- Characteristics of acids
- - Physical properties
- - Ability to generate protons
- - They have hydrogens poor in electron density
- - Strength or acidity constant
- Measurements
- - It has very stable conjugate bases
- - They can have positive charges
- - Their solutions have pH values lower than 7
- - Ability to neutralize bases
- Strong acids and weak acids
- Examples of acids
- Hydrogen halides
- Oxoacids
- Super acids
- Organic acids
- References
The acids are compounds with high tendencies of proton donating or accepting an electron pair. There are many definitions (Bronsted, Arrhenius, Lewis) that characterize the properties of acids, and each of them is complemented to build a global image of these types of compounds.
From the above perspective, all known substances can be acidic, however, only those that stand out far above the others are considered as such. In other words: if a substance is an extremely weak proton donor, compared to water, for example, it can be said that it is not an acid.
Acetic acid, a weak acid, donates a proton (hydrogen ion, highlighted in green) to water in an equilibrium reaction to give the acetate ion and the hydronium ion. Red: oxygen. Black: carbon. White: hydrogen.
This being the case, what exactly are the acids and their natural sources? A typical example of them can be found inside many fruits: such as citrus. Lemonades have their characteristic flavor due to citric acid and other components.
The tongue can detect the presence of acids, just as it does other flavors. Depending on the level of acidity of these compounds, the taste becomes more intolerable. In this way, the tongue functions as an organoleptic meter for the concentration of acids, specifically the concentration of hydronium ion (H 3 O +).
On the other hand, acids are not only found in food, but also within living organisms. Likewise, soils present substances that can characterize them as acidic; such is the case of aluminum and other metal cations.
Characteristics of acids
What characteristics must a compound have, according to existing definitions, to be considered acidic?
It must be able to generate H + and OH - ions by dissolving in water (Arrhenius), it must donate protons to other species very easily (Bronsted) or finally, it must be able to accept a pair of electrons, being negatively charged (Lewis).
However, these characteristics are closely related to the chemical structure. Therefore, by learning to analyze it, its strength of acidity or of a couple of compounds can be deduced which of the two is the most acidic.
- Physical properties
Acids have a flavor, worth the redundancy, acid and their smell often burns the nostrils. They are liquids with a sticky or oily texture and have the ability to change the color of litmus paper and methyl orange to red (Properties of Acids and Bases, SF).
- Ability to generate protons
In 1923, the Danish chemist Johannes Nicolaus Brønsted and the English chemist Thomas Martin Lowry introduced the Brønsted and Lowry theory stating that any compound that can transfer a proton to any other compound is an acid (Encyclopædia Britannica, 1998). For example in the case of hydrochloric acid:
HCl → H + + Cl -
Brønsted and Lowry's theory did not explain the acidic behavior of certain substances. In 1923 the American chemist Gilbert N. Lewis introduced his theory, in which an acid is considered as any compound that, in a chemical reaction, is capable of joining a pair of electrons not shared in another molecule (Encyclopædia Britannica, 1998).
In this way, ions such as Cu 2+, Fe 2+ and Fe 3+ have the ability to bind to pairs of free electrons, for example from water to produce protons in the following way:
Cu 2+ + 2H 2 O → Cu (OH) 2 + 2H +
- They have hydrogens poor in electron density
For the methane molecule, CH 4, none of its hydrogens is electronically deficient. This is because the difference in electronegativities between carbon and hydrogen is very small. But if you replace one of the H atoms by one of fluorine, then there would be a noticeable change in the dipole moment: H 2 FC- H.
H experiences a displacement of its electron cloud towards the adjacent atom bonded to F, which is the same, δ + increases. Again, if another H is replaced by another F, then the molecule becomes: HF 2 C- H.
Now δ + is even greater, because two F atoms, highly electronegative electron density which remove the C, and this latter consequently to the H. If the replacement process continue finally obtained: F 3 C- H.
In this last molecule H presents, as a consequence of the three neighboring F atoms, a marked electronic deficiency. This δ + does not go unnoticed for any species rich enough in electrons to strip this H and, in this way, F 3 CH to become negatively charged:
F 3 C– H +: N - (negative species) => F 3 C: - + H N
The above chemical equation can also be considered this way: F 3 CH donates a proton (H +, the H once detached from the molecule) to: N; or, F 3 CH gains a pair of electrons from H when another pair is donated to the latter from: N -.
- Strength or acidity constant
How much F 3 C: - is present in the solution? Or, how many molecules of F 3 CH can donate acidic hydrogen to N? To answer these questions, it is necessary to determine the concentration of F 3 C: - or H N and, using a mathematical equation, establish a numerical value called the acidity constant, Ka.
The more molecules of F 3 C: - or HN produced, the more acidic F 3 CH will be and the greater its Ka. In this way Ka helps to clarify, quantitatively, which compounds are more acidic than others; and, likewise, it discards as acids those whose Ka are of an extremely small order.
Some Ka can have values that are around 10 -1 and 10 -5, and others, values millionths smaller like 10 -15 and 10 -35. It can then be said that the latter, having said acidity constants, are extremely weak acids and can be discarded as such.
So which of the following molecules has the highest Ka: CH 4, CH 3 F, CH 2 F 2, or CHF 3 ? The answer lies in the lack of electron density, δ +, in their hydrogens.
Measurements
But what are the criteria for standardizing Ka measurements? Its value can vary enormously depending on which species will receive the H +. For example, if: N is a strong base, Ka will be large; but if, on the contrary, it is a very weak base, Ka will be small.
