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Acids are much more common substances than many people realize. They are present in all sorts of places from the food we eat, the liquids we drink, the batteries that power our devices, and more. In addition to being ubiquitous, acids are also very varied when it comes to their properties, the most important of which is, incidentally and precisely, their acidity. In the following sections we will review the concept of acid from different points of view, we will define what strong acids are and we will also see examples of the strongest acids known to science.
What is an acid?
There are several different concepts of acids and bases. According to both Arrhenius and Bromsted and Lowry, an acid is any chemical substance that has the ability to release protons (H + ions ) in solution. Although this concept is appropriate for the great majority of the compounds that we consider to be acids, it is inadequate for other substances that behave like acids and that produce solutions with an acidic pH, but that, despite this, do not even have hydrogen cations in them. its structure.
In view of the above, the broadest and most accepted concept of acid is that of Lewis acids, according to which an acid is any chemical substance deficient in electrons (generally with an incomplete octet) capable of receiving a pair of electrons per part of a base , thus forming a dative or coordinate covalent bond. This concept is much more general than the others, since it allows us to expand the concept of acids and bases beyond the aqueous solutions we are used to.
How is acidity measured?
If we want to talk about strong and weak acids, we must have a way of measuring the relative strength of acids, that is, we must be able to measure their acidity in order to compare. In aqueous solutions, acidity is measured in terms of the ability to generate hydronium ions in solution, either by direct donation of protons to water molecules:
or by coordination of water molecules that produce the loss of a proton to a second water molecule:
In both cases, these are reversible reactions that are associated with an ionic equilibrium constant called the acid dissociation constant or acidity constant ( K a ). The value of this constant, or its negative logarithm, called pK a , is often used as a measure of the acidity of an acid. In this sense, the higher the value of the acidity constant (or the lower the value of its pK a ), the stronger an acid will be, and vice versa.
Another way of measuring the degree of acidity that is similar, although a little more direct, is by experimentally measuring the pH of solutions of different acids, but with the same molar concentration. The lower the pH, the more acidic the substance.
The acidity of superacids
Although the above ways of measuring acidity are suitable for acids in aqueous solutions, they are not useful for cases where acids are dissolved in other solvents (particularly aprotic or non-hydrogen solvents) or much except in the case of pure acids. In addition, water and other solvents have what is called an acid leveling effect, which causes all acids, after a certain level of acidity, to behave in the same way in solution.
To overcome this difficulty, that all strong acids in aqueous solution have the same acidity, other ways of measuring acidity have been devised. Collectively, these are called acidity functions, the most common being the Hammett or H 0 acidity function . This function is similar in concept to pH, and represents the ability of a Bromsted acid to protonate a very weak generic base, such as 2,4,6-trinitroaniline, and is given by:
In this case, pK HB+ is the negative logarithm of the acidity constant of the conjugate acid of the weak base when dissolved in the pure acid, [B] is the molar concentration of the unprotonated base, and [HB + ] is the concentration of its conjugate acid. The lower the H 0 , the higher the acidity. For reference, sulfuric acid has a Hammett function value of -12.
strong acids and weak acids
Strong acids are considered to be all those that completely dissociate in aqueous solution. In other words, they are those for which dissociation in water is an irreversible process. On the other hand, weak acids are those that do not completely dissociate in water because their dissociation is reversible and they have a relatively low acidity constant associated with them.
The Superacids
In addition to strong acids, there are also superacids. These are all those acids that are stronger than pure sulfuric acid. These acids are so strong that they are capable of protonating even substances that we normally think of as neutral, and they can even protonate other strong acids.
List of common strong acids
The most common strong acids are:
- Sulfuric acid (H 2 SO 4 , only the first dissociation)
- Nitric acid (HNO 3 )
- Perchloric acid (HClO 4 )
- Hydrochloric acid (HCl)
- Hydroiodic acid (HI)
- Hydrobromic acid (HBr)
- Trifluoroacetic acid (CF 3 COOH)
There are a few additional examples of strong acids, but most acids are weak.
Fluoroantimonic Acid: The Strongest Acid in the World
The strongest known acid is a superacid called fluoroantimonic acid with the formula HSbF 6 . It is prepared by reacting antimony pentafluoride (SbF 5 ) with hydrogen fluoride (HF).
This reaction generates the hexacoordinated ion [SbF 6 – ] which is extremely stable due to multiple resonance structures that distribute and stabilize the negative charge over 6 fluorine atoms, which is the most electronegative element in the periodic table.
In terms of acidity, this acid has a Hammett acidity function value between –21 and –24, which means that this acid is between 10 9 and 10 12 times more acidic than pure sulfuric acid ( remember Hammett’s acidity function is a logarithmic function, so each change of one unit implies a change of one order of magnitude).
List of other superacids
- Triflic acid or trifluoromethanesulfonic acid (CF 3 SO 3 H)
- Fluorosulfonic acid (FSO 3 H)
- Magic acid (SbF5)-FSO 3 H
References
Brønsted-Lowry Superacids and the Hammett Acidity Function. (2021, October 4). https://chem.libretexts.org/@go/page/154234
Chang, R. (2021). Chemistry (11th ed .). MCGRAW HILL EDUCATION.
Farrell, I. (2021, October 21). What’s the strongest acid in the world? CSR Education. https://edu.rsc.org/everyday-chemistry/whats-the-strongest-acid-in-the-world/4014526.article
Ganninger, D. (2020, October 26). The Strongest Acid in the World–Knowledge Stew . Medium. https://medium.com/knowledge-stew/the-strongest-acid-in-the-world-eb7700770b78#:%7E:text=Fluoroantimonic%20acid%20is%20the%20strongest,a%20host%20of%20other% 20substances
SciShow. (2016, December 19). The Strongest Acids in the World [Video]. Youtube. https://www.youtube.com/watch?v=cbN37yRV-ZY
