<sup>+</sup> ions in an aqueous solution.

This causes the [[protonation]] of water, or the creation of the [[hydronium]] ({{chem2|H3O+}}) ion.{{refn|group=note|More recent [[IUPAC]] recommendations now suggest the newer term "hydronium"<ref>{{cite journal |last1=Murray |first1=Kermit K. |last2=Boyd |first2=Robert K. |last3=Eberlin |first3=Marcos N. |last4=Langley |first4=G. John |last5=Li |first5=Liang |last6=Naito |first6=Yasuhide |date=June 2013 |orig-year=2006 |title=Standard definition of terms relating to mass spectrometry recommendations |journal=[[Pure and Applied Chemistry]] |volume=85 |issue=7 |pages=1515–1609 |doi=10.1351/PAC-REC-06-04-06|s2cid=98095406 |url=https://www.degruyter.com/downloadpdf/j/pac.2013.85.issue-7/pac-rec-06-04-06/pac-rec-06-04-06.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.degruyter.com/downloadpdf/j/pac.2013.85.issue-7/pac-rec-06-04-06/pac-rec-06-04-06.pdf |archive-date=2022-10-09 |url-status=live}} (In this document, there is no reference to deprecation of "oxonium", which is also still accepted, as it remains in the IUPAC Gold book, but rather reveals preference for the term "Hydronium".)</ref> be used in favor of the older accepted term "oxonium"<ref name="iupac_gold">{{cite book |title= IUPAC Compendium of Chemical Terminology (interactive version) |edition= 2.3.3 |year= 2014 |publisher= International Union of Pure and Applied Chemistry |doi= 10.1351/goldbook.O04379 |access-date= 9 May 2007 |url= http://goldbook.iupac.org/terms/view/O04379 |article= oxonium ylides}}</ref> to illustrate reaction mechanisms such as those defined in the Brønsted–Lowry and solvent system definitions more clearly, with the Arrhenius definition serving as a simple general outline of acid–base character.<ref name="miessler_165"/>}} Thus, in modern times, the symbol {{chem2|H+}} is interpreted as a shorthand for {{chem2|H3O+}}, because it is now known that a bare proton does not exist as a free species in aqueous solution.<ref name="Acids, Bases, & Salts">{{cite book |last=LeMay|first=Eugene|title=Chemistry|year=2002|publisher=Prentice-Hall|location=Upper Saddle River, New Jersey|isbn=978-0-13-054383-7|pages=602}}</ref> This is the species which is measured by [[pH indicator]]s to measure the acidity or basicity of a solution. 
* An ''Arrhenius base'' is a substance that dissociates in water to form hydroxide ({{chem2|OH-}}) ions; that is, a base increases the concentration of {{chem2|OH-}} ions in an aqueous solution. 
The Arrhenius definitions of [[acidity]] and [[alkalinity]] are restricted to aqueous solutions and are not valid for most non-aqueous solutions, and refer to the concentration of the solvent ions. Under this definition, pure {{chem2|H2SO4}} and HCl dissolved in toluene are not acidic, and molten NaOH and solutions of calcium amide in liquid ammonia are not alkaline. This led to the development of the Brønsted–Lowry theory and subsequent Lewis theory to account for these [[aqueous solution|non-aqueous]] exceptions.<ref>{{cite web|url=https://www.chemguide.co.uk/physical/acidbaseeqia/theories.html|title=Theories of acids and bases|website=www.chemguide.co.uk|access-date=18 April 2018}}</ref>

The reaction of an acid with a base is called a [[neutralization (chemistry)|neutralization]] reaction. The products of this reaction are a [[salt (chemistry)|salt]] and water.
<math chem display=block>\text{acid} \ + \ \text{base} \ \longrightarrow \ \text{salt} \ + \ \text{water}</math>

In this traditional representation an acid–base neutralization reaction is formulated as a [[salt metathesis reaction|double-replacement reaction]]. For example, the reaction of [[hydrochloric acid]] (HCl) with [[sodium hydroxide]] (NaOH) solutions produces a solution of [[sodium chloride]] (NaCl) and some additional water molecules.
<math chem display=block>\ce{HCl_{(aq)} {} + NaOH_{(aq)} -> NaCl_{(aq)} {} + H2O}</math>

The modifier ([[aqueous solution|aq]]) in this equation was implied by Arrhenius, rather than included explicitly. It indicates that the substances are dissolved in water. Though all three substances, HCl, NaOH and NaCl are capable of existing as pure compounds, in [[aqueous solution]]s they are fully dissociated into the aquated ions {{chem2|H+, Cl-, Na+}} and {{chem2|OH-}}.

====Example: Baking powder====
[[Baking powder]] is used to cause the dough for breads and cakes to "rise" by creating millions of tiny [[carbon dioxide]] bubbles. Baking powder is not to be confused with [[baking soda]], which is [[sodium bicarbonate]] ({{chem2|NaHCO3}}). Baking powder is a mixture of baking soda (sodium bicarbonate) and acidic salts. The bubbles are created because, when the baking powder is combined with water, the sodium bicarbonate and acid salts react to produce gaseous [[carbon dioxide]]. 

Whether commercially or domestically prepared, the principles behind baking powder formulations remain the same. The acid–base reaction can be generically represented as shown:<ref>{{cite book | editor=A.J. Bent | title=The Technology of Cake Making | edition=6 | year=1997 | page=102 | publisher=Springer | isbn=9780751403497 | url=https://books.google.com/books?id=OTy8aIWxHhQC&pg=PA102 | access-date=2009-08-12}}</ref>

<math chem display=block>\ce{NaHCO3 + H+ -> Na+ + CO2 + H2O}</math> 

The real reactions are more complicated because the acids are complicated. For example, starting with sodium bicarbonate and [[monocalcium phosphate]] ({{chem2|Ca(H2PO4)2}}), the reaction produces carbon dioxide by the following [[stoichiometry]]:<ref name="KO">John Brodie, John Godber "Bakery Processes, Chemical Leavening Agents" in Kirk-Othmer Encyclopedia of Chemical Technology 2001, John Wiley & Sons. {{doi|10.1002/0471238961.0308051303082114.a01.pub2}}</ref>

<math chem display=block>\ce{14 NaHCO3 + 5 Ca(H2PO4)2 -> 14 CO2 + Ca5(PO4)3OH + 7 Na2HPO4 + 13 H2O}</math>

[[Image:Calcium dihydrogen phosphate.png|class=skin-invert-image|thumb|220px|[[Monocalcium phosphate]] ("MCP") is a common acid component in domestic baking powders.]]
A typical formulation (by weight) could call for 30% sodium bicarbonate, 5–12% [[monocalcium phosphate]], and 21–26% [[sodium aluminium sulfate]]. Alternately, a commercial baking powder might use [[sodium acid pyrophosphate]] as one of the two acidic components instead of sodium aluminium sulfate. Another typical acid in such formulations is [[cream of tartar]] ({{chem2|KC4H5O6}}), a derivative of [[tartaric acid]].<ref name="KO"/>