A strong acid is any compound capable of completely and irreversibly releasing protons or hydrogen ions, H+. Being so reactive, a large number of species are forced to accept these H+; such as water, the mixture of which becomes potentially dangerous with simple physical contact.
The acid donates a proton to the water, which works as a base to form the hydronium ion, H3OR+. The concentration of the hydronium ion in a solution of a strong acid is equal to the concentration of the acid ([H3OR+] = [HAc]).
In the upper image there is a bottle of hydrochloric acid, HCl, with a concentration of 12M. The higher the concentration of an acid (weak or strong), it is necessary to be more cautious in handling it; that is why the bottle shows the pictogram of a hand injured by the corrosive property of a drop of acid falling on it.
Strong acids are substances that must be handled with full awareness of their possible effects; By working with them carefully, their properties can be used for multiple uses, one of the most common being the synthesis or dissolution of samples..
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A strong acid dissociates or ionizes 100% in aqueous solution, accepting a pair of electrons. The dissociation of an acid can be outlined with the following chemical equation:
HAc + HtwoO => A- + H3OR+
Where HAc is the strong acid, and A- its conjugate base.
The ionization of a strong acid is a process that is usually irreversible; in weak acids, on the contrary, ionization is reversible. The equation shows that HtwoOr is it the one that accepts the proton; however, so can alcohols and other solvents.
This tendency to accept protons varies from substance to substance, and thus the acid strength of HAc is not the same in all solvents..
The pH of a strong acid is very low, being between 0 and 1 pH units. For example, a 0.1 M HCl solution has a pH of 1.
This can be demonstrated by using the formula
pH = - log [H+]
The pH of a 0.1 M HCl solution can be calculated, then applying
pH = -log (0.1)
Obtaining a pH of 1 for the 0.1 M HCl solution.
The strength of acids is related to their pKa. The hydronium ion (H3OR+), for example, has a pKa of -1.74. Generally, strong acids have pKa with values more negative than -1.74, and are therefore more acidic than H itself3OR+.
The pKa expresses in a certain way the tendency of the acid to dissociate. The lower its value, the stronger and more aggressive the acid will be. For this reason, it is convenient to express the relative strength of an acid by the value of its pKa.
In general, strong acids are classified as corrosive. However, there are exceptions to this assumption.
For example, hydrofluoric acid is a weak acid, yet it is highly corrosive and capable of digesting glass. Due to this, it has to be handled in plastic bottles and at low temperatures..
On the contrary, an acid of great strength such as carborane superacid, which despite being millions of times stronger than sulfuric acid, is not corrosive.
As a rightward shift occurs in a period of the periodic table, the negativity of the elements that make up the conjugate base increases.
Observing period 3 of the periodic table shows, for example, that chlorine is more electronegative than sulfur and, in turn, sulfur is more electronegative than phosphorus..
This is in accordance with the fact that hydrochloric acid is stronger than sulfuric acid, and the latter is stronger than phosphoric acid..
By increasing the electronegativity of the conjugate base of the acid, the stability of the base increases, and therefore its tendency to regroup with hydrogen to regenerate the acid decreases..
However, other factors must be considered, since this alone is not decisive.
The strength of the acid also depends on the radius of its conjugate base. The observation of group VIIA of the periodic table (halogens) shows that the atomic radii of the elements that make up the group have the following relationship: I> Br> Cl> F.
Likewise, the acids that form keep the same decreasing order of the strength of the acids:
HI> HBr> HCl> HF
In conclusion, as the atomic radius of the elements of the same group in the periodic table increases, the strength of the acid they form increases in the same way..
This is explained by the weakening of the H-Ac bond by a poor overlap of the atomic orbitals that are unequal in size..
The strength of an acid within a series of oxacids depends on the number of oxygen atoms in the conjugate base..
The molecules that have the highest number of oxygen atoms constitute the species with the highest acid strength. For example, nitric acid (HNO3) is a stronger acid than nitrous acid (HNOtwo).
On the other hand, perchloric acid (HClO4) is a stronger acid than chloric acid (HClO3). And finally, hypochlorous acid (HClO) is the lowest strength acid in the series.
Strong acids can be exemplified in the following decreasing order of acid strength: HI> HBr> HClO4 > HCl> HtwoSW4 > CH₃C₆H₄SO₃H (toluenesulfonic acid)> HNO3.
All of them, and the others that have been mentioned so far, are examples of strong acids..
HI is stronger than HBr because the H-I bond breaks more easily as it is weaker. HBr surpasses HClO in acidity4 because, despite the great stability of the anion ClO4- by delocalizing the negative charge, the H-Br bond remains weaker than the O bond3ClO-H.
However, the presence of four oxygen atoms makes HClO4 more acidic than HCl, which does not have any oxygen.
Next, HCl is stronger than HtwoSW4 because the Cl atom is more electronegative than that of sulfur; and the HtwoSW4 in turn, it exceeds in acidity CH₃C₆H₄SO₃H, which has one less oxygen atom and the bond that holds the hydrogen together is also less polar.
Finally, the HNO3 It is the weakest of all for having the nitrogen atom, from the second period of the periodic table.
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