Ka measurements are made using the most common and weakest of all bases (and acids): water. Depending on the degree of donation of H + to the H 2 O molecules, at 25ºC and at a pressure of one atmosphere, the standard conditions are established to determine the acidity constants for all the compounds.
From this arises a repertoire of tables of acidity constants for many compounds, both inorganic and organic.
- It has very stable conjugate bases
Acids have highly electronegative atoms or units (aromatic rings) in their chemical structures that attract electron densities from surrounding hydrogens, thus causing them to become partially positive and reactive to a base.
Once the protons donate, the acid transforms into a conjugate base; that is, a negative species capable of accepting H + or donating a pair of electrons. In the example of the CF 3 H molecule its conjugate base is CF 3 -:
CF 3 - + HN <=> CHF 3 +: N -
If CF 3 - is a very stable conjugate base, the equilibrium will be shifted more to the left than to the right. Also, the more stable it is, the more reactive and acidic the acid will be.
How do you know how stable they are? It all depends on how they deal with the new negative charge. If they can delocalize or diffuse the increasing electron density efficiently, it will not be available for use in bonding with the base H.
- They can have positive charges
Not all acids have electron-deficient hydrogens, but may also have other atoms capable of accepting electrons, with or without a positive charge.
How is this? For example, in boron trifluoride, BF 3, the B atom lacks an octet of valence, so it can form a bond with any atom that gives it an electron pair. If an anion F - round in its vicinity occurs the following chemical reaction:
BF 3 + F - => BF 4 -
On the other hand, free metal cations, such as Al 3+, Zn 2+, Na +, etc., are considered acidic, since from their environment they can accept dative (coordination) bonds of electron-rich species. Likewise, they react with OH - ions to precipitate as metal hydroxides:
Zn 2+ (aq) + 2OH - (aq) => Zn (OH) 2 (s)
All of these are known as Lewis acids, while those that donate protons are Bronsted acids.
- Their solutions have pH values lower than 7
Figure: pH scale.
More specifically, when an acid dissolves in any solvent (which does not neutralize it appreciably), it generates solutions with a pH lower than 3, although below 7 they are considered very weak acids.
This can be verified by using an acid-base indicator, such as phenolphthalein, universal indicator, or purple cabbage juice. Those compounds that turn the colors to those indicated for low pH, are treated as acids. This is one of the simplest tests to determine the presence of them.
The same can be done, for example, for different soil samples from different parts of the world, thus determining their pH values to, together with other variables, characterize them.
And finally, all acids have sour flavors, as long as they are not so concentrated as to irreversibly burn the tissues of the tongue.
- Ability to neutralize bases
Arrhenius, in his theory, proposes that acids, being able to generate protons, react with the hydroxyl of the bases to form salt and water in the following way:
HCl + NaOH → NaCl + H 2 O.
This reaction is called neutralization and is the basis of the analytical technique called titration (Bruce Mahan, 1990).
Strong acids and weak acids
Acids are classified into strong acids and weak acids. The strength of an acid is associated with its equilibrium constant, hence, in the case of acids, these constants are called acid constants Ka.
Thus, strong acids have a large acid constant so they tend to dissociate completely. Examples of these acids are sulfuric acid, hydrochloric acid, and nitric acid, whose acid constants are so great that they cannot be measured in water.
On the other hand, a weak acid is one whose dissociation constant is low so it is in chemical equilibrium. Examples of these acids are acetic acid and lactic acid and nitrous acid whose acidity constants are in the order of 10 -4. Figure 1 shows the different acidity constants for different acids.
Figure 1: acid dissociation constants.
Examples of acids
Hydrogen halides
All hydrogen halides are acidic compounds, especially when dissolved in water:
-HF (hydrofluoric acid).
-HCl (hydrochloric acid).
-HBr (hydrobromic acid).
-HI (iodic acid).
Oxoacids
Oxo acids are the protonated forms of oxoanions:
HNO 3 (nitric acid).
H 2 SO 4 (sulfuric acid).
H 3 PO 4 (phosphoric acid).
HClO 4 (perchloric acid).
Super acids
Super acids are the mixture of a strong Bronsted acid and a strong Lewis acid. Once mixed they form complex structures where, according to certain studies, the H + "jumps" within them.
Their corrosive power is such that they are billions of times stronger than concentrated H 2 SO 4. They are used to crack large molecules present in crude oil, into smaller, branched molecules, and with great added economic value.
-BF 3 / HF
-SbF 5 / HF
-SbF 5 / HSO 3 F
-CF 3 SO 3 H
Organic acids
Organic acids are characterized by having one or more carboxylic groups (COOH), and among them are:
-Citric acid (present in many fruits)
Malic acid (from green apples)
-Acetic acid (from commercial vinegar)
-Butyric acid (from rancid butter)
-Tartaric acid (from wines)
-And the family of fatty acids.
References
- Torrens H. Hard and Soft Acids and Bases.. Taken from: depa.fquim.unam.mx
- Helmenstine, Anne Marie, Ph.D. (May 3, 2018). Names of 10 Common Acids. Recovered from: thoughtco.com
- Chempages Netorials. Acids and Bases: Molecular Structure and Behavior. Taken from: chem.wisc.edu
- Deziel, Chris. (April 27, 2018). General Characteristics of Acids & Bases. Sciencing. Recovered from: sciencing.com
- Pittsburgh Supercomputing Center (PSC). (October 25, 2000). Recovered from: psc.edu.