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What are acid and base theories?

The theories of acids and bases are a set of conceptual contributions that define and differentiate acids and bases, two conglomerates of substances of immense relevance in the fields of chemistry.

Its roots come from the first characterizations made by Robert Boyle in 1661, who defined acids as sour and corrosive substances that change the color of litmus paper from blue to red; and bases as soapy substances that, unlike acids, change the color of litmus paper from red to blue.

Antoine Lavoisier proposed that all acids contained oxygen atoms, such as H 2 SO 4 and HNO 3 . Centuries later in 1811 Humphry Davy found that many acids did not actually possess oxygen atoms, such as HCl, HF, HBr, etc.

And shortly before the appearance of the famous acid-base triad (top image), Justig Liebig suggested that acids have hydrogen atoms that can be replaced by metal cations.

This empirical knowledge served as inspiration for the current theories of acids and bases: Arrhenius, Bronsted-Lowry and Lewis. As seen in the graph, Arrhenius’s theory is the most restrictive or limited of all, while Lewis’s is the most general and globalized.

Arrhenius theory

Proposed in 1884 by Svante Arrhenius, his theory says that acids when dissolved in water produce H 3 O + or H + ions ; while the bases, when dissolved in water, produce or release OH  ions .

The H 3 O + and OH  ions combine in a reversible reaction that gives rise to water molecules:

3 O +   + OH     ⇌ 2H 2 O

So, an Arrhenius acid is also one that increases the concentration of H 3 O + ions , while an Arrhenius base is one that increases the concentration of OH  ions .

This means that an Arrhenius acid does not necessarily have to contain H 3 O + ions , and an Arrhenius base does not have to have OH  ions in its structure.

Example and limitations

Consider the following dissociations:

HCl (aq) → H + (aq) + Cl  (aq)

NaOH (aq) → Na + (aq) + OH  (aq)

HCl is an Arrhenius acid because when dissolved in water it produces H + ions , or more correctly, H 3 O + ions . And on the other hand, NaOH is an Arrhenius base because when dissolved in water it releases OH  ions . These substances react with each other to produce salt and water:

HCl (aq) + NaOH (aq) → NaCl (aq) + H 2 O

The problem with the Arrhenius theory lies in the fact that it is limited only to aqueous solutions, and therefore applies only to substances that are soluble in water. For example, MgO is very insoluble in water, and yet it is a basic oxide.

Likewise, it is unable to explain by itself how substances such as NH 3 and CaO produce OH ions  dissolved in water, even when they do not have OH  ions in their molecular or crystalline structure (as is the case with NaOH or KOH).

Nor does it explain how CO 2 can release H 3 O + ions in water without having any hydrogen atom in its molecular structure (as is the case with HCl or H 2 SO 4 ).

Brönsted-Lowry theory

Proposed individually in 1923 by Johannes Bronsted and Thomas Lewry, their theory of acids and bases says that an acid is any substance that can donate H + ions , and that a base on the other hand is any substance that is capable of accepting these H + . This theory is more general than Arrhenius’s and covers several of its limitations.

Example

Consider again the neutralization equation between H 3 O + and an OH  to produce two water molecules:

Example of acid and base neutralization in water. Source: Gabriel Bolívar.

Note that the H 3 O + donates an H + that immediately becomes linked or coordinated with the OH  just to the right. When the H + ends up bound to one of the free pairs of electrons of the O in the OH  , two neutral water molecules remain as products.

The H 3 O + therefore is a Bronsted-Lowry acid, for donating the H + , and the OH  is a Bronsted-Lowry base for accepting this H + .

Another example of an acid-base reaction is as follows:

NH 3 (g) + HCl (g) → NH 4 Cl (s)

This reaction, according to Arrhenius, could not be considered of the acid-base type because it does not take place in an aqueous medium but in the vapor phase or in the gaseous state . NH 3 is a Bronsted base because it accepts H from HCl by donating a pair of electrons located on the nitrogen atom; and HCl, logically, is Bronsted acid.

Thus, MgO is a Bronsted base because it is capable of accepting H + from acids to transform into Mg (OH) 2 .

Lewis theory

The Bronsted-Lowry theory explains a wide spectrum of acid-base reactions. However, it ignores those reactions where the H + and OH  ions are not involved at all, as well as a greater participation of the pairs of free electrons in molecular mechanisms.

Thus, in 1923 GN Lewis proposed an acid-base theory that says the following: an acid is any substance that is capable of accepting pairs of electrons, while a base is one that donates these pairs of electrons.

For example, the H + ion is a Lewis acid because it accepts pairs of electrons of any kind around it: from H 2 O to originate H 3 O + , from NH 3 to originate NH + , etc.

Example and advantages

Consider another example in the following reaction:

Example of a neutralization between an acid and a Lewis base. Source: Gabriel Bolívar.

NH 3 donates an electron pair (blue) from the nitrogen atom to the boron atom of BF 3 . Therefore NH 3 is the Lewis base because it donates the electron pair, and BF 3 is the Lewis acid because it accepts them.

Generally, cations and electron-deficient species are Lewis acids, while anions and electron-rich species are Lewis bases.

The advantages of the Lewis acid-base theory is that it encompasses all the others.

For example: H 2 O has pairs of free electrons in oxygen. If you donate one of them to an H + ion , it will act as a Lewis base, with H + therefore being Lewis acid.

According to Bronsted-Lowry, the H 2 O would be acting as a base because it is precisely accepting the H + by donating a pair of its free electrons to it. And finally, H 2 O is also a base according to Arrhenius theory because it is decreasing the concentration of H + ions in the aqueous medium, and consequently increasing the concentration of OH  ions .

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