Acid–base reaction
In chemistry, an acid–base reaction is a chemical reaction that occurs between an acid and a base. It can be used to determine pH via titration. Several theoretical frameworks provide alternative conceptions of the reaction mechanisms and their application in solving related problems; these are called the acid–base theories, for example, Brønsted–Lowry acid–base theory.
Their importance becomes apparent in analyzing acid–base reactions for gaseous or liquid species, or when acid or base character may be somewhat less apparent. The first of these concepts was provided by the French chemist Antoine Lavoisier, around 1776.
It is important to think of the acid–base reaction models as theories that complement each other. For example, the current Lewis model has the broadest definition of what an acid and base are, with the Brønsted–Lowry theory being a subset of what acids and bases are, and the Arrhenius theory being the most restrictive.
The Arrhenius theory describes an acid as a substance that increases the concentration of hydrogen ions in a solution.
A base is a substance that increases the concentration of hydroxide ions in a solution. However, Arrhenius's definition only applies to substances that are in water.
Acid–base definitions
Historic development
The concept of an acid–base reaction was first proposed in 1754 by Guillaume-François Rouelle, who introduced the word "base" into chemistry to mean a substance which reacts with an acid to give it solid form. Bases are mostly bitter in nature.Lavoisier's oxygen theory of acids
The first scientific concept of acids and bases was provided by Lavoisier in around 1776. Since Lavoisier's knowledge of strong acids was mainly restricted to oxoacids, such as and , which tend to contain central atoms in high oxidation states surrounded by oxygen, and since he was not aware of the true composition of the hydrohalic acids, he defined acids in terms of their containing oxygen, which in fact he named from Greek words meaning "acid-former". The Lavoisier definition held for over 30 years, until the 1810 article and subsequent lectures by Sir Humphry Davy in which he proved the lack of oxygen in hydrogen sulfide, hydrogen telluride, and the hydrohalic acids. However, Davy failed to develop a new theory, concluding that "acidity does not depend upon any particular elementary substance, but upon peculiar arrangement of various substances". One notable modification of oxygen theory was provided by Jöns Jacob Berzelius, who stated that acids are oxides of nonmetals while bases are oxides of metals.Liebig's hydrogen theory of acids
In 1838, Justus von Liebig proposed that an acid is a hydrogen-containing compound whose hydrogen can be replaced by a metal. This redefinition was based on his extensive work on the chemical composition of organic acids, finishing the doctrinal shift from oxygen-based acids to hydrogen-based acids started by Davy. Liebig's definition, while completely empirical, remained in use for almost 50 years until the adoption of the Arrhenius definition.Arrhenius definition
The first modern definition of acids and bases in molecular terms was devised by Svante Arrhenius. 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 ionises in water to form hydrogen cations ; that is, an acid increases the concentration of H+ ions in an aqueous solution.
- An Arrhenius base is a substance that dissociates in water to form hydroxide ions; that is, a base increases the concentration of ions in an aqueous solution.
The reaction of an acid with a base is called a neutralization reaction. The products of this reaction are a salt and water.
In this traditional representation an acid–base neutralization reaction is formulated as a double-replacement reaction. For example, the reaction of hydrochloric acid with sodium hydroxide solutions produces a solution of sodium chloride and some additional water molecules.
The modifier 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 solutions they are fully dissociated into the aquated ions and.
Example: 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. Baking powder is a mixture of baking soda 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:
The real reactions are more complicated because the acids are complicated. For example, starting with sodium bicarbonate and monocalcium phosphate, the reaction produces carbon dioxide by the following stoichiometry:
Image:Calcium dihydrogen phosphate.png|class=skin-invert-image|thumb|220px|Monocalcium phosphate is a common acid component in domestic baking powders.
A typical formulation 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, a derivative of tartaric acid.
Brønsted–Lowry definition
The Brønsted–Lowry definition, formulated in 1923, independently by Johannes Nicolaus Brønsted in Denmark and Martin Lowry in England, is based upon the idea of protonation of bases through the deprotonation of acids – that is, the ability of acids to "donate" hydrogen cations otherwise known as protons to bases, which "accept" them.An acid–base reaction is, thus, the removal of a proton from the acid and its addition to the base. The removal of a proton from an acid produces its conjugate base, which is the acid with a proton removed. The reception of a proton by a base produces its conjugate acid, which is the base with a proton added.
Unlike the previous definitions, the Brønsted–Lowry definition does not refer to the formation of salt and solvent, but instead to the formation of conjugate acids and conjugate bases, produced by the transfer of a proton from the acid to the base. In this approach, acids and bases are fundamentally different in behavior from salts, which are seen as electrolytes, subject to the theories of Debye, Onsager, and others. An acid and a base react not to produce a salt and a solvent, but to form a new acid and a new base. The concept of neutralization is thus absent. Brønsted–Lowry acid–base behavior is formally independent of any solvent, making it more all-encompassing than the Arrhenius model. The calculation of pH under the Arrhenius model depended on alkalis dissolving in water. The Brønsted–Lowry model expanded what could be pH tested using insoluble and soluble solutions.
The general formula for acid–base reactions according to the Brønsted–Lowry definition is:
where HA represents the acid, B represents the base, represents the conjugate acid of B, and represents the conjugate base of HA.
For example, a Brønsted–Lowry model for the dissociation of hydrochloric acid in aqueous solution would be the following:
The removal of from the produces the chloride ion,, the conjugate base of the acid. The addition of to the forms the hydronium ion,, the conjugate acid of the base.
Water is amphoteric that is, it can act as both an acid and a base. The Brønsted–Lowry model explains this, showing the dissociation of water into low concentrations of hydronium and hydroxide ions:
This equation is demonstrated in the image below:
Here, one molecule of water acts as an acid, donating an and forming the conjugate base,, and a second molecule of water acts as a base, accepting the ion and forming the conjugate acid,.
As an example of water acting as an acid, consider an aqueous solution of pyridine,.
In this example, a water molecule is split into a hydrogen cation, which is donated to a pyridine molecule, and a hydroxide ion.
In the Brønsted–Lowry model, the solvent does not necessarily have to be water, as is required by the [|Arrhenius Acid–Base model]. For example, consider what happens when acetic acid,, dissolves in liquid ammonia.
An ion is removed from acetic acid, forming its conjugate base, the acetate ion,. The addition of an ion to an ammonia molecule of the solvent creates its conjugate acid, the ammonium ion,.
The Brønsted–Lowry model calls hydrogen-containing substances acids. Thus, some substances, which many chemists considered to be acids, such as or, are excluded from this classification due to lack of hydrogen. Gilbert N. Lewis wrote in 1938, "To restrict the group of acids to those substances that contain hydrogen interferes as seriously with the systematic understanding of chemistry as would the restriction of the term oxidizing agent to substances containing oxygen." Furthermore, and are not considered Brønsted bases, but rather salts containing the bases and.