- Posted by: bakhshi
- Category: News and Articles
Humans have long been dealing with acid and base and have experienced sour tastes and other properties of acids such as vinegar (acetic acid) and lemon (citric acid). Acid has long been known as a hydrogen compound that can be completely or partially replaced by some metals and form salts. For example, in the reaction below, HCl is known as acid because the metal is replaced by hydrogen.
HCl (aq) + NaOH (aq) → H2O (l) + NaCl (aq)
Arrhenius Theory: In the year 9, Arrhenius introduced the following definition for acids and bases while expressing the theory of ion separation:
Acids are compounds that produce water in the solvent of H + ions and bases are compounds that produce water in -OH ions.
HCl acid → H + (aq) + Cl– (aq)
Open NaOH (aq) → Na + (aq) + OH– (aq)
So the common properties of acids, such as having a sour taste, are attributed to the existence of H + ions. The reaction of acids with other substances is known as the result of the H + reaction with those substances. The reaction of zinc metal with some acids, which results in the release of hydrogen gas, can be written as follows:
Zn (s) + H + (aq) → Zn2 + (aq) + H2 (g)
In this way, the common properties of bases (having a taste) are derived from -OH properties. For example, SO2 gas with bases can be written as follows:
SO2 (g) + 2OH– (aq) → H2O (l) + SO32- (aq)
Therefore, the reaction of neutralizing acids and bases is the result of the reaction of + H with -OH
H + (aq) + OH– (aq) → H2O (l)
Arrhenius’s theory of acids and bases has two important properties:
It justifies the common properties of acids and bases.
Provides a scale for calculating and comparing the strength of acids. This means that weak acid is partially dissolved. Whereas strong acid is almost completely dissolved. But it must be acknowledged that, despite being correct, Arrhenius’s theory has some drawbacks. The most important weakness of Arrhenius’s theory is that only the solvent (the reaction medium) considers water.
2. Lori Bronsted Theory: In Lori and Bronsted, they introduced the definition of acid and base independent of the solvent environment. According to this theory, acid is a chemical species (cation and anion molecules) that can produce H + in a reaction medium independent of the nature of the solvent.
HF → H + + F–
NH4 + → H + + NH3
H2PO4– → H + + HPO42-
And again it is a chemical species that, independently of the nature of the solvent, can stabilize the H + particle.
NH3 + + H + → NH4
Since the reaction of proton production by an acid is an equilibrium reaction and the reaction of proton stabilization on one again is an equilibrium reaction, so it can be said that each acid opens with the loss of the proton and again with the acid proton stabilizes. The equilibrium between acid and base can then be considered independent of the solvent type on the basis of the following equilibrium:
Acid = H + + base
HF = H + + F–
HF-conjugated acid HF and -F base conjugate are called HF acid conjugates and -HF / F is a pair of open acid.
Proton Exchange: + H is not available in the free state, in order for an acid to be able to produce a proton in a reactive environment, it must be present in a game that is capable of proton stabilization.
Base (1) + + H = acid (1)
Acid (1) = + H + base (1)
Base (1) + acid (1) = base (1) + acid (1)
According to the above reaction, in a proton exchange reaction, we are faced with two acid-base pairs. If the upper reaction is directed to the right the reaction will not be directed to the left then obviously open (1), weak and acidic (2) acid is weak, so in a pair of acid and base if the acid is strong, the conjugate is weak and Conversely in an acid-base pair, if the acid is strong, the conjugate acid is weak. For example, in the ionic separation of water by the following reaction:
H2O (l) + H2O (l) = H3O + (aq) + OH– (aq)
Base (1) acid (2) acid (2) base (1)
The acid and base pairs are equilibrium – H2O / OH (one pair) and H3O + / H2O (the other pair). So we conclude that the water solvent acts as an acid against the acids and as an acid against the bases.
HF + H2O = F– + H3O +
Acid (1) base (2) base (2) acid (2)
NaOH (aq) + H2O (l) = Na + (aq) + OH– (aq)
Base (1) acid (2) acid (2) base (1)
These include compounds such as water that act as both acid and base ampholyte, such as HCO3- and H2PO4-
HCO3– (aq) + H3O + (aq) = H2CO3 (aq) + H2O
HCO3– (aq) + H2O = CO32- (aq) + H3O + (aq)
2. Lewis Theory: Gilbert Lewis introduced a more comprehensive concept for acids in year 6 that separated the acid-base theme from protons. According to Lewis theory, acid is a substance that can form a covalent bond by taking a pair of electrons out of the base. In Lewis theory, the concept of electron pairs and covalent bond formation is used. Lewis’s definition of acids is much more comprehensive than Lori’s and Bronsted’s suggested that Lewis’s definition of acid and base can also be provided by molecular orbital theory. In general, the acid can get a pair of electrons from the highest empty orbital to the bottom of its empty orbital. Although this is the most comprehensive definition for acids and bases, the Lori-Bronsted definition is more applicable to acid and base. Using this definition, one can determine the strength of an acid. This concept is also used in organic chemistry (for example, in carboxylic acid). Chemical compounds that can play Lewis acid include:
Atoms are molecules that have incomplete octaves.
BH3 + F– → BH4– (aq)
Many simple cations can play the role of Lewis acid.
Cu2 + + 4NH3 → Cu (NH3) 42+
Some metal atoms in the formation of cracks
Ni + 4CO → Ni (CO) 4
Compounds whose central atoms are capable of expanding their valence layer play an acid role in the reactions that make this expansion possible, for example in the reaction, the central atom’s capacity (Sn) layer is extended from 1 to 5 electrons.
SnCl4 + 2Cl– → SnCl62- (aq)
Some compounds are acidic because of having one or more double bonds in the molecule. For example CO۲