The acids they are compounds with a high tendency to donate protons or accept a pair of electrons. 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 this type 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.
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 with 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 of the concentration of acids, specifically the concentration of hydronium ion (H3OR+).
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.
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What characteristics must a compound have, according to existing definitions, to be considered as acidic??
Must be able to generate H ions+ and OH- when dissolving in water (Arrhenius), it has to 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, it is possible to deduce its strength of acidity or of a couple of compounds, which of the two is the most acidic..
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, S.F.).
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 Cutwo+, faithtwo+ and the faith3+ They have the ability to bind with pairs of free electrons, for example from water to produce protons in the way
Cutwo+ + 2HtwoO → Cu (OH)two + 2H+
For the methane molecule, CH4, none of its hydrogens is electronically deficient. This is because the electronegativity difference between carbon and hydrogen is very small. But, if one of the H atoms were replaced by one of fluorine, then there would be a noticeable change in the dipole moment: HtwoFC-H.
H it experiences a displacement of its electron cloud towards the adjacent atom bonded to the F, which is the same, δ + increases. Again, if another H is replaced by another F, then the molecule would look like: HFtwoC-H.
Now δ + is even higher, since they are two highly electronegative F atoms, which subtract electron density from C, and the latter, consequently, from C H. If the substitution process continued, it would finally be obtained: F3C-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 by any species rich enough in electrons to strip this H and thus F3CH become negatively charged:
F3C-H + : N- (negative species) => F3C:- + HN
The above chemical equation can also be considered this way: F3CH donates a proton (H+, the H once detached from the molecule) a: N; o, F3CH gains a pair of electrons from H as another pair was donated to the latter from: N-.
How much F3C:- is it present in the solution? Or, how many molecules of F3CH can donate hydrogen acid to N? To answer these questions, it is necessary to determine the concentration of F3C:- or from HN and, using a mathematical equation, establish a numerical value called the acidity constant, Ka.
The more molecules of F3C:- or HN are produced, the more acid will be F3CH and bigger 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 around 10-1 and 10-5, and others, values millionths smaller like 10-fifteen 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: CH4, CH3F, CHtwoFtwo or CHF3? The answer lies in the lack of electron density, δ +, in their hydrogens..
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 H moleculestwoOr, 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..
Acids have highly electronegative atoms or units (aromatic rings) in their chemical structures that attract electronic densities from surrounding hydrogens, thus causing them to become partially positive and reactive to a base..
Once the protons donate, the acid turns into a conjugate base; that is, a negative species capable of accepting H+ or donate a pair of electrons. In the example of the CF molecule3H its conjugate base is CF3-:
CF3- + HN <=> CHF3 + : N-
Yes CF3- 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 it or diffuse the increasing electron density efficiently, it will not be available for use in bonding with the base H..
Not all acids have electron-deficient hydrogens, but they may also have other atoms capable of accepting electrons, with or without a positive charge..
How is this? For example, in boron trifluoride, BF3, the B atom lacks an octet of valence, so it can form a bond with any atom that gives it a pair of electrons. If an anion F- round in its proximity the following chemical reaction occurs:
BF3 + F- => BF4-
On the other hand, free metal cations, such as Al3+, Zntwo+, Na+, etc., are considered acidic, since from their environment they can accept dative (coordination) bonds of electron-rich species. They also react with OH ions- to precipitate as metal hydroxides:
Zntwo+(aq) + 2OH-(ac) => Zn (OH)two(s)
All of these are known as Lewis acids, while those that donate protons are Bronsted acids..
More specifically, when an acid dissolves in any solvent (that 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..
Arrhenius, in his theory, proposes that acids, by being able to generate protons, react with the hydroxyl of the bases to form salt and water in the following way:
HCl + NaOH → NaCl + HtwoOR.
This reaction is called neutralization and is the basis of the analytical technique called titration (Bruce Mahan, 1990).
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 acidity 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 on the order of 10-4. Figure 1 shows the different acidity constants for different acids.
All hydrogen halides are acidic compounds, especially when dissolved in water:
-HF (hydrofluoric acid).
-HCl (hydrochloric acid).
-HBr (hydrobromic acid).
-HI (iodic acid).
Oxo acids are the protonated forms of oxoanions:
HNO3 (nitric acid).
HtwoSW4 (sulfuric acid).
H3PO4 (phosphoric acid).
HClO4 (perchloric acid).
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+ "Jump" inside them.
Their corrosive power is such that they are billions of times stronger than HtwoSW4 concentrated. They are used to crack large molecules present in crude oil, into smaller, branched molecules, and with great added economic value..
-BF3/ HF
-SbF5/ HF
-SbF5/ HSO3F
-CF3SW3H
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 fatty acid family.
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