Some Characteristics of acids Most important are its physical properties, its strength and its ability to neutralize bases, among others.
Acids are chemicals with the ability to donate a hydronium ion (H 3 OR + ), Or as it is commonly called a proton (H + ), In an aqueous medium, or capable of forming bonds with hydroxide ions, or any substance capable of accepting a pair of electrons.
They often have the general formula of H-A where H is the proton and"A"is the generic term associated with the non-protonic acid part.
Originally, our concepts of acidity came from ancient Greeks who defined"bitter-tasting"substances as Oxein , Which mutated into the Latin word for vinegar, acetum, which was then converted to"acid."
These substances not only had a bitter taste but also had the property to change color the litmus paper.
The theoretical structuring of the acids began when the French chemist Antoine Laurent Lavoisier (1743-1796) turned his attention to the classification of acids and bases. His idea was that all acids contained more or less a particular"essence"that was responsible for their acidity and were not just different.
Unfortunately, Lavoisier mistakenly thought that the substance Oxein-genic Was, as he called it, the oxygen atom. At the beginning of the 19th century, the English chemist Humphry Davy (1778-1829) showed that oxygen could not be responsible for acidity, because there were numerous acids that did not contain oxygen (LESNEY, 2003).
It was decades later that the idea of acidity associated with the presence of hydrogen was proposed by Justus von Liebig (1803-1873). Clarity was brought to the field when, in the 1890s, Svante August Arrhenius (1859-1927) defined acids as"substances that deliver hydrogen cations to the solution"(Encyclopædia Britannica, 1998).
Main Characteristics of Acids
1- Physical Properties
The acids possess a taste, worth the redundancy, acid and its smell often burns the nostrils.
They are liquids with 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.).
2- Proton generation capacity
In 1923, Danish chemist Johannes Nicolaus Brønsted and the English chemist Thomas Martin Lowry introduced the theory of Brønsted and Lowry 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 -
The theory of Brønsted and Lowry did not explain the acid behavior of certain substances. In 1923 the American chemist Gilbert N. Lewis introduces his theory, in which an acid is considered as any compound that, in a chemical reaction, is able to join a pair of electrons not shared in another molecule (Encyclopædia Britannica, 1998) .
In this way, ions such as Cu 2+ , faith 2+ And Faith 3+ Have the ability to bind to pairs of free electrons, such as water to produce protons in the manner:
Cu 2+ + 2H 2 O → Cu (OH) 2 + 2H +
3- Strength of an acid
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, said constants are named Ka acidity constants.
Thus, strong acids have a large acidity constant so they tend to dissociate completely. Examples of these acids are sulfuric acid , Hydrochloric acid and nitric acid, whose acid constants are so large that they can not be measured in water.
On the other hand, a weak acid is one whose dissociation constant is low so that it is in chemical equilibrium. Examples of these acids are acetic acid and lactic acid and nitrous acid Whose acid constants are of the order of 10 -4 . Figure 1 shows the different acid constants for different acids.
Figure 1: acid dissociation constants.
4- pH less than 7
The pH scale measures the level of alkalinity or acidity of a solution. The scale varies from zero to 14. A pH less than 7 is acidic. A pH greater than 7 is basic. The midpoint 7 represents a neutral pH. A neutral solution is neither acidic nor alkaline.
Figure 2: pH scale.
The pH scale is obtained as a function of the concentration of H + In the solution and is inversely proportional to it. Acids, by increasing the concentration of protons, lower the pH of a solution.
5- Ability to neutralize bases
Arrhenius, in his theory, proposes that acids, on being able to generate protons, react with hydroxyls of the bases to form salt and water in the manner:
HCl + NaOH → NaCl + H 2 OR.
This reaction is called neutralization and is the basis of the analytical technique called titration (Bruce Mahan, 1990).
6- Reduction oxide capacity
Given their ability to produce charged species, acids are used as a medium for electron transfer in redox reactions.
Acids also have the tendency to be reduced since they possess the ability to accept free electrons. Acids contain H ions + . They tend to take electrons and form hydrogen gas.
2H + + 2e - → H 2
Metals do not have tight control over their electrons. They abandon them without much struggle and form metallic ions.
Fe → Faith 2+ + 2e -
So when you put an iron nail into an acid, the H ions + They grab the electrons from the iron. Iron is converted to soluble Fe 2 + , And the solid metal gradually disappears. The reaction is:
Fe + 2H + → Faith 2+ + H 2
This is known as acid corrosion. Acids not only corrode metals by dissolving them, they also react with organic compounds such as cellular membrane .
Such a reaction is usually exothermic which causes severe burns on contact with the skin, so this type of substance should be handled with care. Figure 3 is the safety code when a substance is corrosive.
Figure 3: Signaling of corrosive substances.
7- Acid catalysis
Acceleration of a chemical reaction by the addition of an acid is known as acid catalysis. Said acid is not consumed in the reaction.
The catalytic reaction may be acid specific as in the case of the decomposition of sucrose sugar into glucose and fructose in sulfuric acid Or may be general to any acid.
The mechanism of acid and base catalyzed reactions is explained in terms of the Brønsted-Lowry acid and base concept as one in which there is an initial transfer of protons from an acid catalyst to the reagent (Encyclopædia Britannica, 1998).
Generally the reactions where an electrophile is involved are catalyzed in acid medium, either electrophilic additions or substitutions.
Examples of acid catalysis are the nitration of benzene in the presence of sulfuric acid (Figure 4a), the hydration of ethylene to produce ethanol (Figure 4b), esterification reactions (Figure 4c) and ester hydrolysis (Figure 4d) (Clark, 2013).
Figure 4: Examples of acid catalysis.
References
- Bruce Mahan, R.M. (1990). Chemistry university course fourth edition. Wilmington: Addison-Wesley Iberoamericana S.A.
- Clark, J. (2013, December 20). Examples of Acid Catalysis in Organic Chemistry . Retrieved from chem.libretexts.org.
- Encyclopædia Britannica. (1998, July 20). Acid-base catalysis . Retrieved from britannica.com.
- Encyclopædia Britannica. (1998, December 21). Arrhenius theory . Retrieved from britannica.com.
- Encyclopædia Britannica. (1998, July 20). Brønsted-Lowry theory . Retrieved from britannica.com.
- Encyclopædia Britannica. (1998, July 20). Lewis theory . Retrieved from britannica.com.
- LESNEY, M. S. (2003, March). Chemistry Chronicles A Basic History of Acid- From Aristotle to Arnold. Retrieved from pubs.acs.org.
- Properties of Acids and Bases. (S.F.). Retrieved from sciencegeek.net.